Optical methods and data processing in heat and fluid flow

Optical Methods and Data Processing in Heat and Fluid Flow

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Optical Methods and Data Processing in Heat and Fluid Flow Edited by C Greated, J Cosgrove, and J M Buick

Published by Professional Engineering Publishing, Bury St Edmunds and London, UK.

First Published 2002 This publication is copyright under the Berne Convention and the International Copyright Convention. All rights reserved. Apart from any fair dealing for the purpose of private study, research, criticism or review, as permitted under the Copyright, Designs and Patents Act, 1988, no part may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, electrical, chemical, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright owners. Unlicensed multiple copying of the contents of this publication is illegal. Inquiries should be addressed to: The Publishing Editor, Professional Engineering Publishing Limited, Northgate Avenue, Bury St Edmunds, Suffolk, IP32 6BW, UK. Fax: +44 (0) 1284 705271.

© 2002 The Institution of Mechanical Engineers, unless otherwise stated.

ISBN 1 86058 281 8

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Contents xi

Preface Section 1

Doppler Anemonmetry

Chapter 1

Comparison of Injector Sprays for Gasoline Direct-injection Engines J Allen, J Heath, G Pitcher, G Hargrave, and G Wigley

1

Application of Laser Doppler Anemometry and Infrared Thermograph Methods for Measurements of Fluid Flow in a Small Transonic Channel R Dizene, E Dorignac, R Leblanc, and J M Charbonnier

13

An Experimental Investigation of the Flow Produced in a Rectangular Container by a Rotating Disc using LDA V G Meledin, I V Naumov, and V A Pavlov

25

The Design, Development, and Preliminary Results from a High-speed, Optically Accessed, Single-cylinder Engine G Pitcher, P Williams, J Allen, and G Wigley

37

Chapter 2

Chapter 3

Chapter 4

Section 2

Laser Speckle and Holography

Chapter 5

The Reflected Spectrum of Complex Multi-layered Inhomogeneous Highly Scattering Medium I V Meglinsky and S J Matcher

45

Digital Speckle Photography Applied to in Vivo Blood Microcirculation Monitoring N Fomin, C Fuentes, J-B Saulnier, and J-L Tuhault

59

Development of Full-volume Digital Holography for Particle Measurement S Murata and N Yasuda

69

A Particle Imaging and Analysis System for Underwater Holograms J J Nebrensky, G Craig, G L Foresti, S Gentili, P R Hobson, H Nareid, G G Pieroni, and J Watson

79

Chapter 6

Chapter 7

Chapter 8

Section 3

Fluorescence, Phospherence, and Liquid Crystals

Chapter 9

The Application of LIF to Study the Dispersion of a Surface Film due to Wave Breaking using a Two-camera System T Schlicke, A D Arnott, J M Buick, C A Greated, and N H Thomas

93

Chapter 10 Thermographic Phosphor Thermometry - Recent Developments for Applications in Gas Turbines J P Feist, A L Heyes, K L Choy, and J R Nicholls Chapter 11

Chapter 12

Chapter 13

103

Improved Liquid Crystal Thermography by using True-colour Image Processing Technology M Wierzbowski, M Ciofalo, and J Stasiek

125

Development of an Optical Measuring Technique for the Study of Acoustical Phenomena J M Buick, J A Cosgrove, D M Campbell, and C A Greated

133

A Study of the Flow Structure in the Near-wall Region of a Complex-shaped Channel using Liquid Crystals G M Zharkova, V N Kovrizhina, and V M Khachaturyan

143

Section 4

PIV

Chapter 14

Spatio-temporal Reconstruction of the Unsteady Wake of Axisymmetric Bluff Bodies via Time-recording DPIV C Brucker

151

The Measurement of the Velocity Field around a Ship Hull Model in a Towing Tank using PIV Method J Dekowski, M Kocik, J Podlinski, J Wasilewski, J Mizeraczyk, L Wilczynski, J Kanar, and J Stqsiek

161

Velocity Measurements in Impinging Turbulent Jets using Digital Particle Image Velocimetry M Fairweather, G K Hargrave, and T C Williams

185

Application of Particle Image Velocimetry to Helicopter Vortex Interactions R B Green and C J Doolan

197

Chapter 15

Chapter 16

Chapter 17

Section 5

Multi-phase flow analysis

Chapter 18

Recognition of Two-phase Flow Patterns with the use of Dynamic Image Analysis R Ulbrich, M Krotkiewicz, N Szmolke, S Anweiler, M Masiukiewicz, and D Zajac

207

Gas/Liquid Mixing: Simultaneous PIV Measurements of Two Phases Mixing Together; High-pressure Spray Application T Boedec and S Simoens

219

Flame Visualization Enhancement by Image Processing W B Ng, K Y Cheung, and Y Zhang

231

Chapter 19

Chapter 20

Chapter 21 Optical Diagnostics - Automatic Data Processing and Application in Fundamental Studies and Control Systems V S Abrukov, I VAndreev, and P V Deltsov Chapter 22

Chapter 23

Chapter 24

247

Application of Physical Modelling to Study Combustion Processes and Flow Patterns in Large-scale Boilers and Furnaces. J Baranski, W Blasiak, and J Stqsiek

267

Pulsed Laser Particle Image Velocimetry using a Fibre-optic Delivery System S S Coltman and C A Greated

279

Automated Fringe Analysis for Profilometric Mass-transfer Experiments J J Nebrensky

295

Authors' Index

305

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xi

Preface It is difficult to overstate the importance of metrology in science, it provides the data on which theories are accepted or rejected and can force a rethinking of ideas formerly held true for many centuries. This book contributes to the science of measurement and is concerned with the use of optical techniques and data processing to quantify fluid mechanic and heat transfer properties as well as closely related topics. The range and diversity of fluid and heat applications necessitates a correspondingly large range of measurement techniques and devices, some of which are not optically based but in the current context provide useful data for comparison purposes. Examples of both optical and non-optical techniques and devices include, thermocouples, strain-gauges, pitot-static tubes, piezoelectric transducers, infrared thermal imaging, laser Doppler velocimeters, hot-wire probes holography, schlieren, nuclear magnetic resonance, and condenser microphones. The rapid development of these techniques over the last few decades ensures the list of methods continually increases and, of course, provides the raison d'etre of this current collection of Chapters. Optical methods have been used for centuries to qualitatively examine flows. The sketches of Leonardo da Vinci capturing the three-dimensional swirls of flowing water provides proof of this in a most aesthetically pleasing way. Why then the huge increase in fluid and heat measurement over the last three or four decades? The advent of sophisticated numerical techniques, particularly in the area of image processing and the rapid advances in the associated hardware, have undoubtedly served as a catalyst. However, prior to this, the development of the laser and laser technology was perhaps even more fundamental. In so many optical measuring techniques applied to fluid and heat measurement: laser Doppler velocimeters, laser speckle techniques, particle tracking velocimeters, holography, fluorescence-based techniques, and infrared techniques, the laser is a fundamental tool. It is perhaps initially surprising that the use of this ubiquitous tool in fluid measurement came about in an almost inadvertent way. Cummins, Knable, and Yeh (1) were investigating the Brownian motion of a colloidal suspension by observing the broadening of the laser light spectrum. In doing this they observed a net shift in the frequency of the light, generated by small convection currents set-up in their measurement volume. Yeh and Cummins (2) then proceeded with this concept, directly using it to measure fluid velocities and hence the field of laser Doppler velocimetry (LDV) was born. The fact that this was a non-intrusive technique, it provided an unambiguous measurement of one or more of the components of the velocity vector and it was largely unaffected by the thermophysical properties of the fluid ensured its rapid growth in the fluid measurement community. Almost in parallel with this, starting with Burch and Tokarskii experiments (3) on fringe formation from multiple scattering sources in 1968, speckle metrology developed, and over the next two decades the field grew and subdivided, creating several new measurement techniques such as particle image velocimetry (PIV) and dynamic speckle methods. More recently with the advances in CCD technology and computer hardware, digital PIV, DPIV, and holographic PIV, HPIV have emerged. Processing the raw experimental data has also seen dramatic changes in the last few decades. For example, in early studies using PIV, the data recorded on photographic negatives was optically correlated based on a Young's Fringes method. This has now largely been replaced

by numerical correlation schemes applied directly to the raw data retrieved from CCD sensors. The development of sophisticated analysis schemes has now become a large research field with the difference in the methods of numerical analysis often largely differentiating between schemes. For example PIV and particle tracking velocimetry (PTV) can use identical recording techniques however the analysis differs. In PIV a statistical correlation is performed yielding an average particle displacement, whereas particle tracking attempts to identify and track individual particles from frame to frame. These analysis techniques are increasingly borrowing from the fields of image processing and are used, for example, to differentiate between the phases in multiphase flows, to examine droplet formation, and to analyse flame phenomena. The harnessing of optical phenomena for heat and temperature measurement continues apace ranging from infrared emissions to the temperature dependence of the phospherence of certain materials. This collection brings together a selection of Chapters detailing some of the latest advances in the many differing aspects of laser measurement. Section 1 is concerned with investigations using measurement techniques based on the Doppler principle. These include methods of improving signal analysis and velocity measurements of rotating and transonic flows as well as flows in combustion engines. An interesting application of phase Doppler anemometry (PDA) to the sizing of droplets in a combustion process is detailed. The laser speckle techniques of Section 2 indicate how these are playing increasingly useful roles in the important field of medical diagnostics. Holographic methods are also presented here with the very interesting application to the automated classification of small marine organisms, as well as the use of holography for the measurement of the three-dimensional positions of particles. In Section 3 an indication of the ever broadening use of techniques, based on fluorescence and phospherence, is given with applications to pollutant dispersion due to ocean wave action, an excellent Chapter on stateof-the-art temperature measurement using thermographic phosphor thermometry and the use of fluorescence in the studies of acoustic phenomena. Two interesting liquid crystal applications are also included in this section. It becomes evident from a perusal of Section 4 that PIV is now well established as a powerful velocity measurement technique in the engineering world with applications to the wake and vortex formation of helicopters and ships. The final section of the book indicates, perhaps the most difficult, problems from a measurement point of view and is titled by flow type, i.e multi-phase flow, rather than measurement technique, since in these challenging flows more than one technique, either on the optical or the analysis side, has to be employed to separate the phases. Thus Section 5 includes a gas-liquid mixing application that combines PIV, LIF, and mie-scattering diffusion, image processing techniques to enhance flame visualization, detonation, turbulence, combustion applications, and mass transfer measurements. (1) (2) (3)

Cummins, H. Z., Knable, N., and Yeh, Y. 'Observation of Diffusion Broadening of Rayleigh Scattered Light', Physical Review Letters, Vol. 12, pp. 150-153, 1964. Yeh, Y. and Cummins, H. Z. 'Localized Fluid Flow Meeasurements with a He-Ne Laser Spectrometer', Applied Physics Letters, Vol. 4, pp. 176-178, 1964. Burch, J. M. and Tokarskii, J. M. J. 'Production of Multiple Beam Fringes from Photographic Scatterers', Optica Acta, Vol. 15 (2), pp.101-111, 1968.

1 Comparison of Injector Sprays for Gasoline Direct-injection Engines J Allen, J Heath, G Pitcher, G K Hargrave, and G Wigley

Abstract The spray characteristics of a variety of injectors designed for use in gasoline direct injection engines have been measured using phase Doppler anemometry and high-speed digital imaging techniques in atmospheric and elevated pressure regimes. The interpretation of these results and their application to the design of a direct-injected spark-ignition engine are considered.

1.1

Introduction

Spark ignition gasoline engines equipped with direct injection have already reached the market place, installed in mass produced passenger cars (1). This technology has the potential to deliver fuel economy and emissions benefits. Direct injection fuel systems are evolving along three paths. 1. Single fluid - high fuel supply pressure injection. 2. Dual fluid - low fuel pressure with air-assisted injection. 3 Single fluid - low fuel supply pressure with solenoid assisted injection As part of Lotus' gasoline direct injection research programme, examples of each of these three types of injectors were assessed using a range of complimentary optical diagnostic techniques. The results from this spray characterization have allowed comparison of the relative merits of these injectors and provided direction for suitable combustion chamber designs and fuelling strategies for each type tested.

Optical Methods and Data Processing in Heat and Fluid Flow

2

1.2

Spray measurement techniques

In conjunction with the Faculty of Engineering at Loughborough University, Lotus has established a series of optical diagnostic rigs to allow the comprehensive measurement and imaging of injector sprays, using gasoline as the test fluid. High-speed imaging techniques allow visualization of the fuel sprays while phase Doppler anemometry (PDA) techniques have been used to measure droplet velocities and diameters. The facility allows sprays to be imaged and measured in both atmospheric conditions and at elevated pressures.

1.3

Injector spray tests into atmosphere

Four injectors were tested: one air-assisted, two single fluid with high fuel supply pressure, and one single fluid with a low supply pressure and solenoid-assisted injection. These are listed in Table 1.1. The injectors were each tested at four fuel-delivery rates. These fuel loads, and the equivalent operating points for a four-cylinder, 2.2-litre engine, are given in Table 1.2.

Table l.1 Injector types tested Injector name Injector type Fuel pressure Air pressure Type 1 50 bar Single fluid Type 2 100 bar Single fluid Air assist 7.2 bar 6.5 bar Low pressure dual fluid Solenoid Supply 4 bar* Single fluid solenoid injection 'The solenoid injector is supplied with fuel at 4 bar. The solenoid injection system raises this to approximately 30 bar during injection

Table 1.2 Fuelling conditions Fuel load

Fuel injected per cycle Equivalent engine (mg/injection) BMEP (bar) Load A 6.5 Idle Load B 11 2 21 Load C 4 Load D 53 (33 for solenoid injector)* 11.5(7)* *The maximum amount of fuel from the solenoid injector in one injection event is equivalent to 7 bar brake mean effective power (BMEP) at 4400 r/min.

Engine speed (r/min) 750 2000 4000 4400 33 mg/injection

Images taken of the spray from each injector at fuel load B, using a laser sheet or diffuse xenon flash back lighting for illumination, are shown in Fig. 1.1. These images have been chosen to illustrate the wide range of spray structures found with these injectors. The timing of each image with respect to the start of injection is indicated in the figure. The type 1 and type 2 single fluid injectors both have a cone shaped spray structure. There is a leading central jet of fuel issued from the type 1 injector before the cone structure has time to

Comparison of Injector Sprays for Gasoline Direct-injection Engines

3

form. The high penetration rate of this central part of the spray structure could lead to piston crown or cylinder wall wetting, depending upon the injection timing strategy used. Cylinder wall wetting would be undesirable from both a combustion and lubrication perspective. With the type 2 injector, which is also a single fluid design, there is no central jet of fuel evident.

Fig. 1.1 Spray structure diversity

The air-assist injector has a shaped nozzle extension that has the effect of containing the spray in a plume below the nozzle. The solenoid in the solenoid injector is used to drive a plunger that pressurizes the fuel. In the nozzle is a sprung poppet valve that is opened by the fuel pressure acting behind it - this ensures that the fuel has reached the required pressure before the valve opens. The amount of fuel injected is determined by the duration of the signal to the solenoid and thus the stroke through which the plunger operates. It can be seen that the fuel appears to leave the injector as thin streams of liquid jets, forming a narrow cone angle. Imaging of the spray gives useful information on its size, structure, and speed of penetration. To obtain a measurement of the droplet diameters and more detailed velocity data, the phase Doppler anemometry (PDA) measurement technique has been used. As will be seen later, combining these techniques can give a better understanding of the spray than any one of these when used in isolation. Two fuel loads are presented; load B is 11 mg of fuel per injection event and represents a BMEP value of 2 bar for a 2.2-litre, four-cylinder engine running at 2000 r/min. Load D is 53 mg/injection and represents full-load, peak-torque (with a power enrichment fuelling strategy) in the same engine design. Results from the solenoid injector are not shown for fuel load D since this injector can not deliver the required quantity of fuel in a single injection. The PDA droplet diameter data recorded for each injector is shown in histogram format in Fig. 1.2. The data presented is both spatially and temporally averaged for the whole spray. It can be seen that the air-assist injector has the largest proportion of small diameter droplets, with a rapid drop in the number of samples measured as the droplet diameter increases. At the opposite end of the range is the type 1 injector which has a far lower number of small droplets. It can also be seen that the increase in fuel load from B to D causes a reduction in the peak of small diameter droplets and a subsequent shift toward the larger sizes.

4

Optical Methods and Data Processing in Heat and Fluid Flow

Fig. 1.2 Droplet size comparison at fuel load B and D

Further analysis of the PDA results yields statistical data. The D10 (arithmetic mean) and the D32 (Sauter mean) droplet diameter, plotted against fuel load for each injector, is shown in Fig. 1.3. The air-assist injector is seen to have the smallest D10 droplet diameter of between 3 and 4.5 um, depending on the fuel load. The type 2 single fluid injector has a mean droplet size of between 1.5 and 3.5 um larger than the air-assist, while the solenoid and type 1 injectors produce even larger D10 droplet diameters.

Fig. 1.3 Effect of fuel load on arithmetic mean and Sauter mean droplet diameters

As the fuel load is increased, the air-assist injector shows a rise in D10 droplet diameter of 1.5 um between 11 mg/injection (load B) and 21 mg/injection (load C). Beyond this, the droplet diameter levels off. The type 2 injector displays a continuing rise in D10 droplet diameter as the fuel load is increased. The type 1 injector also shows a continuing rise in droplet diameter as the fuel load is increased, but to a lesser degree than the type 2 injector. The rise in droplet size from fuel load A (6.5 mg/injection) to load B (11 mg/injection), however, is more pronounced. The solenoid injector shows very little variation in droplet diameter as the fuel load is changed. The Sauter mean diameter (SMD or D32) is calculated from the ratio of droplet volume to surface area and so represents the ability of the droplets in the spray to vapourize. A high

Comparison of Injector Sprays for Gasoline Direct-injection Engines

5

SMD indicates that the fuel spray contains some large droplets and so will not vapourize as readily as a spray with a low SMD value. The SMD is very sensitive to, and increased significantly by, the presence of even a very small percentage of large droplets. For this reason it is essential that the droplet diameter measurement range used with each spray being analysed is consistent. At low fuel loads the air-assist injector exhibits the lowest SMD seen for all of the test injectors. It can be seen that this rises rapidly as the fuel load is increased from B (11 mg/injection) to C (21 mg/injection), and continues to climb further as the fuel load is increased to D (53 mg/injection). The SMD for the type 2 and type 1 injectors appears to be fairly insensitive to fuel load, while the solenoid injector appears to show a reduction in SMD with increasing fuel load. These results require careful consideration for the reasons given in the following paragraphs. The change in the PDA D10 droplet diameter as the axial distance from the nozzle is increased is shown for fuel loads B (11 mg/injection) and D (53 mg/injection) in Fig. 1.4. The results given in Figs 1.4 are temporally averaged over the complete injection period and spatially averaged for each of the downstream traverse positions.

Fig. 1.4 Effect of distance from nozzle on D10 droplet diameter (fuel loads B and D)

In Fig. 1.4 the air-assist and type 2 injectors show a consistent mean droplet diameter along the spray axis. The type 1 injector shows an initial rise in D10 droplet diameter up to 20 mm downstream of the nozzle, with a consistent diameter of 8 um thereafter. The solenoid injector shows a gradual increase in D10 droplet diameter downstream of the nozzle. The simplest mechanisms to explain this are: • coalescence of droplets; • complete vapourization of the smallest droplets. The effects seen can not, however, be due to coalescence. This is because of the drop sparsity found in the sprays. The geometry of the cone structure causes the spray to become even less dense further from the nozzle, making coalescence even less likely.

6

Optical Methods and Data Processing in Heat and Fluid Flow

The vapourization of smaller droplets could cause an increase in the percentage of larger droplets observed, but this would be expected to be apparent in the results measured for the air-assist injector at fuel load B (11 mg/injection). At this condition this injector is extremely effective at atomizing the fuel into droplets of below 2 um, as shown in Fig. 1.4. Despite the large percentage of droplets of this size, no increase in the measured droplet size was experienced as the distance from the nozzle was increased. Conversely, it could be argued that the droplet diameter would reduce, not increase, as the spray is measured further from the nozzle, for the following reasons. • Aerodynamic effects acting on the largest droplets would be expected to break them up into smaller droplets. The Weber number determines the extent to which this happens. • Droplets further from the nozzle will have had more time for vapourization to occur, therefore would be reduced in size. The phenomenon of increasing droplet size with increased axial distance is even more pronounced at fuel load D (53 mg/injection). This is seen even with the air-assist and type 2 injectors which didn't exhibit this characteristic at the lower fuel load. A basic understanding of the PDA measurement method (2), and cross-referencing of the results with images taken of the sprays, is required in order to explain these results. The PDA technique measures the diameters of spherical droplets and their velocities. If the fuel is not in droplet form, the signal processor will reject that data. Images of the type 1 and solenoid injector fuel sprays are shown in Fig. 1.5.

Fig. 1.5 Images of fuel from the type 1 and solenoid injectors showing fuel filaments

Filaments of unatomized fuel are clearly visible in the spray structure. The image of the type 1 spray was taken showing the area between 10 and 20 mm downstream of the nozzle, fuel load D (53 mg/injection), while the image from the solenoid injector shows 0-30 mm downstream at its maximum fuel load of 33 mg/injection. Study of these images in conjunction with the PDA data allows the correct interpretation of these results. As the measurement position is

Comparison of Injector Sprays for Gasoline Direct-injection Engines

7

moved axially downstream from the nozzle, more of the fuel that was originally in filament form (and thus not measured by the PDA measurement technique) breaks up into droplets and so can now be measured. This gives the impression of increased droplet diameters further down the spray. From the work performed in atmospheric conditions, it can be seen that the techniques used for studying the sprays are complimentary. Combining results from high-speed imaging of the sprays and from the PDA measurement technique can give a greater understanding of the spray characteristics than either one of these techniques on its own. The dual fluid air-assist injector is the most effective at atomizing fuel at the lower fuel loads, but is seen to produce some larger droplets as the fuel load is increased. The type 2 injector is a more effective single fluid atomizer than the type 1, which in turn is more effective than the solenoid injector. As well as their effectiveness at atomizing the fuel, the spray shape and penetration velocity of each of the sprays is important when considering their application in an engine.

1.4

Cylinder pressure effects

The effect of elevated cylinder pressure on spray penetration at fuel load B (11 mg/injection) for the type 1 and air-assist injectors is shown in Fig. 1.6. The tests were performed in a pressurized chamber with fused silica windows for optical access. The chamber was pressurized using nitrogen gas. It can be seen that the general trend is for a more compact spray as the chamber pressure is increased. It is the increased density of the atmosphere in the chamber that causes the fuel droplets to impart their momentum to their surroundings more rapidly.

Fig. 1.6 Effect of pressure: air assist (2.0 ms after SOI) type 1 (1.2 ms after SOI)

With the single fluid type 1 injector the largest effect on the spray envelope was seen as the chamber pressure was increased from 0-6 bar (gauge). Increasing the pressure up to 15 bar appears to have had a lesser effect. The air-assist injector spray was imaged injecting into chamber pressures of up to 5 bar. At this chamber pressure the pressure drop across the nozzle is reduced to just 1.5 bar. It can be seen that the size of the fuel spray plume at this condition is much reduced. This is due to a combination a lower initial spray velocity and the increased chamber pressure causing the fuel to decelerate more rapidly.

8

Optical Methods and Data Processing in Heat and Fluid Flow

The PDA measurement technique was used in conjunction with the pressure chamber to ascertain the effect on droplet diameter. The results, using the type 1 injector, were taken on the spray centreline, 18 mm downstream of the nozzle and are shown in Fig. 1.7. It can be seen that the spread of droplet diameters widens as the chamber pressure is increased, reducing the concentration of small diameter droplets.

Fig. 1.7 Effect of cylinder pressure on droplet size, type 1 injector, spray centreline, 18 mm downstream

1.5

Application to combustion chamber design

Results from the spray analysis have allowed consideration of the application of each injector type into combustion chamber designs and the fuelling strategies that will need to be employed. Delivering the fuel to the spark plug at the correct time during the cycle is essential when operating under stratified conditions. In-cylinder air motion, such as tumble or swirl, can be used to do this. Piston crown shape can also be used to influence the air (and thus fuel) motion (3). Alternatively, with a finely atomized fuel spray, the fuel can be aimed directly at the spark plug (without risk of plug fouling) and the injection timing used to ensure a stratified mixture is delivered at the correct time, thus minimizing surface wetting and a possible source of hydrocarbon emissions. This latter system is preferred by Orbital using their dual-fluid injector (4). The injection timing strategy used with each injector to achieve stratified operation is dependent upon the pressure drop across the nozzle, the spray penetration rate, the spray envelope, and the dropsize distribution. It has been reported that droplet size is the factor limiting how late into the compression stroke injection can continue, while still leaving time for the fuel to vapourize (5). Therefore, a low-pressure air-assist system with good spray atomization, despite the lower pressure drop across the nozzle, may be suited to injection later into the compression stroke than a high-pressure single fluid system with less effective atomization. When selecting an injector for our own direct-injection research and demonstration engine, our choice was influenced by the air-assist injector's ability to produce a well contained and finely atomized spray. Its requirement for a low pressure fuel and air system could also lead to

Comparison of Injector Sprays for Gasoline Direct-injection Engines

9

a faster implementation. On these grounds a packaging study of the air-assist injector into a contemporary 1.8-litre, four-cylinder, sixteen-valve engine has been undertaken. An issue with stratified operation is the exhaust after-treatment system. Conventional catalytic converters require the engine to run at stoichiometric air-fuel ratios. When running with very lean stratified mixtures, the conversion of the oxides of nitrogen in the exhaust gases can be poor, especially when operating on European specification fuels which have a high sulphur content. Work in the field of exhaust after-treatment of lean bum engines is still ongoing, and for this reason the decision was made to implement a direct-injection scheme using homogeneous charge at stoichiometric air-fuel ratios. The aim of the conversion to direct injection has been to produce a homogeneously charged engine with minimal changes to the cylinder head and port architecture. This will allow investigation into the effects on volumetric efficiency and transient fuelling and emissions control. Two concepts were studied that would allow these criteria to be met; these were central and side injector positions. A number of central injector schemes were considered to address the issue of packaging both the injector and spark plug. The final design concept chosen was a side injection scheme using injectors with an 80 degree cone angle. The injectors were mounted under the (unmodified) intake ports. The angle of installation of the injector was constrained by the inlet ports, head gasket sealing faces, and clearance to the piston at TDC. The PDA data from the 80 degree cone angle injectors were processed and put into an animated vector file. These vectors were then superimposed on to a schematic of the combustion chamber to show the sort of interaction that could be expected between the spray with the physical boundaries of the combustion chamber and piston. Frames from this animation, showing an engine running at 2000 r/min with fuelling to produce a brake mean effective pressure (BMEP) of 2 bar, can be seen in Fig. 1.8.

Fig. 1.8 Frames from PDA animation showing droplet velocities and sizes

10

Optical Methods and Data Processing in Heat and Fluid Flow

Fig. 1.8 Frames from PDA animation showing droplet velocities and sizes (cont)

The injection event is timed to avoid the fuel contacting the piston, while still occurring early enough in the intake stroke to allow thorough mixing for a homogeneous mixture preparation. Fuel is first seen 5 mm below the nozzle at 7.25 ms after top dead centre (ATDC) on the induction stroke; this equates to 87 crank angle degrees. The final timing of injection events will be decided from the results of a series of calibration experiments being undertaken on the test engine. It can be seen that the shape of the nozzle keeps the fuel spray contained, preventing it from wetting the combustion chamber roof and spark plug. Some intake valve wetting will occur (these are not shown on the model), although it is envisaged that the air flow across the valves will keep this to a minimum. At the end of injection (12.35 ms ATDC), the fuel has reached the far side of the combustion chamber and could cause wall wetting and bore washing. Study of the corresponding droplet sizes shows these to be below 2 um in diameter and so these would be expected to have vapourized before penetrating this far in a running engine. The engine has been built, installed on a dynamometer and is currently undergoing testing. At the time of writing the test programme has not yet been completed, although initial brake specific fuel consumption and emissions results are encouraging.

1.6

Conclusions

Optical diagnostic techniques have been used to characterize the sprays from a variety of gasoline direct injector designs. The measurement techniques used have been found to be complimentary and have led to a greater understanding of the processes involved than any technique used in isolation. Imaging techniques have allowed the sprays' envelopes to be defined in atmospheric and elevated pressure conditions. The spray measurements have shown the air-assist injector to produce the smallest arithmetic mean droplet diameters of all the injectors tested, with diameters of between 3 and 4.5 um, dependent upon fuel load. The type 2 injector is the most effective single fluid atomizer,

Comparison of Injector Sprays for Gasoline Direct-injection Engines

11

producing droplets of between 4.5 and 8 um mean diameter, 1.5-3 um smaller than those from the type 1 injector. The solenoid injector appears to produce similar mean droplet diameters to those from the type 1 injector, although closer study of the results indicates that a large proportion of the fuel may be in filament, not droplet, form. The PDA measurement results will enable the effect of droplet size and velocity to be ascertained on the performance of engines utilizing direct injection of gasoline. Use of the PDA technique in conjunction with Lotus' optically accessed engine will also allow validation of in-cylinder airflow and spray models. The results from the measurement of the fuel sprays have been used to package direct injectors into the cylinder head of a contemporary 1.8-litre, sixteen-valve, four-cylinder engine.

References (1)

(2)

(3)

(4)

(5)

Iwamoto, Y., Noma, K., Nakayama, T., Yamauchi, T., and Ando, H. Development of a Gasoline Direct Injection Engine, SAE International Congress and Exposition, Detroit, 1997. SAE Paper Number 970541 Wigley, G., Hargrave, G., and Heath, J. A High Power, High Resolution LDA/PDA System Applied to Gasoline Direct Injector Sprays, Particle and Particle System Characterisation, vol. 16. Wiley-VCH, 1999. Lake, T., Stokes, J., Whitaker, P., and Crump, J. Comparison of Direct Injection Gasoline Combustion Systems, SAE International Congress and Exposition, Detroit, 1998. SAE Paper Number 980154. Houston, R. and Cathcart, G. Combustion and Emission Characteristics of Orbital's Combustion Process Applied to Multicylinder Automotive Direct Injected 4-Stroke Engines, SAE International Congress and Exposition, Detroit, 1998. SAE Paper Number 980153 Dodge, L. G. Fuel Preparation Requirements for Direct-Injected Spark-Ignition Engines, SAE International Congress and Exposition, Detroit, 1996. SAE Paper Number 962015

J Allen, J Heath, and G Pitcher Lotus Engineering, Hethel, Norwich, UK G K Hargrave and G Wigley Loughborough University, UK © With Authors 2002

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2 Application of Laser Doppler Anemometry and Infrared Thermograph Methods for Measurements of Fluid Flow in a Small Transonic Channel R Dizene, E Dorignac, R Leblanc, and J M Charbonnier

Abstract This Chapter presents a comparison of laser Doppler anemometer and Pitot-static sensing techniques for the measurement of the mean velocity and the turbulence properties in flows in a small transonic channel. The work deals with turbine blade cooling. The measurements have been done in a turbulent boundary layer which develops in jets/cross flow interaction. Comparison of precision measurement results between the two methods were made by conducting a series of tests and experiments, where the influence of the seeding over the measurements was shown. The mean and standard deviation estimated from the data are subject to both systematic and statistical uncertainty, and an analysis of these uncertainties is given. Another method of optical processing was investigated and used for stationary measurements of the temperature and the heat flux densities. This method consists of an application of an infrared thermograph technique. Conclusions are presented by the analysis of comparison results of wall temperature measurements with those taken from located thermocouples sensing.

Notation D

injection tubes diameter (mm)

R

blowing rate ratio

14

Optical Methods and Data Processing in Heat and Fluid Flow

Re u, v T X, Y, and Z

reynolds number longitudinal and vertical mean velocity components (m.s-1) local mean temperature (°K) longitudinal, vertical, and transversal distances (mm)

9

Non dimensional wall temperature

Lower symbols e external flow gj stagnation jet j Jet w wall

2.1

Introduction

Turbine airfoil metal temperatures in many current high temperature gas turbines are maintained at acceptable levels by means of film cooling. In order to predict the interaction behaviour between the coolant and the mainstream, and the exterior temperature and convection heat flux distribution on cooled turbine blade, a series of measurements was conducted in the closed rectangular cross section of a transonic wind tunnel. A detailed investigation based on a number of tests have been made on two-dimensional film cooling and have been concerned with the determination of velocity, temperature and pressure fields, and the determination of wall temperature. The results of this investigation are presented in Dizene et al. (1) and (2). The present study is concerned with the measurement techniques that would lead to predictions of both the interaction phenomena in the flow field and the heat-transfer coefficients in the neighbourhood and the downstream of a single row of injection holes with air as the film coolant and with a mainstream of air. Although a number of different geometries for the injection holes are possible, a system is chosen that approximates one used in many applications. This is a row of circular holes inclined at 45 degrees to the surface and spaced apart, centre to centre, by three diameters. Figure 2.1, which shows the complete test apparatus, gives an indication of this geometry in part of the figure. In order to study film cooling problem from a single slot, film cooling from a single row of holes, or from other system of injection, it requires a source of high temperature for practical exposure times. To simplify the test set, and to save more energy, the thermal problem was inverted. Hence, the problem remains like a cross flow study developed on a heated flat plate through which warm air was injected in the ambient mainstream. In both problems, real film cooling or the inverted thermal problem, the plate is taken at intermediary temperatures and the study is not modified as the Richardson number, which is defined as the gravitational forces to inertia forces ratio, remains negligible. The need of several measurements to take them around and behind holes, velocity and turbulence intensity were measured by a one-component laser anemometer system. The flow field temperature was measured using a thermocouple probe, and the wall temperature distribution was measured using an infrared technique, developed by Dorignac (3).

Application of Laser Doppler Anemometry and Infrared Thermograph Methods for Measurements of Fluid Flow in a Small Transonic Channel

15

Fig. 2.1 Wind tunnel equipment and injection model

2.2

Experimental apparatus and measurement techniques

2.2.1 Wind tunnel and test model The experiments were conducted in the University of Poitiers Aerodynamic study laboratory. All the measurements were made in rectangular cross section transonic wind tunnel. The dimensions of the tunnel test section are 40 mm high, 80 mm wide, and 600 mm long. The side walls were set to diverge slightly to maintain a uniform free stream speed of the air in the tunnel. The mainstream velocity and the boundary layer profiles were determined using a total and static pressure probe, again with static wall taps. With a normal free stream speed of 235 m/s, the boundary layer displacement thickness at the upstream edge of the injection holes was about 15 mm. The Reynolds number based on this velocity and diameter D (5 mm) was 0.78 x 105. Air was injected through five tubes spaced three diameters apart across the span. The tubes are long enough to assure fully developed turbulent flow at the exit in the absence of a mainstream flow. The flow inside the wind tunnel is developed by a supersonic ejector placed downstream of the sonic throat, as shown in Fig. 2.1. The overall flow was determined by measuring the pressure drop across an orifice plate. The injection abscise (X = 0) is located at 380 mm from the tunnel test section inlet. The dimensions of the measurement surface are X = 20 D long and Z = -1.5 D wide. The thermal test conditions are inverted by using a wall temperature of Tw = 313 K which is lower than the jets temperature Tgj = 333 K but above than mainstream temperature Tge = 286 K. Hence, a temperature difference between main and coolant flows of approximately 45 oC was obtained by heating the coolant approximately 40 °C above room temperature. The test plate was 600 mm long and was formed in the lower wall of the test section. The lower surface wall is maintained at a constant temperature by using a heated water circulation system. Seven thermocouples are embedded in the test plate

16

Optical Methods and Data Processing in Heat and Fluid Flow

at Y = 0.8 mm, and these measure and control the wall temperature and are located along the axe of symmetry (Z = 0). In these test conditions and as previously discussed, we will observe a heat flux from the wall to the flow. Then, in order to obtain an optimal cooling effectiveness, the heat flux will be negative and therefore the wall will be heated by jets. 2.2.2 Measurement techniques The flow field characteristics and wall surface flow studies were realized by taking some velocity and temperature measurements. Velocity and turbulence were measured using a onecomponent laser anemometer system. The magnitude of the velocity vector was then estimated and the flow angle determined. The temperature above the plate was measured using chromel alumel thermocouples having a recovery factor equal to 0.88. Detailed measurements were performed using cross flow momentum flux ranging from about 3-5 times the jets momentum fluxes. For data processing, the output voltages of the sensors were stored numerically on a computer. An amplifier multiplexed intelligent card for data acquisition collects the output voltages. 2.2.2.1 Laser Doppler anemometer system In numerous flow investigations, the variables required are the components of the mean velocity vector and the turbulence intensities. In our configuration, further information on the flow which is included in the Reynolds shear stresses and distribution excesses, is often of little interest in these investigations. The longitudinal and vertical mean velocity distributions were measured using a one-component LA system with its optical arrangement shown in Fig. 2.2. The beam splitter device is of an auto corrector model. The following numbers indicate : 1 the beam expander, 2 the dicrotic beam splitter, 3 deviation prism, 4 the Brag cell, 5 the collimator lens. The elements 2, 3, and 4 are interdependent and mounted on a pivoting arbour. The collimator lens assure, the beam convergence. Because of the relatively high compressible flow velocity, all measurements were made by a forward scattering set-up. The transmitter-receiver optical displacement is realized by an electrical motor moving step by step with a resolution of 0.1 mm. Fringe defilement is realized by Brag cell at a frequency of 40 MHz.

Fig. 2.2 Optical arrangement shame of LA system

The seeding method consists of an injection into the mainstream of pulverized vegetable oil particles 1 um in diameter size. The use of the seed injection into the flow field in order to

Application of Laser Doppler Anemometry and Infrared Thermograph Methods for Measurements of Fluid Flow in a Small Transonic Channel

17

enable LA measurements can result in the accumulation of seed particles on the windows inner surfaces. This deposition is due to large particles which do not follow the flow and are therefore scotched on the surface of the window. The photo multiplier can be saturated because of reflected light entering the collection optics. Reflections from window surfaces can be reduced by using anti-reflection coatings on the window. The mean velocity components u and v are respectively measured. The measurement direction is changed when the fringes are oriented to measure the other component. Data processing is performed using a TSI processor 1990 model. The output signal is received on an intelligent CTM 05 interface card, manufactured by Metrabyte located in a computer. The uncertainties in the measured velocity were approximately ±4 m/s. 2.2.2.2 Infrared technique set-up A second measurement technique was developed to obtain detailed film effectiveness data using a scanning camera. Data for this investigation were obtained in the same open circuit transonic wind tunnel. A view of the tunnel installation in the region of the test model is presented in Fig. 2.3. The surface temperature and heat fluxes on the flat plate model were determined using an AGEMA infrared scanning camera SWB 880. This camera had a depth of field of approximately 5 cm when focused on the test section and 35 cm from the camera. The infrared viewed the test surface through a scuttle of vinyl chloride which is used as an infrared window in the test section up wall. The camera was calibrated and the emissivity coefficient obtained was 0.87. The test surface of the model was coated with high emissivity flat-black paint. The thermocouple measurements provided accurate temperature measurements at seven locations and the infrared scanning camera provided isotherm contours at selected temperature levels which would have been possible only with a multitude of thermocouples. For some coolant flow rates, four separate isothermal levels were mapped along the X direction with a 250 x 250 pixel resolution. In the span wise direction which the length covered is about X = 40 D. The flux density is deduced from an application of a numerical method developed by Dorignac (3), based on a stationary conduction model. The validation method is obtained by comparing the isotherm contour value with the value resulting from thermocouple measurement at the same position. The measured temperature is influenced by numerous parameters like emissivity coefficient and radiation issued from the surroundings. The data are presented in a plane view of the flat plate with the coolant holes. The hole position on the isotherm display is difficult. This position was realized using high temperature variations. Then the isotherm contours were positioned with an uncertainty of ±0.26 mm. The uncertainties in measured temperatures taken by the scanning camera were approximately 10 per cent (3).

18

Optical Methods and Data Processing in Heat and Fluid Flow

Fig. 2.3 IR scanning camera in test cell

2.3

Results and Discussions

2.3.1 Channel flow fields laser anemometer measurement 2.3.1.1 Seed particle considerations The trade-offs between seeding the entire flow field, using a point source of seed, or not seeding are briefly discussed below. When designing a laser anemometer experiment, we must decide whether to seed the entire flow field and jet flow (full coverage) or not to use seed in the jet flow (without injection). Both methods are used in our experiments and compared to see by which method the signal strength can be improved relative to the background noise. The method chosen is dependent on the flow field characteristics. The seeder used has been designed in the aerodynamic studies laboratory (LEA in FRANCE) which uses natural vegetable oil. This seed material is formed by pulverizing a particle vegetable oil which is injected into the flow field. The seeder is placed far enough from the upstream to enable the decay of the wake shed before the measurement point is reached. The results of these two methods are shown in Fig. 2.4. When not using seed, the light reflected from solid surfaces is orders of magnitude higher in intensity than the light scattered by the particles which are naturally present in atmospheric air, so this dominates the PMT signal, i.e. the signal to noise ratio of the PMT signal remains minimum or may drop to zero. Seed particles are injected locally into the upstream flow, thus only a comparatively few of the particles may penetrate the wake flow where measurements are being taken So, this is not sufficient to yield adequate data rates and we observe a low turbulence intensity in all measurement planes. When seed is used (full coverage), the signal to noise ratio remains constant, so we observe a high turbulence intensity in all planes than without seeding and validate reference ratio remains 100 per cent.

Application of Laser Doppler Anemometry and Infrared Thermograph Methods for Measurements of Fluid Flow in a Small Transonic Channel

19

Fig. 2.4(a) Axial mean velocity jets/mainstream seeded o flow field seeded

Fig. 2.4(b) Axial velocity fluctuation jets/flow field seeded o flow field seeded

2.3.1.2 Velocity component measurements Our optical configuration has been constructed for measuring one velocity component. The simplest method of measuring velocity magnitude and flow angle is to acquire measurements at three different fringe orientations. The statistical error, measured with a single component system is not greater than that resulting from a two-component system as was described in (1). The magnitude of the turbulence components u' and v' are also determined with measurements taken in the X and Y co-ordinate directions, and compared in measurement planes located without distortions, with that resulting from a two-component system (4) in Fig. 2.5. We observe satisfactory agreement between both measurement results.

20

Optical Methods and Data Processing in Heat and Fluid Flow

Fig. 2.5(a) Mean velocity profiles measured using LA-system

Fig. 2.5(b) Turbulence intensity profiles measured using LA-system one - component LA-system o two-components LA-system

Application of Laser Doppler Anemometry and Infrared Thermograph Methods for Measurements of Fluid Flow in a Small Transonic Channel

21

In addition, the magnitude of velocity vector V and flow angle a measured with a onecomponent laser anemometer system are presented and discussed in Fig. 2.5. The measurement velocity components are made along the symmetrical axe (Z/D = 0) and in lateral positions located at Z/D = 6 and 20. Results are presented respectively in Figs 2.6(a) and 2.6(b) and compared with that resulting from a single Pitot probe tube. Great differences in magnitude are observed in the symmetry plane until X/D = 2, i.e. where the flow angle remains large (about a = 15 degrees).

Fig. 2.6 Comparison of Pitot/LDV velocity vector profiles

22

Optical Methods and Data Processing in Heat and Fluid Flow

We observe here that the velocity vectors resulting from a laser anemometer system measurement are usually much larger in magnitude than the velocity vector resulting from Pitot tube measurements. Use of Pitot tube probe results in two greater limitations. The first one is its sensitivity to accurate estimations of mean velocity vector when the flow angle is greater than 15 degrees. The measurement error is estimated in this way at about 20 per cent. Figure 2.6(b) illustrates great differences particularly in symmetry plane (Z/D = 0). For the downstream flow field (Z/D = 20) when the flow angle is small, the values resulting from the two measurement techniques are in good agreement. The second one is obviously due to acoustic effects which have been present outside and more inside the turbulent boundary layer. From the above results and taking into account the order of magnitude of the instantaneous flow angle, it appears clearly that for the flow configuration of interest here, the scale pressure mean can not be measured with an acceptable accuracy by using a classical Pitot static and Pitot total tube. The brievely flow description obtained with a one and two-component velocity measurements and presented here show that the statistical error in velocity measured with a single component system will, in most applications, be to date. Single channel laser anemometer systems have both aerodynamic and turbomachinary applications. The advantages of a single channel system are its simplicity and the fact that the available laser power is concentrated into a single fringe system. 2.3.2 IR camera temperature measurements In studying the two-dimensional film cooling from a single row of holes, a convenient means of analysing the problem has been to consider the wall temperature and the heat transfer coefficient as separate quantities to be determined. The wall temperature can be put in a convenient dimensionless form 6w as defined previously. As for aerodynamic fields, the thermal quantities have been determined in regions around holes, behind the holes, and downstream the holes. The measured wall temperature evolution is shown in Fig. 2.7. The figures in the span wise planes (X, Z) are given in order to appreciate the phenomena in the Y direction.

Fig. 2.7 Isotherm contours and flux density IR camera evolution

Application of Laser Doppler Anemometry and Infrared Thermograph Methods for Measurements of Fluid Flow in a Small Transonic Channel

23

The plane located at position X = 0 and at very close station measurements, the thermal effect, induced by the warm jet exit, may be seen. Downstream, points out the mixing and the thermal diffusion of the jets. Wall temperatures were measured in planes with the thermocouple probe located at positions X/D = 1; 2 and 10 along the injection tubes and in the mainstream. The profiles close the wall (Y/D < 1) are only presented in Fig. 2.8. These results are compared to heat gradients from the surface wall to the turbulent boundary layer that are indicated by the infrared camera. The temperature values indicated by the camera are below the point value which is close to the wall and the value which is measured by using a thermocouple probe. Hence, the slope is considered positive and we can think there are some points located near the wall, which are not measured by the thermocouple probe position, where the temperature profiles presents a minimum value. This phenomena is an unfavourable effect on the heat transfer between the jets and the wall.

Fig. 2.8 Temperature gradient effects

2.4

Conclusion

A series of tests and experiments in a small laboratory transonic wind tunnel has been conducted using a laser Doppler anemometer system and an infrared thermograph measurement technique. The method of seeding applied to the LA measurement system is presented, along with the results from classical, static, and total pressure tube measurements to estimate the velocity in a turbulent boundary layer and both methods are compared. The results show that for the flow configuration considered here, the classical method does not

24

Optical Methods and Data Processing in Heat and Fluid Flow

allow measurement of the in-stream mean pressure Pitot tube with an acceptable accuracy. Some results issued from the method of a one-component LA system at two different fringe orientations are compared with that from a two-component LA system for mean and fluctuation velocity measurements. Statistical error in the velocity measured with the onecomponent LA system is acceptable.

References (1)

(2)

(3)

(4)

Dizene, R., Charbonnier, J. M., Dorignac, E., and Leblanc, R. Etude experimentale d'une interaction de jets obliques avec un ecoulement transversal compressible. I. Effets de la compressibilite en regime subsonique sur les champs aerothermiques. International Journal of Thermal Science, March 2000, vol. 39 N°3, pp 390-03. Dizene, R., Dorignac, E., Charbonnier, J. M., and Leblanc, R. Etude experimentale d'une interaction de jets obliques avec un ecoulement transversal compressible. II. Effets du taux d'injection sur les transferts thermiques. International Journal of Thermal Science, May 2000, vol. 39 N°5, pp 571-581. Dorignac, E. Contribution a 1'etude de la convection forcee sur une plaque en presence de jets parietaux dans un ecoulement subsonique. These de Doctorat, Universite de Poitiers, 1990. Bousgarbies, J. L., Foucault, E., Vuillerme, J. J., and Dirignac, E. Elude de 1'interaction jets/ecoulement en paroi plane. Refroidissement des aubes de turbines par jets - rapport final, contrat DRET, Decembre 1991.

Bibliography Blair, M. F. and Lander, R. D. New techniques for measuring film cooling effectiveness. Journal of Heat Transfer, November 1975, pp 539-543. Goldstein, R. J. and Taylor, J. R. Mass transfer in the neighborhood of jets entering a crossflow. Journal of Heal Transfer, November 1982, vol. 104, pp 715-721.

Acknowledgements The authors wish to express their thanks to DRET (DGA) institution for their financial help and Mr Henry Garem for his contribution with the conduction of all experiments. The authors wish to express their thanks to Professor David Zeitoun from IUSTI (Marseille, France) for his interest in this work.

R Dizene Department of Mechanical Engineering, USTHB University, Algiers, Algeria E Dorignac and R Leblanc Laboratoire D'etudes Thermiques, ENSMA, Futuroscope, France J M Charbonnier CNES, Toulouse, France

3 An Experimental Investigation of the Flow Produced in a Rectangular Container by a Rotating Disc using LDA V G Meledin, I V Naumov, and V A Pavlov

Abstract The swirled flow produced in a rectangular container is investigated experimentally using a laser measuring apparatus. The apparatus consists of the following units: an optoelectronic module of two-dimensional LDA with adaptive temporal selection of the velocity vector, a two-channel ADC for input of a quadrature Doppler signal, a computer with software, a device to generate a stable specialized vortex flow, and a CCD camera for registration of the flow structure. The rectangular container was created from optical quality plexiglas. The swirled flow is generated by a rotating disc (diameter 118 mm) placed in to the top lid of the container. The working fluid was a water-glycerine mixture. The optical module of the twodimensional LDA was a differential Doppler anemometer. The optoelectronic module of the LDA generated a quadrature pair of the Doppler signals. The measurement algorithm of a Doppler frequency is based on a spectral method of signal processing. This system allows us to carry out noncontact measurements of the swirled flows with velocity range of 10" 4 -10''m/s. The distribution of the axial velocity component at the axis of the rotating disc was measured for Reynolds numbers in the ranges 1500 ... 6000. It was found that the spiral vortex breakdown occurred at Re > 4000 in confined rotating flow inside the rectangular container. As seen from the measurement results, because of precession of a vortex core, the region of reversed axial flow does not arise steady in time and vortex breakdown bubble is not formed.

26

3.1

Optical Methods and Data Processing in Heat and Fluid Flow

Introduction

Researches into swirled flows play the important role in modern hydro- and aero-dynamics. These researches are connected to hopes of the construction of an experimentally founded physical models of the evolution of the vortical structures (1, 2). It is known that vortex flows are exposed to a large number of structural changes. Among these changes vortex breakdown phenomenon takes a significant place. The vortex breakdown is characterized by the existence of an internal stagnation point on a vortex axis behind which a region with reversed axial flow is located. The phenomenon of vortex breakdown is observed in vortex flows associated with stall from the leading edge of wings, in intensively swirled flows in tubes and in closed cylinders with a rotating lid (3). Most frequently there are two types of vortex breakdown. One of them is named axisymmetrical (or 'bubble-like') and the other is termed spiral. It has been observed in several experiments that the vortex breakdown location is not steady and exhibits fluctuations. The change of position of these regions makes it extremely difficult to perform measurements in its immediate vicinity. Besides, vortex breakdown is sensitive to external disturbances. Periodic oscillations were observed in a variety of swirling flows after breakdown occurred (4, 5). LDA has the advantage of being a non-contact technique. Previously (4) the velocity profiles and power spectra of velocity fluctuations were obtained in the region of vortex breakdown in a vortex-tube experiment. It was reported that the flow motion in this region was not steady but had coherent low frequency oscillations. The dominant frequency of oscillation was approximately 2 Hz for the fixed fluid consumption in the experimental setup. Vortex flow is good for studying in closed containers with a rotating lid. The experiments executed for the closed cylindrical container have provided the qualitative information based on the flow structure visualization (6, 7). It is known that one of the main factors influencing the intensity of the vortical structure is the effect of a peripheral boundary layer. In the case of a rectangular container the interaction of vortex flow with the container walls is reduced. Previously (8) the precession of a vortex core in the closed rectangular container was investigated. The experimental setup created a swirled flow with high stability that allowed an investigation of a vortex core precession in the range of Reynolds numbers 1500-9000. Spatial-temporal measurements of axial and tangential velocity components were carried out using laser Doppler anemometer. It was found that the value of the first low-frequency peak of the spectral density of the velocity components was proportional to the frequency of the external action (disk rotation). The constant of proportionality was equal to 1/17 in the flow investigated. The purpose of this chapter is to report the results of the experimental investigation and identification of vortex structures in the closed rectangular container induced by a rotating disc placed in the top lid using LDA and further investigation of periodic oscillations of the flow velocity in the region of the vortex breakdown.

An Experimental Investigation of the Flow Produced in a Rectangular Container by a Rotating Disc using LDA

3.2

27

Experimental setup and measurement procedure

The investigation of the flow structure was carried out in a closed rectangular container. The evolution of the vortical structure was studied using three-dimensional flow visualization. The measurements of the axial velocity component on the vortex axis in the closed rectangular container were carried out for the quantitative investigation of the vortical structure. A schematic of the experimental apparatus is shown in Fig. 3.1. The rectangular container, 1, was made from a plexiglas of optical quality. The size of the closed container is 120 x 120 x 120 mm. The swirled flow is generated by a rotating disc, 2, placed in to the top lid of container. The disc radius R is 59 mm. The constancy of the flow in the volume investigated was provided by a frequency-phase system of the stabilization of angular velocity of the rotating disc. The range of the stabilization of the disc velocity is 3-100 r/min with an error no more than 0.2 per cent.

Fig. 3.1 Schematic of the laser measuring apparatus

The working fluid was a glycerine/water mixture (approximately 40 per cent) having a kinematic viscosity v= 3.1 -10-5 m2/s at 25 degrees C. During the experiment the viscosity was checked frequently using a viscometer. The temperature of the working fluid was checked by the digital thermometer. The fluctuation of the temperature during the experiment did not exceed ±0.2 degrees C. The experiments were carried out in the range of Reynolds numbers 1500-6000 and height-to-radius ratio H/R = 2. The Reynolds number for this flow system is Q . R2

defined as Re =

, where Q - angular velocity of rotating disc, R- radius of a disc, vv kinematic viscosity of the working fluid.

28

Optical Methods and Data Processing in Heat and Fluid Flow

The visualization of the flow structure was performed by injecting a fluid containing a fluorescine dye. A small quantity of the water solution of the fluorescine dye moved from the tank 3 through a 1 mm tube 4 and this was injected in to the container through a 0.5 mm inner diameter hole at the geometrical center of the bottom of the rectangular container. The speed of the dye injecting was adjusted by varying the altitude of the tank bottom. The dye was moved by current along the vortex axis. The image of the flow distribution was recorded by a CCD video camera 5 placed perpendicular to the container. The videotape recording of the flow motion was analysed frame by frame, then the frame was processed and stored on a computer 6. The measurement of the axial velocity component was carried out using a Doppler anemometer which was adapted in-house with an adaptive temporal selection of the velocity vector (LDA ATS) (9). The optical module LDA ATS 7 is carried out based on the twofrequency differential optical configuration with a frequency pre-shift 80 MHz. LDA allows two orthogonal components of a velocity vector to be determined by temporal acousto-optical separation of the measuring channels. The wavelength of the laser radiation was 514 run. Two laser light beams were intersected in a focal length F = 500 mm forming an interference pattern in the probe field. The measuring volume was approximately cylindrical, with a diameter of 0.05 mm and a length of 0.5 mm. Back-scattering mode was used to measure the axial velocity component which was perpendicular to the direction of interception of the two laser beams. The flow was seeded with polystyrene particles of 10 um mean diameter used as the tracer particles in measurements of the flow velocity. The spatial positioning of the measuring volume in the flow was carried out on a coordinate-measuring table on which the optical module LDA ATS was installed. The receiving lens of the LDA received the superposition of the light waves scattered from particles during their motion inside the measuring volume. The Doppler frequency shift is a linear function of the particle velocity and the duration is equal to the time of the particle motion through an interference field. The LDA was modified to accommodate an expansion of its dynamic range for measurements of low velocities using computer processing of narrow-band Doppler signals generated as quadrature pairs. The filtration and transposition of a photo-electric signals in the area of the intermediate (zero) frequency is performed. The information about the velocity module contains in the frequency and the information about the velocity sign contains in the relative phase of the pair output signals. After low-pass filtering (LPF) 8, the analytical Doppler signal is introduced in to a computer 6 through the standard two-channel ADC interface 9 with sampling frequency up to 44 kHz. The experimental signal from the particles after an application of a cubic spline-interpolation is shown in Fig. 3.2. The synchronous discretization of the quadrature pair in the interface saves a precise phase relation between cosine and sinusoidal components of the Doppler signal. The measurement algorithm of Doppler frequency is based on the fast Fourier transformation (FFT) - method of signal processing. The algorithm is adapted for specific properties of LDA signals and allows information about the direction and magnitude of the flow velocity to be obtained. The measuring error of the velocity in the range 10"4 H- 10"' m/s did not exceed 1 per cent.

An Experimental Investigation of the Flow Produced in a Rectangular Container by a Rotating Disc using LDA

29

Fig. 3.2 Analytical signal from scattered particles

3.3

Doppler signal processing

The Doppler frequency shift of LDA signals is determined by a new algorithm based on the spectrum integration of the Doppler signal. In a differential LDA configuration with Gaussian beams a signal from a single particle moving through the center of the probe's optical field in a time ti (without the noise and pedestal) is modelled by expression (10):

where A is amplitude coefficient and E is a coefficient dependent on the configuration of the 2 probe optical field. In particular, for a differential optical configuration of LDA: E, = —-—-; where Mis the number of spatial periods in the probe field at a level exp(-2) from the intensity in the centre of a laser beam, COD is the Doppler shift of frequency: a>D =y-vx; vx is the measured projection of velocity vector; r is a known constant determined by the configuration of the optical schematic and the wavelength of the laser radiation. The structure of the probe interference field is considered as a priori knowledge. The velocity of the scattering particle is determined as a result of the frequency measurement of the signal (3.1). The quadrature pair of the Doppler signal is determined as u(t) = Re[x(t)] and v(t) = Im[x(t)]. The information about the input signal is represented in the discrete form as the arrays from N of instantaneous countings {VA} and {uk}, where uk = u(kT0) and v* = v(kT0); k = 1,...^V ; and TO is the period of the sampling frequency. The duration required for signal processing is equal T= Tg-N and the spectral resolution is Aa> = —. The analytical Doppler signal is: Xk = u^-jv/,. In practice, an additive non-stationary Gaussian noise s(t) is presented with a useful LDA signal at the output of the low-pass filter. Using digital methods of Doppler signal processing the additional contribution to the noise component is introduced by noise of the analog-to-digital converter Ek. The analytical LDA signal after discretization can be performed as:

30

Optical Methods and Data Processing in Heat and Fluid Flow

It is known that for spectral measurements the reduction of the Doppler frequency shift error is usually reached by a statistical average. Using a classical spectrum analysis the considerable storage time provided a filtration of the Doppler signal but results in an essential distortion of the true dynamic picture of the flow motion. The application of the analytical signal allows, essentially, the production of the storage time to retain a precision of measurements. The value of the Doppler frequency shift may be calculated by an estimation of the spectral density maximum position. The input information is contained in sample (3.2) - a mixture of signal and noise and the spectral density is determined as the discrete FFT:

A

A

Let 7(m-A&>) be the maximum value of the spectral density Y(ca), where m is counting number of FFT sampling near WD. The spectral density of the Doppler signal in a point m is modeled as:

The quality criterion of the one-particle LDA signal is the conformity of the measured spectrum form (3.3) and model (3.4). Let 77 be the factor defined by a signal and hardware widening of a Doppler spectral peak O>D. Using a rectangular function n(«-Aa>) which is equal to 1 on the interval (m-r\-£m, m+r\-^-m) and zero outside of this interval we have:

Let K be the counting number of FFT sampling for a true position of a>D, K = —&-. The value Aft< a>r> can be estimated from the condition:

The frequency wD measuring error is incurred by the non optimal low-pass filtering and the discretization error of the signal. At non optimal filtration LPF deforms the spectrum of the signal. Discretizating spectral density Y ( w ) is a sequence of the Gaussian images of initial spectrum shifted along the frequency axis on value k • w0. There is spectral foldover if the sampling rate w0 is approximated to the double frequency of the Doppler shift. There are distortions of the spectrum biasing the estimation of the spectral density maximum. The spectral foldover depends on the width of the Doppler spectral peak.

An Experimental Investigation of the Flow Produced in a Rectangular Container by a Rotating Disc using LDA

31

After filtration by the rectangular function in the interval [-wo/2,coo/2] the displacement of the center frequency of the Doppler spectral peak is given by:

For minimization of frequency (a>o) error it is necessary to determine the sampling frequency by the real width of the spectrum. In view of the signal and hardware, the hardware broadening of the Doppler signal spectrum is a constant. It depends on parameters of LDA optic. For LDA ATS, in case of a single particle intersecting the center of the probe optical field the hardware broadening is equal 10 per cent at level exp(-2) from the maximum of the spectral density Y(w). The signal broadening is dependent on the orientation of the flow with respect to the interference fringes formed in the measuring volume. Thus, in the measuring apparatus considered, the total broadening of the Doppler spectral peak may be 10-100 per cent depending on the orientation of the flow velocity vector and the LDA measurement volume. Using computer simulations, an estimation of the displacement of the Doppler spectral peak as a function of sampling frequency was carried out. It was found that at wO = 2wD the displacement of the center frequency of a Doppler spectral peak (3.5) with respect to its true position is placed in the interval 2-21 per cent and depends on the flow orientation with respect to the measurement region. At coo>2.6a>D the definition error of the central Doppler frequency does not exceed 0.1 per cent independently of the widening of a Doppler spectral peak.

3.4

Experimental results and discussion

In the experimental investigation of swirled flow in a closed rectangular container the previously described LDA measuring apparatus was used. The distribution of the axial velocity component at the axis of the disc rotation in the closed rectangular container for Re 1500, 2000, 2500, 3000, and 4000 are shown in Fig. 3.3. On the X axis is the distance from the bottom of the container (mm), on Y is the value of the axial velocity component (mm/s). The profile measurements of the axial velocity component were carried out in random time without the registration of the influence of the vortex core precession. The access time in a measurement point was equal to 10 s. From the experimental data it is obvious that the fluctuations of the axial velocity component grow at increasing Reynolds number. However, at the same Reynolds numbers and aspect ratio the region of reversed axial flow does not occur in contrast to the cylindrical container. It is obvious that the zone of the maximum value of the velocity fluctuations is developed on a 50-110 mm segment from the bottom of the container (zone I). The value of the velocity fluctuation is increased as the Reynolds number is increased.

32

Optical Methods and Data Processing in Heat and Fluid Flow

Fig. 3.3 Distribution of the axial velocity component at the axis of disc rotation in the closed rectangular container for different Reynolds numbers

An examples of flow visualization of the vortex breakdown for Re equal to 5000 and 6000 are shown in Fig. 3.4. The visualization of the flow structure was carried out by injecting fluid with a fluorescent dye at the geometrical center of the bottom of the rectangular container. The dye entered the container bottom and was lifted by the flow along the vortex axis. It is obvious that at Re = 5000 the steady picture of the spiral vortex breakdown has occurred. For Re = 6000 it is obvious that the stratification of the vortex breakdown takes place as a result of vortex core precession.

Fig. 3.4 Flow visualization of the vortex breakdown in the closed rectangular container (H/R = 2, Re = 5000 and 6000)

An Experimental Investigation of the Flow Produced in a Rectangular Container by a Rotating Disc using LDA

33

For the investigation of the evolution of spiral vortex breakdown the measurements of the axial velocity component at Reynolds numbers 5000 and 6000 were carried out. The measurements were performed as follows: • The measurements of the profile of the axial average velocity component were performed. The access time in a measurement point was equal to 5 s. The average value of the velocity was calculated by four measurement values of the axial velocity component in one point in a random time moment:

• In the zone of maximum velocity fluctuations (zone I, Fig. 3.3) the measurements of maximum and minimum values of axial velocity components were carried out. The access time in a measurement point was equal to 5 s. The distribution of the axial velocity component at Re = 5000 is shown in Fig. 3.5. As shown in the picture the significant changes of the axial velocity component in comparison with Re V 1500....4000 show up in the zone II (70-100 mm). In that zone -^^- > 2 in contrast to zones I min

and III where the flow velocity fluctuations did not exceed Fmax - Fmin < 30%(F).

Fig. 3.5 Distribution of the axial velocity component at the axis in the closed rectangular container (H/R = 2, Re = 5000)

In Fig. 3.6 the measured distribution of the axial velocity component for Re = 6000 is shown. The experimental results show that zone II has increased in comparison with zone II in Fig. 3.5 and it equals 50-110 mm. Based on experimental data it is obvious that on the container axis the value of the axial velocity component is reversed in local points and short time.

34

Optical Methods and Data Processing in Heat and Fluid Flow

However because of vortex core precession the region of reversed axial flow does not arise steady in time and the vortex breakdown bubble is not formed.

Fig. 3.6 Distribution of the axial velocity component at the axis in the closed rectangular container (H/R = 2, Re = 6000)

3.5

Conclusion

The experimental investigations of changes of the vortical structure in a closed rectangular container at a range of Reynolds numbers from 1500....6000 were carried out. The measurement algorithm of Doppler frequency, based on the spectral method of signal processing was described. The algorithm is adapted to specific properties of LDA signals and allows information about direction and the velocity magnitude of non-stationary flow to be obtained. The spatial distributions of the axial velocity component on the axis of the disc rotation were obtained. The experimental results show that the frequency and amplitude of the velocity fluctuation is increased as the Reynolds number is increased. The velocity measurements give additional information about particular characteristics of the vortex flow inside the closed container. It was found that the spiral vortex breakdown occurred at Re > 4000 in a confined rotating flow inside the rectangular container. As seen from the measurement results, because of the precession of the vortex core the region of reversed axial flow does not arise steady in time and the vortex breakdown bubble is not formed. It was confirmed that the swirled flow is unsteady with vortex structure precession. It is necessary to take in to account the physical modelling of these processes.

Acknowledgments The authors thank Prof. Yu.N. Dubnistchev and Prof. V.L. Okulov for useful discussion of this work.

An Experimental Investigation of the Flow Produced in a Rectangular Container by a Rotating Disc using LDA

35

The work was carried out at partial support of Russian Foundation for Basic Research (grants 99-02-17123 and 99-02-16702).

References (1)

Sarpkaya T. 1971: On stationary and traveling vortex breakdowns. J. Fluid Mech. 45, p.545. (2) Alekseenko, S. V. and Okulov, V. L. 1996: The swirled flows in engineering applications (review) II Thermophysics & Aeromechanics. 3, p.101. (3) Leibovich, S. 1978: Vortex breakdown. Ann. Rev. Fluid Mech. 4, p.185. (4) Faler, J. H. and Leibovich, S. 1978: An experimental map of the internal structure of vortex breakdown. J. Fluid Mech. 86, p.312. (5) Gursul, I. and Yang, H. 1995: On fluctuation of vortex breakdown location. Phys. Fluids. V.7, l, p.229. (6) Escudicr, P. 1984: Observation of the flow produced in a cylindrical container by a rotating end wall. Exp. in Fluids 2, p.189. (7) Spohn, A., Mory, M., and Hopfinger, E. J. 1998: Experiments on vortex breakdown in a confined flow generated by a rotating disc. J. Fluid Mech. 370, p.73. (8) Dubnishchev, Yu. N., Meledin, V. G., and Naumov, I. V., et al. The laser diagnostics of a low-speed swirled flows. Proc. of the 9th (Millennium) International Symposium on Flow Visualization. UK, Edinburgh. 20-25 August. 2000. (9) Belousov, P. Ya., Dubnishchev, Yu. N., and Meledin, V. G., et al. 1990. Laser Doppler anemometry with adaptive temporal selection of the velocity vector. Optica Applicata. V.20, 3, p. 187. (10) Belousov, P. Ya., Dubnishchev, Yu. N., and Meledin, V. G. 1996: Optic Methods of Flow Studying by selecting of the spatial-temporal structure of the scattered light. SPIE V. 2773, p.l47.

V G Meledin, I V Naumov, and V A Pavlov Institute of Thermophysics SB RAS, Novosibirsk, Russia

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4 The Design, Development, and Preliminary Results from a High-speed, Optically Accessed, Single-cylinder Engine G Pitcher, P Williams, J Allen, and G Wigley

Abstract With increasing legislation on emissions, greater expectations in performance and fuel economy and an ever decreasing time required from concept to production engines, the necessity to understand the fundamental principles in all aspects of engine performance has never been higher. Lotus has designed and built an optically accessed single cylinder research engine to allow observation and measurement of the different phenomena occurring inside the engine cylinder. The engine has a full transparent fused silica cylinder, so the whole engine stroke can be observed, and a sapphire window in the piston crown, which can be accessed via a 45 degree mirror in an extended piston.

4.1

Introduction

Manufacturers are continually being driven to improve the combustion process of the internal combustion engine to reduce engine-out emissions and fuel consumption by ever more stringent emissions legislation, environmental, and customer pressures. A number of new technologies have become available that appear to offer manufacturers the benefits and improvements they seek. Amongst these technologies are:

38

Optical Methods and Data Processing in Heat and Fluid Flow

• Gasoline direct injection. GDI technology has been demonstrated to offer lean bum fuel economy benefits and the possibility of reduced emissions during transient and cold start operating conditions due to better control of fuel-air mixing (1, 2). • Controlled auto ignition. AT technology has been shown to give benefits to fuel consumption, emission formation, and engine stability (3). Many of these new technologies require detailed understanding of the combustion process and unique architectures of the combustion system to ensure correct and robust combustion. These solutions can no longer be derived from the years of existing development data and experience gained from conventional manifold or port fuelled homogeneous combustion systems. A further pressure on the manufacturer is to remain competitive with reduced time-to-market and aggressive timing associated with today's rapidly changing market place. In order to meet these demands manufacturers must have access to a means to rapidly develop new combustion strategy, and thus, enable effective design of improved combustion systems. To further advance its powertrain research and development activities Lotus Engineering has designed a single-cylinder, high-speed, optical engine with full optical access and identical internal geometry. With un-obscured optical access up through the piston crown, cylinder bore, and pent-roof, this engine allows full use of a wide range of optical techniques including laser induced fluorescence (LIF), phase Doppler anemometry (PDA), and high-speed imaging. The authors believe that this latest generation of optical engines will move from the pure research field into the early phase of engine development programmes. This will give engineers the ability to combine laser diagnostic techniques and calibration skills to ensure new robust combustion systems are developed and can be applied to real specific engine models in acceptable time frames for the engines of the future. Optical diagnostic techniques can allow data to be gained on in-cylinder phenomena involving interactions of fuel and air and the formation of combustion products. Data on combustion processes, ignition, flame kernel growth and propagation, flame structure, soot formation, and wall quenching can also be gained.

4.2

Optical engine requirements

There are several, sometimes conflicting, requirements that the engine has been designed to meet. The most basic of these is that the single-cylinder engine should mimic the geometry and performance, in terms of speed and load, of a modern multi-cylinder petrol engine. Added to this capability is also the necessity to be able to represent small high-speed direct injection diesel engines with their increased compression pressures. These criteria have been met by designing the engine with primary and secondary balancing to run at speeds greater than 5000 r/min under full load conditions for a petrol engine, with the glass components being designed to handle the pressures and forces generated. Design flexibility has been built into the engine, to require only minimum modification, allowing any engine geometry up to a bore and stroke of 100 mm by 100 mm to be used, and fast prototyping of the head geometry to keep as close as possible to the actual engine configuration.

The Design, Development, and Preliminary Results from a High-speed, Optically Accessed, Single-cylinder Engine

39

These most basic design criteria have to be complementary to the optical access demanded by the various laser diagnostic and imaging techniques. The requirement was to have full access to the engine cylinder, Fig. 4.1, including any pent roof, and this was achieved by having the complete cylinder built of fused silica. Additionally, access was required through the piston crown, and here an optical sapphire window was fitted into the top of the piston. Even with the optical windows in place, additional care had to be taken in the design of the peripheral components, i.e. timing belts, to ensure that these did not interfere with the access so generated. This access is not only a question of being able to see all parts of the cylinder, but imposes further constraints when high-powered lasers are utilized. There is then the necessity of running the engine optical cylinder dry, i.e. oil free, to prevent oil being burnt on to the glass surface and also giving an unknown refractive effect to the laser beams transmission due to the presence of oil films.

Fig. 4.1 View of the engine showing full length glass cylinder

The final requirement was one of maintenance, for both the mechanics of the engine and also for mounting/de-mounting the optical components. The challenge here was designing the engine in such a way that the optical components could be removed very quickly for cleaning, and then rebuilt into the engine, to minimize the down time during experimental work. The object was to ensure that this down time did not become the dominant time during the investigative work.

4.3

Engine design

4.3.1 Optical components The main optical component is the fused silica cylinder in which the special piston runs. This allows optical access to the full bore and stroke. Principle parameters considered during its design were its strength to withstand high cylinder pressures and the optical suitability of its geometry and material. A hydraulic platform is used to hold the cylinder up to the head to facilitate removal and replacement of the optical liner. Increasing cylinder wall thickness

40

Optical Methods and Data Processing in Heat and Fluid Flow

distorts light passing through it. Thinner walls however limit the maximum pressure that can be used. Thermally toughened glasses cause distortion of polarized coherent laser beams used in phase Doppler anemometry and chemically toughened glasses can absorb and fluoresce under high-power UV wavelength lasers used in Laser Induced Fluorescence. Although not the strongest optical material available, fused silica was chosen for a general-purpose cylinder due to its good optical and mechanical properties and relative cost. The cylinder has a wall thickness of 14 mm, calculated to allow cylinder pressures of up to 40 bar to be used with a significant safety factor. The top of the optical cylinder and its mating surface in the cylinder head has a specially developed contoured profile to provide optical access directly into the pent-roof combustion chamber from the side without compromising the valve seat or port geometry. A circular window made of 12 mm thick hem-sapphire crystal (Al2O3) is mounted in the crown of the piston. Every effort was made to maximize the useful optical area while minimizing weight and stress concentration. The final design 'encases' the sapphire window with titanium whose coefficient of linear thermal expansion is almost identical to that of the sapphire itself. The conical section window is fixed to the piston body by a titanium retainer with a mating cone shape. The window is separated from the piston body by a titanium washer. This window allows access up to the cylinder head and the valve seat area, Fig. 4.2.

Fig. 4.2 Access through the piston

4.3.2 Mechanical components The cylinder head was machined from cast iron, which was selected as a material for its rigidity and thermal stability over aluminium. The port and valve seat geometry has been detailed to ensure the same air motion characteristics as the engine that formed the basis for this design. The cylinder head design is sufficiently modular and flexible that alternative port geometry can be manufactured and evaluated rapidly with minimum engine down time in order to ensure optimum air and fuel motion is achieved. The cylinder head is mounted on four columns above the optical section of the engine. These columns are spaced in such a way that there is no obstruction to the various 'optical envelopes' required for planar imaging and scanning single point measurements from the whole bore and stroke. Two camshafts in separate housings actuate the valve gear. The

The Design, Development, and Preliminary Results from a High-speed, Optically Accessed, Single-cylinder Engine

41

camshaft housings are readily removable and are to be replaced by the Lotus Active ValveTrain system to give the engine a fully variable hydraulically actuated valve system. The cylinder block consists of two sections. The upper section provides mounting for the cylinder head, hydraulic platform, 45 degree mirror, and steel cylinder. This mounts on to a machined-from-solid aluminium lower crankcase containing a cross-drilled crankshaft running on three pressure fed main bearings. Although the elongated piston is manufactured from lightweight materials, the reciprocating mass at 1235 gms is considerable when compared to that of an equivalent production piston at 300-400 gms. As the engine was intended for high-speed use, and to be used in close proximity to mechanically sensitive optical devices, it was decided that the added cost and complexity of adding balance shafts was justified. Two contra-rotating primary and secondary balance shafts with adjustable balance weights are built into the crankcase and are gear driven from the crankshaft. An elongated and bifurcated piston design was adopted, as has become common practice in optical engines to allow optical access into the combustion chamber through the piston via a fixed 45 degree mirror. The design allows the glass liner and piston crown to be removed quickly without dismantling the engine. The glass liner is first lowered on its hydraulic platform and the piston gudgeon pin is removed through an aperture in the crankcase. This allows the piston to be lowered to clear the optical cylinder allowing the glass to be removed from its seat. With the cylinder removed the piston crown can be unscrewed from the piston assembly and the window cleaned or replaced. To minimize the reciprocating mass resulting from the long piston assembly and window, advanced materials were considered. However, aluminium and titanium were selected for ease of manufacture and thermal compatibility with sapphire. 4.3.3 Piston ring/glass cylinder The piston was designed to run un-lubricated in the glass bore and at the high design speeds sealing without excessive friction was a concern. A considerable amount of rig testing was carried out to identify the best material for the piston compression ring. A carbon/carbon matrix was found to be the best material exhibiting both low friction and high temperature capability.

4.4

Results

4.4.1 Air flow measurements The base measurement set required for future work with this research engine, is the incylinder air flow generated while the engine is being motored. This data is used for comparison with regard to fuel flow and combustion propagation. The airflow was measured using laser Doppler anemometry, and the first data set was collected with the engine running at 1500 r/min. Three vertical planes were measured; one diameter between the inlet valves, the orthogonal Diameter, and one further plane through the centre of one of the inlet valves, to measure the maximum tumble component. The horizontal scans were performed with a 5 mm matrix of points, with 10 mm between each vertical location. At each position the axial and radial components of velocity were measured. It is planned to measure a few horizontal

42

Optical Methods and Data Processing in Heat and Fluid Flow

planes, where the swirl/radial components will be measured, giving some locations where all three velocity components will be available. The velocity vectors, measured on a plane through the centre of the cylinder and in between the inlet valve pair, i.e. the symmetry plane, are shown in Fig. 4.3.

Fig. 4.3 Velocity vectors at 90, 130, and 180, degrees after TDC

These vector fields indicate how the inlet flow develops and then decays by bottom dead centre. There is also evidence of the three-dimensional nature of the flow field, most noticeably where adjacent vectors are directly opposed. The flow fields and subsequent tumble analysis are fully described in (7). 4.4.2 Combustion imaging Although the engine has been designed with direct injection as one of the main operating conditions for research, there was also a requirement to run the engine with port injection. For this purpose the standard production injector was mounted into the inlet manifold, directed on to the rear of the inlet valves and injected gasoline with 3 bar pressure. The principle object of this work was to capture some combustion images with the engine running in conventional spark ignition and auto-ignition mode. It also served as an opportunity to prove the engine design in firing mode. The first images were obtained with the engine operating with convention spark ignition, at 1000 r/min, and an example image is shown in Fig. 4.4. The camshafts were then substituted to allowed internal exhaust gas trapping and the engine was then run at 2000 r/min. Under these conditions the engine can be made to run with autoignition. In this mode of operation, there are multiple ignition sites, and these are shown in Fig. 4.5. It is these multiple sites that are believed to be responsible for the reduction in peak pressure fluctuations observed in this mode of operation.

The Design, Development, and Preliminary Results from a High-speed, Optically Accessed, Single-cylinder Engine

43

Fig. 4.4 Combustion from spark ignition

Fig. 4.5 Multiple ignition sites with auto-ignition

4.5

Conclusions

This Chapter has described the design, major components, and early commissioning of a single-cylinder research engine for laser diagnostics. The engine has been built to allow laser diagnostics to be used to investigate in-cylinder flow structure, air fuel mixing, and combustion, has finished its commissioning and is used on a day-to-day basis. Initial work has been to characterize the in-cylinder air motion, using laser Doppler anemometry, as a basis for further research into airflow/spray interaction and combustion. The engine has also been fired in both spark-ignition and auto-ignition modes, and combustion images obtained. Future work will be looking at the break-up of direct injection sprays in the engine cylinder for both homogeneous and lean burn operation.

44

Optical Methods and Data Processing in Heat and Fluid Flow

References (1)

(2)

(3)

(4)

Wigley, G., Hargrave, G., Law, D., Pitcher, G., Durell, E., and Allen, J. Air Flow and Fuel Spray Characterisation - Diagnostics for 21st Century Engines, Proceedings of the 21st Century Emissions Conference, IMechE, London, UK, December 2000. Law, D., Kemp, D., Allen, J., Kirkpatrick, G., and Copland T. Controlled Combustion in an 1C Engine with a fully Variable Valve Train, Proceedings of the SAE 2001, Advances in Combustion, Detroit, USA, March 2001. Law, D., Allen, J., Kemp, D., and Williams, P. 4 Stroke Active Combustion (Controlled Auto-Ignition) Investigations using a Single Cylinder Engine with Lotus Active Valve Train (AVT), Proceedings of the 21st Century Emissions Conference, IMechE, London, UK, December 2000. Pitcher, G. and Wigley, G. LDA Analysis of the Tumble Flow Generated in a Motored 4 Valve Engine, Ninth International Conference Laser Anemometry Advances and Applications, University of Limerick, Ireland, 2001.

Bibliography Maly, R. R. Progress in Combustion Research. IMechE Combustion Engines Group Prestige Lecture 8/10/98. Kuwahara, K. et al. Mixing Control Strategy for Engine Performance Improvement in a Gasoline Direct Injection Engine, Proceedings of the SAE 1998, Advances in Combustion, Detroit, USA, 1998, SAE report number 980158. Kuwahara, K. et al. Diagnostics of In-Cylinder Flow, Mixing and Combustion in Gasoline Engines - Proceedings of VSJ-SPIE98 6-9/12/98.

G Pitcher, P Williams, and J Allen Lotus Engineering, Norwich, UK G Wigley Loughborough University, Loughborough, UK e With Authors 2002

5 The Reflected Spectrum of Complex Multi-layered Inhomogeneous Highly Scattering Medium I V Meglinsky and S J Matcher

Abstract We use the optical/near-infrared (NIR) reflectance spectroscopy to non-invasively measure the haemoglobin saturation in living human skin. The difficulties in the clinical application of this technique for skin tissue oxygenation monitoring are due to the complexity of extracting the information of chromophore distribution and their concentrations from the reflectance spectra in the case of multiple scattering of light. We have developed a computational model of human skin and Monte Carlo technique for simulation of the reflectance spectra of skin in the visible and near-infrared spectral region. The computational model of skin contains several layers with wavy inter-layered boundaries corresponding to the structure of human skin. Our model takes into account probe geometry, variations of spatial distribution of blood vessels, various levels of blood oxygen saturation, volume fraction of water, oxy- and deoxyhemoglobin, melanin content, and chromophores of interest. The small source-detector separation (250, 400, and 800 um) required due to the shallow (100-150 um under skin surface) spatial location of skin capillary loops is our main interest. Comparison of the results of spectra simulation and experimental results made in vivo are presented. As the experimental system we use the spectrometer in conjunction with the two-dimensional array of a CCD camera.

5.1

Introduction

The unique characteristics of optical methods are of great interest to researchers working in different areas of biology and medicine. Nevertheless, the problem of implementing optical diagnostic methods in the clinical practice in order to solve a wide range of actual diagnostic tasks remains unresolved. The difficulties in the clinical application of these techniques for

46

Optical Methods and Data Processing in Heat and Fluid Flow

monitoring the human body are due to highly anisotropy scattering of the probing radiation in most of biological tissues. Furthermore, random inhomogeneous variations of the optical properties of the skin layers act like a screen, which keeps the optical radiation from deep penetration of the human body. The in vivo spectral reflectance measurements of human skin can serve as a valuable supplement to standard non-invasive techniques for diagnosing various skin diseases, such as venous ulcers, skin necrosis, interstitial oedema, etc. However, quantified analysis of the reflectance spectra is complicated by the fact that skin has a complex multi-layered nonhomogeneous structure (1, 2) with a spatially varying absorption coefficient, mainly determined by melanin pigmentation, oxygen saturation of cutaneous blood, index of erythema, bilirubin and B-Carotine, and other chromophores (3). In clinical applications given the measured reflectance spectra, we need to extract the concentrations of various chromophores of interest, such as oxy- and deoxy- hemoglobin, water, and melanin. Various approaches exist but in our initial work we have applied the simplest method - the modified Beer-Lambert law (4). This method attempts to account for the spectral distortions introduced by multiple scattering via a simple linearized equation which relates overall tissue attenuation -ln(I/I 0 ) to the tissue absorption coefficient ua:

where p is the source-detector spacing and a is a scaling factor, the 'differential pathlength factor' which accounts for the path lengthening effect of the random walk experienced by photons as they propagate through the multiple scattering medium. G is an offset term which can account for two effects. Since equation (5.1) represents a linear relationship between -ln(I/I 0 ) and ua (G is assumed to be determined purely by us and geometrical factors) then the technique of multi-linear regression can be used to estimate the relative concentrations of chromophores in the skin, provided the wavelength dependence of G is known. Since G is determined by us, then we will assume that it is dominated by terms which are either independent of wavelength A. or are linearly related to A. ua is given by the sum of absorption coefficients for each separate chromophore, which in turn are determined by the absolute concentration C and specific absorption coefficient e of each chromophore. Given N chromophores, equation (5.1) can thus be re-written:

By making measurements of A at a minimum of N+3 wavelengths, solving equation (5.2) for a, b, and C,'s becomes an exercise in multi-linear regression (5) which we solve using standard algorithms (6). Typically we fit all wavelengths from 550-770 nm in 0.7 nm steps; hence the number of wavelength greatly exceeds N.

The Reflected Spectrum of Complex Multi-layered Inhomogeneous Highly Scattering Medium

47

It should always be borne in mind however that equation (5.1) is simply an approximation. The modified Beer-Lambert law was originally introduced to analyse near-infrared transmission spectra of brain and muscles and, in particular, was designed to analyse relative changes in tissue oxygenation. During such changes G and a can, to first order, be assumed to remain unchanged so that equation (5.2), in the form:

is used to estimate in oxy- and deoxy- hemoglobin concentration from changes in attenuation A/4. The equation is strictly only valid in the limit that AC, —»0, otherwise the fundamentally non-linear relationship between A and //„ introduces errors. This effect has been studied using the diffusion equation for the specific case of detecting cytochrome-oxidase redox changes in the presence of large changes in hemoglobin absorption (7). For this reason, it is the general goal of our Monte Carlo modelling to investigate the validity of the simple multi-linear regression approach. This is done simply by comparing the chromophore concentrations determined by equation (5.2) applied to a Monte Carlo calculated reflectance spectrum with the actual chromophore concentrations used to perform the Monte Carlo calculation. In this Chapter we present a computational model of skin as a complex multi-layered medium with wavy boundaries corresponding the skin tissues structure, and method for skin reflectance spectra simulation, that is done using the Monte Carlo technique. The optical properties of the skin tissues, simulated and experimental skin reflectance spectra are presented.

5.2

Monte Carlo technique

Our simulation is based on the standard Monte Carlo technique (8-11), i.e. the simulation is performed as a sequential three dimensional tracing of photon packets between scattering events from the point of the radiation entering a medium, to the receiving area where the photon leaves the medium. The random pathlength that a photon packet moves for the j-th step is given by:

where Ej is a uniformly distributed random number between 0 and 1. The scattering event is simulated by generating two random angles O and 9 in respect to the Henyey-Greenstein angular probability density function (12). Internal reflection on the medium boundary is taken into account allowing the photon packet to split into a reflected and a transmitted part (13). The statistical weight of the reflected and transmitted parts of the photon packets are attenuated according the Fresnel reflection coefficients (14):

48

Optical Methods and Data Processing in Heat and Fluid Flow

where a, and at are the angles of the photon packet incidence on the medium boundary and angle of transmittance, respectively, n\ is the refractive index of the first layer of the medium and «o represents the refractive index of the ambient medium. The probability of the photon packet being detected can then be described as follows (15):

where Wg is the initial weight of the photon packet, M is the number of photon packet partial reflections on the medium boundary. The individual trajectory of each detected photon packet is stored in a data file, and then we include the absorption of the medium layers according to the Beer-Lambert law:

where Wj is the final weight of the j-th photon packet equation (5.6), Kj is the number of scattering events for the j-th photon packet, Uai and li are the local absorption coefficient of the medium and the photon packet pathlength for the i-th step, respectively. The total diffuse reflectance on the medium boundary is defined as the normalized sum of statistical weights of the photons reaching the detector area:

Here, Nph is the total number of detected photon packets, typically 105-106. The simulation of a photon packet is stopped if the statistical weight fall bellow 0.0001, or if a total number of scattering events exceeds 10 000.

5.3

Skin model

As an object of investigation skin represents a complex heterogeneous medium, where the spatial distribution of blood and chromophores content are variable with the depth.

49

The Reflected Spectrum of Complex Multi-layered Inhomogeneous Highly Scattering Medium

Nonetheless, it is possible to define the regions in skin, where the gradient changing of skin cells structure, chromophores, and blood amounts with a depth increasing roughly equals zero. This allows us to approximate skin as a multi-layered medium. Thus, following earlier work aimed at optical modeling of the skin (10, 11, 16, 17) we have considered the skin as a three-dimensional half-infinite medium divided into seven layers. The first layer in our model corresponds to the layer of desquamating flattened dead cells mainly containing keratin, which is 20 um thick, and known as the stratum corneum. The second layer we call living epidermis. It is 80 um thick and is assumed to contain primarily living cells: a fraction of dehydrated cells, laden cells with keratohyalin granules, columnar cells, and also melanin dust, small melanin granules, and melanosoms (1, 2). Given the inhomogeneous distribution of the blood vessels and skin capillaries within the skin (18) we sub-divide the dermis into four different layers, with different blood volumes. These layers are: pappilary dermis (150 um thick), upper blood net dermis (80 um thick), dermis (1500 um thick), deep blood net dermis (80 um thick). The deepest layer in our model is the subcutaneous fat (6000 um thick). Of course, some variability in thickness is expected from region to region of the body and between individuals, and histological evidence suggests this can be of order 30-40 per cent (1, 2, 18). However, we assume that these values for layer thickness are typical of Caucasian adults. To try to represent the observed histological structure of real skin we model the boundaries of the layers as periodic surfaces (15):

where Bk(x,y) is the depth of the k layer at (x,y), Zt(xo,yo) is the mean depth of the boundary, k is the layer index, Akx, Aty, a^, a^ are amplitude coefficients, wkx, co^y, cu'h,, 6)'ty are scale lengths of the roughness, and $tx, <4>, '&, 'ty are arbitrary phase offsets, respectively, in x and y directions. The values of these parameters presented in Table 5.1 produce boundaries comparable to the observed structure of real skin (1, 2, 18—22). Table 5.1 The values of the layers boundaries parameters used in the simulation *

Layer boundary

Ai^.Aky, fan

atj,,ak/, fan

(n/

\

VtaL 0 1 2 3 4 5 6 7

Skin surface Startum comeum/Living epidermis Living epidermis/Pappilary dermis Pappilary dermis/Upper blood dermis Upper blood dermis/Reticular dermis Reticular dermis/Deep blood dermis Deep blood dermis/Subcutaneous fat Subcutaneous fat/Muscles

2 2.5 20 2 2 2 5 5

0.2 0.25

0.2 0.2 0.2 0.2 0.5 0.5

100 80 50 20 20 20 20 25

(n/

}

Zk(xa,yo), fan

Vco'iy 150 80 45 40 50 50 50 30

0 20 100 350 430 1930 2100

8000

Such boundaries (Fig. 5.1) are closer to the structure of observed histological sections than plane boundaries. This is important as the statistics of photon reflections at the boundaries will be affected (15).

Optical Methods and Data Processing in Heat and Fluid Flow

50

Fig. 5.1 The example of wavy surface simulate the junction between skin layers in our model, corresponding to a cross-section of a real image of the epidermal boundary

5.4

Skin optics simulation

The different cells structure, density and distribution of blood vessels, pigments, and water content of skin affect their tissue optical properties (3, 23-24), which makes it difficult to define the optical events in a simulation. Based on the results of recent experimental data (25-26) we have assumed the wavelength independent of skin layers scattering properties. These optical data for different skin layers used in the simulation are represented in Table 5.2.

Table 5.2 Scattering and refractive properties of skin layers used in the simulation k 1 2 3 4 5 6 7

Layer Stratum corneum Living epidermis Papillary dermis Upper blood net dermis Dermis Deep papillary net dermis Subcutaneous fat

/Js, mm'

a

100 60 30 35 25 35 15

0.9 0.85 0.9 0.95 0.8 0.95 0.75

1

n 1.5 1.34 1.39 1.4 1.39 1.4 1.44

These data (see Table 5.2) are collected from a wide range of literature, quoted for Ax 600 nm. So, scattering coefficients //s and anisotropy factors g are taken from (23-26), and refractive indices n from (23, 27, 28). The taken refractive index of the ambient medium is no = 1. Whereas, the absorption properties of skin (absorption coefficient jua) have been calculated taking into account the spatial distribution of blood within the skin, the oxygen saturation S, and total hemoglobin volume fraction in the blood y.

The Reflected Spectrum of Complex Multi-layered Inhomogeneous Highly Scattering Medium

51

where /if 40 '(A) and u™(A) are the absorption coefficients of oxy- and deoxy-hemoglobin respectively (see Fig. 5.2), CBlood is the volume fraction of blood in the layer, ublood-free-tissue ^-^ is the absorption coefficient of blood free tissues including water content:

Here,CH2Ois the volume fractions of water, U(f2°(/l) is the absorption coefficients of water (see Fig.5.2),UTissue(A) is the absorption coefficient dependent on the cells structure of the skin layers and on the absorption properties of other chromophores, which is not of interest in the study.

Fig. 5.2 Absorption spectrum of the oxy-, deoxy- hemoglobin, and water, re-calculated from their molar extinction coefficient spectra

We have calculated r assuming that hemoglobin is contained in the erythrocytes only, i.e.:

where Ht is the haemotocrit, FRBC is the volume fraction of erythrocytes in the total volume of the blood cells, FHb is the volume fraction of hemoglobin in erythrocytes. The values of Csiooii, S, Ht, FHb, FRBC, and CHi0 generally are different from layer to layer. In our simulations we used the data presented in the Table 5.3, which are collected from the literature (1, 2, 18, 23, 29-32).

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Optical Methods and Data Processing in Heat and Fluid Flow

Table 5.3 The blood parameters used in the simulation for determination of absorption coefficient of a layer k

Layer

CBlood

S

Ht

FHb

FRBc

1 2 3 4 5 6 7

Stratum corneum Living epidermis Papillary dermis Upper blood net plexus Dermis Deep blood net dermis Subcutaneous fat

0 0 0.04 0.3 0.04 0.1 0.05

0 0 0.6 0.6 0.6 0.6 0.6

0 0 0.45 0.45 0.45 0.45 0.45

0 0 0.99 0.99 0.99 0.99 0.99

0 0 0.25 0.25 0.25 0.25 0.25

CH2°

0.05 0.2 0.5 0.6 0.7 0.7 0.7

Concerning the blood free skin layers Stratum corneum, and Living epidermis, we assume that UTissue (A) for these layers are described as (24, 33):

For the other layersMTissue(A) is defined as U Baseline (A) [24]:

The absorption coefficients of skin layers are presented in Fig. 5.3.

Fig. 5.3 Skin layers absorption coefficients - numbers indicate skin layers (see Table 5.2)

The Reflected Spectrum of Complex Multi-layered Inhomogeneous Highly Scattering Medium

5.5

53

Results and discussion

The results of the Monte Carlo simulation are quite close to the results of experimental measurements of the skin reflectance spectra (Fig. 5.4) on the spectrophotometer system similar to those described in details earlier (34). The difference in the results of simulation and experimental data (see Fig. 5.4) could be explained by our choice of the optical properties of the skin tissues for the simulation, and experimental errors.

Fig. 5.4 Simulated and measured reflectance spectra of skin: line is a result of the Monte Carlo simulation and symbols indicate the experimental results Using the technique described above we simulated reflectance spectra of skin in case when blood volume in some layers changed from 0 to 40 per cent, with the constant amount of blood in other layers (see Table 5.3). Figure 5.5 presents the results of the reflectance spectra for different source-detector spacings: 250, 400, and 800 um. The results show how the reflectance spectra are sensitive to the blood volume changes in the pappilary and upper blood net dermis [see Fig. 5.5(a)], and in the deep blood net dermis [see Fig. 5.5(b)]. These results are agreed with the conclusions of our recent studying of spatial detector depth sensitivity for different small source-detector spacings (15).

54

Optical Methods and Data Processing in Heat and Fluid Flow

Fig. 5.5 The simulated reflectance spectra for the different source-detector fiber spacing: 250 fan, 400 fan, and 800 fan (center - to center): (a) the blood volume in pappilary dermis, upper blood net dermis is changed from the 0 to 40 per cent; (b) the blood volume in pappilary dermis, deep blood net dermis is changed from the 0 to 40 per cent — source and detector fiber diameters are 200 fan, and 50 fan, respectively, with the numerical aperture 0.2

The results of both simulations show that the changes of the blood volume in the Deep blood net dermis do not effect the reflectance spectra [see Fig. 5.5(b)]. The detector collects the signal mainly from the top layers (about 100 um in depth, see Table 5.1) for 250 um sourcedetector separation, whereas the skin blood oxygenation measurements require us to sample deeper layers of skin, because capillary loops are typically located at a depth of about 100150 um under the skin surface (1-2, 18). Increasing the source-detector separation to 400 um increases the difference in spectra (see Fig. 5.5), as the detected area increases. In this case detected signal is collected mainly from top blood layers: pappilary dermis and upper blood net dermis. Increasing the source-detector spacing to 800 um increases the sampling volume so that it reaches the middle of the dermis (15), that produces the maximum difference in spectra (see Fig. 5.5).

5.6

Conclusions

Figures 5.4 and 5.5 present the results of the simulation of the reflected spectrum from the skin simulated as a multi-layered highly scattering complex medium with wavy boundaries between the layers. Comparison of the skin spectrum measured in vivo and reflectance spectra simulated as described above (see Fig. 5.4) demonstrates the accuracy of our skin model and Monte Carlo technique. The results presented in Fig. 5.5 show how the skin reflectance spectra are sensitive to the different source-detector spacing in fibre optic probe. We predict that probe spacings 400-800 um sample primarily the papillary dermis and upper dermis plexus, whereas probe with 250 um source-detector spacing refer mainly to blood free skin

The Reflected Spectrum of Complex Multi-layered Inhomogeneous Highly Scattering Medium

55

layers. In a subsequent paper we will present experimental results obtained on real skin at all three probe spacings to validate this prediction. We acknowledge financial supports of EPSRC grant GR/L89433. The authors also would like to thank to Prof. A Shore and to Dr P Collier for useful and helpful discussions concerning human skin structure and its properties.

References (1) (2) (3) (4) (5) (6) (7) (8) (9)

(10) (11) (12) (13) (14) (15)

Stenn, K. S. 'The skin', in Cell and Tissue Biology, Ed. L. Weiss, Baltimore: Urban&Shwarzenberg, pp.541-572, 1988. Odland, G. F.'Structure of the skin', in Physiology, Biochemestry and Molecular Biology of the Skin, Edited by L.A. Goldsmith, Oxford: Oxford University Press, I, pp.3-62, 1991. Young, A. R. 'Chromophores in human skin', Phys. Med. Biol, 42, 789-802, 1997. Delpy, D. T., Cope, M., van der Zee, P., Arridge, S. R., Wray, S., and Wyatt, J. 'Estimation of optical pathlength through tissue from direct time of flight measurement', Phys. Med. Biol., 33, No. 12, pp.1433-1442, 1988. Andersen, P. H. and Bjerring, P. 'Noninvasive computerized analysis of skin chromophores in vivo by reflectance spectroscopy', Photodermatol. Photoimmunol. Photomed., 7, pp.249-257, 1990. Press, W. H., Teukolsky, S. A., Vetterling, W. T., and Flannery, B. P. Numerical Recipes in C, 2-nd ed. Cambridge University Press, Cambridge, 1992, 994 p. Matcher, S. J., Elwell, C. E., Cooper, C. E., Cope, M., and Delpy, D. T. 'Performance comparison of several published tissue near-infrared spectroscopy algorithms', Anal. Biochem., 227, pp.54-68, 1995. Bonner, R. F., Nossal, R., Havlin, S., and Weiss, G. H. 'Model of photon migration in turbid biological media', JOSA A, 4, No.:3, pp.423-432, 1987. Flock, S. T., Wilson, B. C., Wyman, D. R., and Paterson, M. S. 'Monte Carlo modeling of light propagation in highly scattering tissues—I: Model predictions and comparison with diffusion theory', IEEE Trans. Biomed. Eng., 36, No.12, pp.1162— 1168, 1989. Jacques, S. L. and Wang, L. 'Monte Carlo modeling of light transport in tissues', in Optical-Thermal Response of Laser-Irradiated Tissues, Edited by A. J. Welch and M. J. C. van Gemert; Plenum Press, New York, pp.73-100, 1995. Keijzer, M., Jaques, S. L., Prahl, S. A. and Welch, A. J. 'Light distribution in artery tissue: Monte Carlo simulation for finite-diameter laser beams', Laser Surg. Med., 9, pp.148-154, 1989. Henyey, L. G. and Greenstein, J. L. 'Diffuse radiation in the galaxy', Astrophys. J., 93, pp.70-83, 1941. van der Zee, P. 'Measurement and modelling of the optical properties of human tissue in the near infrared', Ph.D. Dissertation, University of London, 1992, 313 p. Born, M. and Wolf, E. Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, 6th edn., Pergamon Press, 1986. Meglinsky, I. V. and Matcher, S. J. 'Modeling the sampling volume for skin blood oxygenation measurements', Medical & Biological Engineering & Computing, 2000 (to be published).

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Optical Methods and Data Processing in Heat and Fluid Flow

(16) Yaroslavsky, I. V. and Tuchin, V. V. 'Light propagation in multilayer scattering media: Modeling by the Monte Carlo method', Optica & Spectrosc., 72, pp.505-509, 1992. (17) Meglinsky, I. V. and Matcher, S. J. 'The application of the Monte Carlo technique for estimation of the detector depth sensitivity for the skin oxygenation measurements', Monte Carlo Methods and Appl., 6, pp. 15-25, 2000. (18) Ryan, T. J. 'Cutaneous Circulation', in Physiology, Biochemestry and Molecular Biology of the Skin; Edited by L.A.Goldsmith, Oxford: Oxford University Press, II, pp.1019-1084, 1991. (19) Maibach, H. T. and Lowe, N. L. Models in Dermatology, N.Y.: Karger, 1987. (20) Corcuff, P., Bertrand, C., and Leveque, J. L. 'Morphometry of Human Epidermis In Vivo by Real-Time Confocal Microscopy\Arch. Dermatol. Res., 285, pp.475-481, 1993. (21) Holbrook, K. A. 'Structure and functions of the developing human skin', in Physiology, Biochemistry, and Molecular Biology of the Skin. Ed. by L. A. Goldsmith. Oxford: Oxford Univ. Press. 1. pp.63-112, 1991. (22) Serup, J. and Jemec, G. B. E. Eds., 'Skin surface Contour Evaluation', in: Noninvasive Methods and the Skin, Boca Raton, CRC Press, Inc., Chapter 5, pp.83-131, 1995. (23) Tuchin, V.V. Lasers and fiber optics in biomedical investigations, Saratov: Saratov State University, 1998, 384 p. (24) Jacques, S. L. Skin optics, 1998. (Published on the personal website http://omlc.ogi.edu /news/jan98/skinoptics. html) (25) Doornbos, R. M. P., Lang, R., Aalders, M. C., Cross, F. M., and Sterenborg, H. J. C. M. 'The determination of in vivo human tissue optical properties and absolute chromophore concentrations using spatially resolved steady-state diffuse reflectance spectroscopy', Phys. Med. Biol., 44, pp.967-981, 1999. (26) Simpson, C. R., Kohl, M., Essenpreis, M., and Cope, M. 'Near-infrared optical properties of ex vivo human skin and subcutaneous tissues measured usingthe Monte Carlo inversion technique', Phys. Med. Biol., 43, pp.2465-2478, 1998. (27) Gonzalez, S., Rajadhyaksha, M., and Anderson, R. R. 'Non-invasive (Real-Time) Imaging of Histologic Margin of a Proliferative Skin Lesion In Vivo', Int. Invest. Dermat., 111, No.3, pp.538-539, 1996. (28) Tearney, G. J., Brezinski, M. E., Southern, J. F., Bouma, B. E., Hee, M. R., and Fujimoto, J. G. 'Determination of the refractive index of highly scattering human tissue by optical coherence tomography', Optics Lett., 20, pp.2258-2260, 1995. (29) Bull, R., Ansell, G., Stanton, A. W. B., Levick, J. R., and Mortimer, P. S. 'Normal Cutaneous Microcirculation in Gaiter Zone (Ulcer-Susceptible Skin) versus Nearby Regions in Healthy Young Adults', Int. J. Microcirc., 15, pp.65-74, 1995. (30) Braverman, I. 'Ultrastructure and Organization of the Cutaneous Microvasculature in Normal and Pathologic States', J. Invest. Dermatol., 93, pp.2S-9S, 1989. (31) Ikeda, A., Umeda, N., Tsuda, K., and Ohta, S. 'Scanning Electron Microscopy of the Capillary Loops in the Dermal Papillae of the Hand in Primates Including Man', Journal of Electron Microscopy Technique, 19, pp.419-428, 1991. (32) Jaap, A. J., Shore, A. C., Stockman, A. J., and Tooke, J. E. 'Skin Capillary Density in Subject with Impaired Glucose Tolerance and Patients with Type 2 Diabetes', Diabetic Medicine, 13, pp.160-164, 1996. (33) Meglinsky, I. V. and Matcher, S. J. 'Determination of absorption coefficient of skin melanin in visible and NIR spectral region', in Lasers in Surgery: Advanced Characterization, Therapeutics, and Systems X, R. R. Anderson, K. E. Bartels, L. S.

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57

Bass, C. G. Garrett, K. W. Gregory, N. Kollias, H. Lui, R. S. Malek, G. M. Peavy, H.D. Reidenbach, L. Reinish, D. S. Robinson, L. P. Tate, E. A. Trowers, T. A. Woodward, Editors, Proceedings of SPIE, Vol.3907, pp.143-150, 2000. (34) Mourant, J. R., Bigio, I. J., Jack, D. A., Johnson, T. M., and Miller, H. D. 'Measuring absorption coefficient in small volumes of highly scattering media: sourcedetector separations for which path lengths do not depend on scattering properties', Applied Optics, 36, No.22, pp.5655-5661, 1997. I V Meglinsky School of Physics, University of Exeter, UK and Department of Physics, Saratov State University, Russia S J Matcher School of Physics, University of Exeter, UK

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6 Digital Speckle Photography Applied to in Vivo Blood Micro-circulation Monitoring N Fomin, C Fuentes, J-B Saulnier, and J-L Tuhault

Synopsis This Chapter deals with blood micro-circulation monitoring using CCD recording of the sequences of the dynamic bio-speckle patterns produced when a tissue under study is illuminated by a laser beam with subsequent cross-correlation analysis of these patterns. The time-space cross-correlation analysis of the temporal evaluation of bio-speckle patterns is shown to be a means of tissue blood micro-circulation monitoring. Digital processing of biospeckle patterns records yields two-dimensional maps exhibiting the blood flow's temporal and spatial variations. This might be used for bio-medical diagnostic purposes for detecting, for example, microscale deviation from the normal case.

6.1

Introduction

A map of the blood micro-circulation activity in living tissues is of considerable importance for many diagnostic purposes. The skin micro-circulation fulfils the function of thermal and biochemical exchanges and is important, for example, from the point of view of thermal regulation of the body (1). Optical monitoring of the blood flux has an obvious advantage due to its non-intrusive character. Various methods have been studied for measuring this blood micro-circulation by analysis of the light scattered from moving blood using such optical phenomena as the Doppler effect and the dynamic behavior of laser speckles (2). The application of these phenomena is particularly attractive and promising because of the noncontacting and non-disturbing character of measurement and the possibility of implementation of these techniques in vivo.

60

6.2

Optical Methods and Data Processing in Heat and Fluid Flow

Speckle photography technique

First implementation of speckle photography for blood micro-circulation monitoring used photographic speckle patterns recordings. In this way, Fersher and Briers first measured twodimensional maps of the blood micro-circulation in a human retina by using single exposure speckle photography and optical filtering techniques (3). Fujii et al. have proposed a method of skin blood monitoring by analysis of temporal dynamic behavior of laser speckles recorded by a photomultiplier (PM) at a fixed point in space (4). Using similar PM recording, Ruth (5) has performed an analysis of the influence of the movement of a surrounding tissue on the dynamic spectrum of the bio-speckles and de-composed the movement of the red blood cells (RBCs) and the surrounding skin during the measurements. Further investigations showed the usefulness of the bio-speckle based methods for clinical purposes (6-8). The present paper deals with blood micro-circulation monitoring using CCD recording of the sequences of the dynamic bio-speckle patterns produced when a tissue under study is illuminated with a laser beam with subsequent cross-correlation analysis of these patterns.

6.3

Bio-speckle formation and its dynamic behaviour

Coherent light scattered from diffuse objects produce a random granular interference structure some distance away from the object, which is called speckle pattern (9). Such a pattern can also be observed when a living semi-transparent tissue is illuminated by a laser light. The visible laser light penetrates in to the human skin to a depth of about 200-1000 um and is multiply scattered by the RBCs flowing inside the smallest candelabra capillaries as well as by surrounding tissue. So, the image of the tissue illuminated with the light of laser differs from an image taken under white light illumination by the speckle pattern that is superimposed on the surface features of the tissue. As the scatterers (RBCs) move, the speckles also move and change their shape. The dynamic (time-dependent) bio-speckle pattern is formed as a superposition of some moving speckles with different dynamics, including static speckles. These bio-speckles play a dual role as a source of noise in tissue images and as a carrier of useful information about biological or physiological activity of living tissues, such as subskin blood flow and general tissue-structure motility (10). Dynamics of speckle patterns produced by moving rough surfaces have been extensively studied for velocity measurements (11). However, the spatio-temporal properties of biospeckle are essentially different from those of the speckle patterns formed by a moving rough surface due to the effect of the multiple scattering and different velocities of the scatterers. To describe this effect, Okamoto and Asakura (12) analysed the dynamic properties of multiplyscattered speckles from a series of strong diffusers, each of which completely randomizes the phase of the incident light and moves with its own velocity. They found some typical relationships between the motion of diffusers and the resultant speckle fluctuations. It has been shown that the velocity difference between the phase screens have a strong effect on both decorrelation of the resulting speckle patterns and the fluctuating speed of the speckle intensity. For a statistical description of dynamic speckle patterns it is convenient to use a space-time cross correlation function. The normalized cross-correlation function of the fluctuating component A/ = / -{/) of the speckle intensity is

Digital Speckle Photography Applied to in Vivo Blood Micro-circulation Monitoring

61

Dynamic speckles formed both by single phase screen and by multiple phase screens have two fundamental motions of speckles. In the first type of the speckle motion, called 'translation', the speckles move as a whole and their shape remains unchanged for a considerable displacement. In the second type of speckle motion, speckles deform, disappear, and reappear without appreciable displacement of their positions. This type of speckle motion is called 'boiling' of speckle. In both cases, the speckle's behaviour depends not only on the motion of the scatterers but also on the parameters of the optical scheme used for the speckles observation. In most cases, dynamic bio-speckle mode is mixed and speckles translate gradually changing the structure. One of the main factors characterizing the dynamic behaviour of speckle patterns is the shape of the illuminating wavefront. For the Gaussian beam illumination, the radius of the beam spot in the object plane, a> and the radius of the wavefront curvature, p, are expressed as functions of a distance z from the position of the beam waist

where z0 = nco^ I A. and coa is the spot radius at the beam waist. Two parameters, the correlation time, TC and the lapse time, rd have been introduced by Asakura and Takai (11) to describe the dynamic behaviour of speckles composed of boiling and translation

where f = f2 - 7l , r = t2-t,, and the lapse time , rd depends on r . For a single phase screen moving with a constant velocity v , tc and, -cd are expressed via parameters of the optical imaging scheme. Thus, for a single lens (L2) arrangement shown in Fig. 6.1, they are (11)

where d0 is the distance from the object plane to the imaging lens, dt is the distance from the lens to the observation plane, e = l/d0+l/di-\/r

is a defocusing parameter, D is the

imaging lens diameter, and
62

Optical Methods and Data Processing in Heat and Fluid Flow

with 0 being the angle between the vectors of v and f. Equation 6.4 shows that the correlation time of the intensity fluctuations in speckle pattern is inversely proportional to the velocity of a phase screen which creates this pattern. This relation is widely used for the velocity measurement of rough surfaces by dynamic speckle methods (11). Both experimental and theoretical investigations show that a similar equation can be used for double and triple scattering from a moving rough object. Numerical investigation of Okamoto and Asakura (12, 13) shows that the value of rc is close to a linear increase as the averaged scatterers velocity increases in the case of multiple scattering as well. Thus, the value of l/r r is seen to be proportional to the velocity of scatterers even in more complicated cases including multiple scattering.

Fig. 6.1 Bio-speckle formation under illumination of living tissue by Gaussian beam and experimental installation for blood micro-circulation monitoring via cross correlation analysis sequences of the bio-speckle patterns

6.4

Cross correlation analysis and results

The sequence of the bio-speckle patterns obtained by a laser light scattered from a living human tissue (finger) is shown in Fig. 6.2. The translation and boiling of speckles are observable to the naked eye both on the observation screen and on the TV screen when the sequence of speckle patterns has been recorded. Many parameters are introduced in the practice of measurements to quantitatively characterize such speckle pattern variations. Konishi and Fujii (6) measuring retinal blood flow use the average rate of change of speckles

Digital Speckle Photography Applied to in Vivo Blood Micro-circulation Monitoring

63

called AD - 'the average derivative', and the reciprocal value, BR - 'the blur rate' of the speckle intensity variations. Oulamara et al. (14) studying biological activity of botanical specimens by bio-speckle patterns decorrelation analysis introduce a parameter defined as the decorrelation mean speed (DMS) of the temporal speckle signals. The parameter has been computed as an averaged value of the squared difference between the speckle signals, the first being taken as a reference. This parameter is equivalent to the so called structural function introduced by A. N. Kolmogorov, widely used for statistical characterization of the intensity of turbulent fluctuations in fluid mechanics (9). The same parameter is used in our approach (15) for characterization of speckle pattern decorrelation. The structural function is directly related to the correlation function and is evaluated as

where k and n are the numbers of speckle patterns from the recorded sequence.

Fig. 6.2 The time sequence under specklegram recordings using two types of lasers and examples of bio-speckle patterns obtained by laser light scattered on a living human tissue [original speckle fields are color (red)]

The evaluation of the structural function at each interrogation area has been performed with the use of SCION software. The obtained Dk_n values are coded as gray or color variations and displayed at a monitor.

64

Optical Methods and Data Processing in Heat and Fluid Flow

Fig. 6.3 Averaged structural function of the intensity variations in bio-speckle patterns formed in laser light scattered from an apple tissue (4), human tissue in vivo (•), and a fixed ground glass (A). Numbers indicate chosen frames with time interval between subsequent frames equal to 40ms

Two different tissues have been analysed using this function. Figure 6.4 shows the structural function variations in bio-speckle patterns obtained under illumination of an apple surface by a He-Ne laser. The speckles movement in such patterns are due to movement of the chloroplasts and amyloplasts (small particles of mean size 1 /m) in the cytoplasm medium of the apple tissue, as well as due to the movement of small mineral particles inside the apple cell vacuoles (14). As the speed of all of these scatterers is less than 1 mm min"1, there is an obvious linear dependence of the value of the evaluated structural function on the time interval between frames. Results for another type of tissue are displayed in Fig. 6.5. The speckle pattern variation in this case is due to RBC movement in a living human tissue under study. Such a movement can be two orders of magnitude faster than that in an apple tissue, so no linear increase of the structural function with the time interval between successive exposures is observed. Instead of this, random fluctuations of the blood flow velocity are recorded. The values of the structural function obtained are proportional to the blood flow velocity, but due to the complicated processes of the light scattering in a living tissue, direct experimental calibration is necessary for evaluation of the absolute values of the velocity.

Digital Speckle Photography Applied to in Vivo Blood Micro-circulation Monitoring

65

Fig. 6.4 Local intensity variations in bio-speckle patterns formed in laser light scattered from an apple obtained by cross correlation analysis - numbers indicate chosen frames with time intervals between subsequent frames equal to 40 ms (original pictures are color)

Fig. 6.5 Examples of blood flow patterns in a living tissue obtained by cross-correlation analysis of speckle patterns shown in Fig. 6.1 (original pictures are color)

6.5

Conclusions

The time-space cross-correlation analysis of the temporal evaluation of bio-speckle patterns is shown to be a means of tissue blood micro-circulation flow visualization. Digital processing of bio-speckle patterns records yields two-dimensional maps exhibiting the blood flow temporal and spatial variations. This might be used for bio-medical diagnostic purposes for detecting, e.g. micro-scale deviation from the normal case. Although the cross-correlation data obtained is seen to be proportional to the velocity of the scatterers, the effect of multiple scattering causes difficulties in measuring the absolute value of the blood velocity. Therefore, the monitoring is qualitative and the blood flowmaps are presented in arbitrary units.

66

6.6

Optical Methods and Data Processing in Heat and Fluid Flow

Acknowledgments

The research described in this publication was supported partly by the joint grant of the Belarusian Government and INTAS (INTAS-BELA Project N 97-0082) and INTAS grant INTAS 00-0135. Prof. N.Fomin gratefully acknowledges receipt of a PAST grant from French Ministry of Research and Education.

References (1) (2) (3) (4) (5) (6) (7)

(8)

(9) (10) (11) (12) (13) (14)

Dittmar A. et al. Estimation of skin blood from the measurement of thermal conductivity,- Innovation et Technologic en Biologie et Medicine: Special Issue, v.12, N. l, pp. 121-137, 1991. Briers J.D. Monitoring biomedical motion and flow by means of coherent light fluctuations, - in CIS Selected Papers: Coherence Domain Methods in Biomedical Optics. SPIE Proc., V.V. Tuchin ed., v.2732, pp. 2-15, 1996. Fercher A.M. and Briers J.D. Flow visualization by means of single-exposure speckle photography. Opt. Communications, v.37, pp. 326-330, 1981. Fujii H., Asakura T., Nohira K., Shintomi Y., and Ohura T. Blood flow observed by time-varying laser speckle. Optical Letter, v.10, pp. 104-106, 1985. Ruth B. Non-contact blood flow determination using a laser speckle method. Optics and Laser Technology, v.20 (6), pp. 309-316, 1988. Konishi N. and Fujii H. Real time visualization of retinal microcirculation by laser flowgraphy Optical Engineering, v.34(3), pp. 753-757, 1995. Ruth B. Dynamic speckle patterns: a tool for determining the mean velocity of biological objects. - In V.V. Tuchin, editor, CIS Selected Papers: Coherence-Domain Methods in Biomedical Optics, volume 2732 of SPIE Proceedings, pp. 16-26, Bellingham, Washington. SPIE Press, 1996. Briers J.D. and He X.-W. Laser speckle contrast analysis {(LASCA)} for blood flow visualization: improved image processing., - A.V. Priezzhev, T. Asakura, and J.D. Briers, editors, In Optical Diagnostics in Biological Fluids III. Proc. SPIE, v.3252, pp. 26-33., SPIE Press, Bellingham, 1998. Fomin N. Speckle Photography for Fluid Mechanics Measurements, Springer Verlag, Berlin, 1998. Aizu Y. and Asakura T. Bio-speckles. A. Consortini, editor, In Trends in Optics, pp. 27-49. Academic Press, Orlando, 1996. Asakura T. and Takai N. Dynamic laser speckle and their application to velocity measurement of diffuse object. Journal of Applied Physics, v.25, pp. 179-194, 1981. Okamoto T. and Asakura T. The statistics of dynamic speckles. - E.~Wolf, editor, In Progress in Optics, volume XXXIV, chapter~3, pages 183-248. Elsevier Science B.V., 1995. Okamoto T. and Asakura T. Effect of imaging properties on dynamic speckles produced by a set of moving phase screens. Waves in Random Media, v.2, pp. 49-65, 1992. Oulamara A., Tribillon G., and Duvernoy J. Biological activity measurements on botanical specimen surfaces using a temporal decorrelation effect on laser speckle. Journal of Modem Optics, v.36(2), pp. 165-179, 1989.

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(15) Fomin N., Fuentes C., Saulnier J.-B., and Tuhauit J.-L. In vivo diagnostics of blood microcirculation by speckle patterns cross correlation analysis, - In book of Abstracts, Eurotherm 57 Seminar on Microscale Heat Transfer, Poitiers, France, July 8-10, 1998. (16) Fomin N., Fuentes C., Saulnier J.-B., and Tuhauit J.-L. Blood microcirculation diagnostics by speckle pattern cross correlation analysis, - In book of Abstracts, 5th International Conference on Flow Diagnostics, Moscow, July 23-25, 1999, p. 170, Moscow Power Institute Press, 1999.

N Fomin Convective and Wave Processes Laboratory, HMTI, Minsk, Belarus C Fuentes, J-B Saulnier, and J-L Tuhauit Laboratoire d'etudes thermique, ENSMA, France

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7 Development of Full-volume Digital Holography for Particle Measurement S Murata and N Yasuda

Abstract This Chapter presents a digital method for detecting the position of small particles distributed in three-dimensional space from in-line hologram patterns. The detection is carried out by numerically reconstructing the real image of the particles from observed hologram patterns. In the present method that is called full volume digital holography, two-dimensional real images are reconstructed at a constant small interval in depth to obtain the three-dimensional light intensity distribution of the real images. Three different types of template are tested for searching local minimums in the three-dimensional light intensity distribution which express particle positions in three-dimensional space. The measurement accuracy of the present method is examined in numerical simulation and in experiment, and it is found that the template of circular cylinder gives better results than the other templates and the RMS error is about 1.8 mm using a binary template.

Notation d d' D SF h, H Id Iz J x,y

xz,yz *o, yo, zo t

depth of a tracer particle detected depth of a tracer particle diameter of template a Gaussian function light amplitude on image plane height and width of template light intensity on hologram plane (Transparency function) light intensity on image plane imaginary unit Cartesian co-ordinates on hologram plane Cartesian co-ordinates on image plane Cartesian co-ordinates in three-dimensional template thickness of template

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T Zmin Zmax

Az X

7.1

Optical Methods and Data Processing in Heat and Fluid Flow

template function in three-dimensional template co-ordinate originating from hologram plane and perpendicular to it minimum limit of measurable particle depth maximum limit of measurable particle depth size of spatial discretization along z-axis wave length of illuminating light

Introduction

Recent developments in computer and image processing are remarkable, hence they enable us to realize the computational processing and analysis that were considered to be impossible only five years ago. Digital holography is such a technique that requires large computational power and resources. It is well known that conventional holography has been used for the recording of a three-dimensional object and the measurement of its shape, displacement, deformation, and vibration in various fields of engineering. In digital holography, hologram patterns are obtained experimentally with a CCD camera without development of a sensitive plate and the procedure of image reconstruction is carried out numerically and digitally on a digital computer in order to make holography more suitable for on-line measurement. Kreis and Juptner's research group (1-3) was the first to propose the technique of digital holography for three-dimensional objects of large volume. Quite recently, Owen et al. (4) developed the hardware and software systems for a holographic sensor to monitor small marine particulates and successfully operated the system underwater on a remotely operated vehicle. Besides the research, the importance of digital holography has been increasing in practical applications. In parallel with these researches, the authors (5, 6) also have proposed a digital holographic method for particle measurement and examined the computational performance and the measurement accuracy of the method for the application to three-dimensional particle tracking velocimetry (3D-PTV) in fluid engineering. This method is based on in-line holography for easy camera setting in experiment, and in-plane position on hologram plane and position in depth are measured sequentially in the first and second steps, respectively. Furthermore, the computation of light intensity of the real image is one-dimensional only along the axis of illumination direction originating from the center of each particle on the hologram plane for reducing computing time. However it has a disadvantage on measurement accuracy - it is difficult to accurately detect the particle position on the hologram plane in the first step if a lot of small particles are recorded on the hologram plane and the interference fringes for the particles are strongly overlapped to each other, hence this point has been a target to be attacked for higher measurement accuracy. In this Chapter, a new digital holographic method has been developed for accurately detecting the position of small particles distributed in three-dimensional space from two-dimensional observed hologram patterns. Since the position of each particle is measured from the light intensity distribution in the three-dimensional volume which is obtained using a number of twodimensional real images discretely and numerically reconstructed at a small interval in depth. The present method is called 'Full volume digital holography' to distinguish it from the method reported in our previous papers. In the present method, the three-dimensional position is directly measured from the results of full volume reconstruction, so the information on in-plane position on the hologram plane is not needed for particle depth measurement.

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71

Basic concept

Image reconstruction is carried out digitally and numerically from the observed hologram pattern, expressed by Id, using Fresnel diffraction formula, and the light amplitude of reconstructed image, hz, can be represented by the following equation,

where A. is the wave length of illuminating light and j denotes the imaginary unit. The Cartesian co-ordinates on the hologram plane and on image plane are denoted by (x,y) and (xz,yz), respectively, and z is the distance backward measured from the hologram plane, as shown in Fig. 7.1. From equation (7.1), the light intensity at an arbitrary point (x z ,y z, z) within a reconstructed volume is obtained as follows:

Fig. 7.1 Full volume digital reconstruction

Since the hologram patterns are optoelectronically obtained with a CCD camera without development of a sensitive plate, the input information Id can be directly transferred into a computer and calculations of particle measurements are immediately carried out on it. In practice, the integral in equation (7.1) is discretized with a simple trapezoidal formula or a FFT algorithm to obtain the light intensity I2. From Kreis and his co-researchers' publications, it is pointed out that the reconstructed image is enlarged or contracted according to the reconstruction depth, z, by conventionally applying the FFT algorithm to equation (7.1) and the formulation with a convolution is useful for keeping the size of the reconstructed image constant. This formulation, presented by Kreis, can be written by

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where ^denotes Fourier transform and gp is a Gaussian distribution expressed by

In the full volume reconstruction, a number of two-dimensional real images are numerically reconstructed at a small interval Az along z-axis. Figure 7.1 illustrates the light intensity distribution reconstructed in the three-dimensional volume. There are locally some spatial regions with lower light intensity corresponding to the three-dimensional positions of the small particles. For particle depth measurement, we have only to detect the three-dimensional positions where the light intensity is locally a minimum as shown in Fig. 7.1. In order to obtain the light intensity in a three-dimensional full volume, it is required that the scale of a two-dimensional reconstructed image is constant at any co-ordinate z. As described above, the size of a reconstructed image does not change according to the co-ordinate z by using equation (7.3). However it should be noted that the above approach requires one Fourier transform and one inverse Fourier transform each to obtain one two-dimensional reconstructed image, hence the computational effort is twice that for solving equation (7.1) using a conventional FFT algorithm. In this study, it is confirmed that the FFT algorithm, programmed in FORTRAN, can be carried out only once in about 5 sec on a standard CPU (AMD K6-II 450MHz) for the size of image 512 x 512 pixels and so the above FFT approach is acceptable for real particle measurement. There are a number of variations in detecting the center of local three-dimensional space with lower light intensity. As shown in Fig. 7.2, we prepared three templates to search the threedimensional position of a particle using a cross-correlation function, at the position where the local pattern on a reconstructed three-dimensional volume is very similar to a template. The shapes of the templates are (a) circular cylinder, (b) annular cylinder, and (c) three-dimensional cross. In Fig. 7.2, D, t, and H denote the diameter, thickness, and height (or width) of the template, respectively. In each template, the template function T is given by a Gaussian distribution as follows:

where xo,yo,zo are the Cartesian co-ordinates in a three-dimensional template whose origin is the geometrical center of the template and a x y , az denote the standard deviations in the x (ory) and z directions, respectively. This template is nearly a binary template if axy , az is much larger than the size of the template.

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Fig. 7.2 Three different three-dimensional templates

7.3

Numerical simulations

In numerical simulations, hologram patterns are generated numerically using the Fresnel diffraction formula in the same way as digital image reconstruction. The size of one pixel is 0.01 mm and the size of spatial discretization along the direction z is Az — l mm in this test. The wavelength of illuminating light used in the numerical simulations is 632.8 run, that of a He-Ne laser. Figure 7.3 shows an example of full volume reconstruction for numerically generated in-line hologram pattern. These results are obtained for the case in which there are only three particles of 0.32 mm diameter which are positioned in depth at d = 250, 500, and 750 mm. Digital image reconstruction is performed by solving equation (7.1) with a simple trapezoidal formulation in this case. Since it is somewhat difficult to show the results of the full volume reconstruction in two-dimensional plane, the hologram pattern at z = 0 mm and only three reconstructed images sliced at z = 250, 500, and 750 mm are depicted in Fig. 7.3 for visibility. From Fig. 7.3, we can clearly recognize the difference among the three reconstructed images and the shades of the particles at d = 250, 500, and 750 mm are focused on the images at z = 250, 500, and 750 mm, respectively. Therefore we have only to search the positions of local minimal intensity in a reconstructed volume, using an appropriate template.

Fig. 7.3 A digital hologram and its full volume digital reconstruction

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In order to check the difference in measurement accuracy among three templates shown in Fig. 7.2, performance test was carried out using a numerical hologram pattern obtained for ten small particles. The diameter of the small particles is 0.32 mm, and their depths are given by random numbers within the range from zmin = 200 mm to zmax= 300 mm. We carried out the FFT image reconstruction at all the positions from zmin = 200 mm to zmla= 300 mm with an interval Az = 1 mm. As an example, Fig. 7.4 shows the distribution of cross-correlation function value on the x2 -y2 plane at z = 226 mm and its profile along z-axis for the particle positioned in x = 176 pixel, y = 316 pixel, and d = 226 mm. These results are obtained using a template of circular cylinder. In Fig. 7.4(a), the darker gray level indicates lower cross-correlation function value. Since the cross-correlation function takes a local minimum at each particle position, it is seen that there are ten circular regions with lower cross-correlation function value on the xz -yz plane. In Fig. 7.4(b), it is shown that a local minimum is taken at just the same z-co-ordinate as its particle depth d = 226 mm. Table 7.1 summarizes the RMS error of detected particle position for ten particles. The circular cylinder template gives the better results and the RMS error is less than 2 mm except for axy = 3.5 pixel and az = 0.25 pixel. Table 7.2 shows the influence of template size on RMS error for the template of circular cylinder. In this case, the diameter of small particle is 0.32 mm (32 pixel) and the diameter of the template, D = 29 pixel, gives better results. Since the RMS error will change according to the combination of particle diameter and axy, it will be needed as a next target to develop a new template or an algorithm that is valid for any particle diameter.

Fig. 7.4 Computed cross-correlation function

Table 7.1 Comparison of RMS error among various templates RMS error (mm) axv (pixel) 3.5 7.0 7.0 7.0 14.0

oifp/xe/; 0.25 0.125 0.25 0.50 0.25

Circular cylinder 4.47 1.92 1.84 1.85 1.53

Annular cylinder 5.33 18.02 18.02 18.02 19.01

Three-dimensional cross 6.80 4.81 4.81 4.81 2.52

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Table 7.2 Influence of template size on RMS error D (pixel) 23 29 29 29 35

7.4

t (pixel) 3 1 3 5 3

RMS error(mm) 5.55 1.22 1.13 1.46 5.55

Experiments

This section shows experimental results to confirm the reliability of the numerical simulations in this Chapter. The experimental setup used for the observation of in-line hologram patterns is shown in Fig. 7.5. All optical equipment and optics are set up in line on an optical rail to easily measure the position of each apparatus in depth. The small particles attached to the glass plate are illuminated by He-Ne laser light (wave length = 632.8 nm, output = 2 mW) through a spatial filter and a laser collimating lens. Diffracted light and plane light are recorded by the BAY CCD camera (SONY XC-55), and interference fringes are observed directly on the CCD element without a camera lens. The diameter of the expanded laser beam is about 35 mm and the size of the CCD element is 1/3 inch with 659(H) x 494(V) resolution (cell size 7.4 x 7.4 jim/pixel). The tracer particle is colored white and its mean diameter is 0.32 mm. Observed images are captured into the personal computer (DEC VENTURIS 5100, AMD K6-II 400MHz) with the image capture board (CYBERTEK, CT3001RGB). The resulting digital image is composed of 640 x 480 pixel with 8-bit grey level resolution for a green image.

Fig. 7.5 Optical setup

Table 7.3 shows the comparison of RMS error between binary template and Gaussian template (
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10 ~ 20mm, hence it can be said that Ml volume digital reconstruction is useful and effective for improving the measurement accuracy of digital holographic technique for particle measurement.

Table 7.3 RMS error for experimental results

Binary Gaussian

x-direction

y-direction

4.68um 17.51nm

4.68um 10.46nm

z-direction 1.84mm 3.44mm

Figure 7.6 demonstrates an example of reconstructed particle image at the detected depth z = d' — 289 mm for d = 290 mm using (a) trapezoidal formulation and (b) FFT algorithm based on equation (7.3). It is found from Fig. 7.6 that the particle image can be successfully reconstructed with either numerical method. However it should be noted that the image is disturbed near the boundaries by the assumption of periodicity over a sampling window because of FFT algorithm and this may cause error in searching local minimums in a reconstructed volume. On the other hand, as described in Section 7.2, the approach of solving equation (7.1) with a simple trapezoidal formulation prevents the reconstructed image from being affected by the assumption of the periodicity.

Fig. 7.6 Comparison of a two-dimensional reconstructed real image

7.5

Conclusions

In this Chapter, full volume digital holography for particle measurement has been examined in numerical simulations and in experiments. Three different templates have been tested for searching local minimums in a three-dimensional reconstructed volume of light intensity, minimums which mean the position of small particles. From the test results, it was found that the template of circular cylinder was more useful than the others for searching the local minimums. Furthermore, it was shown in experiments that the RMS error of the present method was 1.8 mm for a binary template.

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The material presented in this contribution was presented the at the International Conference on Laser Anemometry - Advances and Applications on the 12th September 2001, at the University of Limerick, Ireland, organized by the European Association for Laser Anemometry and cosponsored by the Institution of Mechanical Engineers, Published in the proceedings of the same name.

7.6 (1) (2) (3) (4) (5)

(6)

References Schnars, U., Kreis, T. M., and Jiiptner, W. P. O. Direct recording of holograms by a CCD target and numerical reconstruction, Applied Optics, 1994, Vol.33, No.2, pp.179-181. Kreis, T. M., and Jiiptner, W. P. O., Principles of digital holography, Fringe '97, 1997, Akademie Verlag, pp.353-363. Adams, M., Kreis, T. M., and Juptner, W. P. O. Particle size and position measurement with digital holography, Proc.of SPIE, 1997, Vol.3098, pp.234-240. Owen,R.B. and Zozulya, A. A. In-line digital holographic sensor for monitoring and characterizing marine particulates, Optical Engineering 2000, Vol.39, No.8, pp.2187-2197. Murata, S. and Masuda, M. Detection of the depth of tracer particles using numerical reconstruction from in-line hologram patterns, Proc. of 5th Triennial International Symposium on Fluid Control, Measurement and Visualization, 1997, Vol.2, pp.923-928. Murata, S. and Masuda, M. Detection of the depth of tracer particles using numerical reconstruction from in-line hologram patterns, Proceedings of 8th International Symposium on Flow Visualization, Paper No. 177, 1998, pp. 177.1-177.6.

S Murata and N Yasuda Department of Mechanical and System Engineering, Kyoto Institute of Technology, Sakyo-ku, Japan

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8 A Particle Imaging and Analysis System for Underwater Holograms J J Nebrensky, G Craig, G L Foresti, S Gentili, P R Hobson, H Nareid, G G Pieroni, and J Watson

Abstract Pulsed holography is an important technique for the study of particle fields: it allows instantaneous, non-invasive high-resolution recording, and the later replay of real images from which one can obtain the size, shape, position, and - if multiple exposures are made - velocity of every object in the sample volume. This Chapter will discuss various issues encountered during the design of a hologram replay machine, with particular emphasis on the physical design of the instrument, and on the volume scanning and image processing techniques needed to pick out the useful information from the terabyte amounts of raw data produced on each hologram.

8.1

Introduction

Pulsed laser holography is an extremely powerful technique for the study of particle fields as it provides instantaneous, non-invasive high-resolution records, avoiding the distortion of the flow field associated with inserted probes, while from the replayed real images one can obtain the size, shape, position and - if multiple exposures are made - velocity of every object in the sample volume. The value of such experiments depends crucially on the quality of the reconstructed image: not only will poor resolution degrade size and shape measurements, but aberrations such as coma and astigmatism can change the perceived centroid of a particle, affecting position and velocity measurements. The Holomar collaboration (1) is currently working on both an underwater holocamera uniquely incorporating simultaneous in-line and off-axis holography - for the in-situ recording of plankton species and distributions and 'HoloScan', an associated hologram reconstruction

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and analysis instrument primarily designed for the study of underwater holograms such as those produced by the Holomar project or in HPIV studies. This Chapter discusses various issues associated with the high-fidelity replay of underwater holograms appropriate to the design of the HoloScan replay machine. It commences with a brief description of the sources of some of the aberrations and noise that degrade the reconstructed images, and then considers the design of the scanning machine and the implementation of neural net software for the identification and classification of marine particles. It will be assumed that the recording conditions have already been optimized; this aspect is covered in depth in Hobson and Watson (2).

8.2

Sources of aberrations and noise

8.2.1 Plate position and illumination A hologram records the shape of an object wavefront relative to a reference beam. For perfect reconstruction the reference beam at replay must therefore be an exact duplicate of that used in recording, i.e. the wavefront phase distribution at reconstruction must be the same as at exposure. This means that the replay beam must have the same degree of collimation (usually exactly parallel) and, if of the same wavelength as at recording, encounter the holographic plate from exactly the same direction. The replay machine must allow the position of the hologram to be precisely adjusted and hold it securely during the scanning process. In practice it is also possible to cancel out some of the aberrations due to other causes by deliberately introducing an equal but opposite aberration by slightly tilting the hologram plate. As many aberrations depend on the illuminated aperture of the hologram, it is also possible to reduce many aberrations by illuminating only a smaller area of the hologram, but this also reduces the maximum resolution of the image and also its brightness. 8.2.2 Transfer optics In some applications it is necessary to use transfer optics between the holographic plate and the object to be recorded. These will often introduce aberrations in to the object beam which will then be recorded by the hologram, so that even perfect replay would suffer from them. One possibility is to replay the real image back through the transfer optics, which then undo the aberrations to give a corrected real image. Examples of such optics are the fish-eye lenses used for holographic recording in the large bubble chambers used in high-energy physics. 8.2.3 Refractive index changes A change in the medium enclosing the hologram or object implies refraction effects; for example, if the hologram and objects are submerged in water during recording then when replaying in air the rays forming the image will be refracted through a greater angle leaving the hologram plate than they would have been in water, and so the image points will be located closer to the hologram plate. Thus for a hologram directly in a medium a distortion of the image field results from a change in the ambient refractive index. This can be corrected by changing the illumination wavelength: as the hologram acts as a diffraction grating shorter wavelength illumination will be diffracted less, so that by careful choice of wavelength the distortion can be corrected. In practice the situation will be complicated by a glass window and air space that act as unconnected transfer optics and thus introduce extra aberrations. Since it is not practical to replay the holograms in to large tanks of water, it is necessary to try to

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reduce these aberrations by other means: in particular by appropriate choice of the holocamera's window and air space thicknesses. 8.2.4 Wavelength changes There is only a limited number of possible (and affordable) laser wavelengths, not all of which are available in both pulsed and continuous operation with suitable powers. This means that in some applications it may not be possible to play back the hologram at the ideal wavelength, and a nearby wavelength must be used instead. It is possible to limit the effects of a wavelength mismatch by changing the reference beam angle to reproduce the required phase distribution across the hologram, but this can drastically reduce the replay efficiency in some situations where the Bragg condition is no longer satisfied within the emulsion. 8.2.5 Viewing angle For in-line holography, information about the object is recorded on the holographic plate in the vicinity of the projected centre of the object, so that during replay the light forming the image is all more-or-less parallel to the optical axis and different regions of the image can be investigated simply by moving the camera along three axes. In off-axis holography, however, the image is recorded over the whole plate and the field of view can be much wider, so it becomes necessary not only to translate the camera but also to rotate it as it moves away from the optical axis, otherwise it is necessary to use a much wider aperture camera lens and the focal plane of the object may also lie at an angle to that of the detector (Fig. 8.1).

Fig. 8.1 Comparison of viewing geometries and camera movement for in-line (above) and off-axis (below) replay

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One interesting development is the In-line Recording and Off-axis Viewing (IROV) technique, in which an in-line hologram is viewed using magnifying optics at a steep angle, so that the light forming the image is only that diffracted from the high-spatial frequencies in the hologram (3). This scheme provides a reduced depth of focus for the particle images and reduced speckle noise compared with conventional in-line reconstruction, at least for small volumes. 8.2.6 Background noise 'Background noise' refers to intensity variations in the image, often on relatively large length scales, that may obscure details in the object. There are many possible sources of this noise, including, for example, uneven illumination (say from dirt in the reference beam) and the formation of interference patterns due to stray light or light multiply reflected within the hologram plate or optical components. There is also the possibility of substantial scattering of light within the holographic emulsion, while in particle field holography there may also be out of focus particle images within the field of view. 8.2.7 Speckle noise The coherent nature of laser light gives rise to the phenomenon of 'speckle', which causes the images to have a speckled or grainy appearance. This leads to difficulties when viewing holograms of particle fields: large bright particles such as bubbles may be seen as 'large bubble moons against a background of speckle noise stars' (4), but smaller, fainter particles will be lost among the speckles. The phenomenon is aperture dependent: speckle size is reduced (and image quality improved) as the aperture is opened, which means that the optimum aperture is a compromise between increased aberrations and increased speckle. Another way to reduce the speckle visibility is to insert a rotating ground-glass diffuser in to the illumination beam.

8.3

The 'HoloCam' underwater holocamera

The Holomar collaboration has developed an underwater holocamera, contained within a pressure housing designed for operation down to 100m (1). Uniquely, the camera incorporates both the 'in-line' and 'off-axis' holographic geometries: in-line holography can record organisms in the 5 to 250 [an range at concentrations up to several thousand cm"3 while off-axis holography is better for organisms bigger than 100 um and at much higher concentrations. The use of both geometries with overlapping sample volumes (of several litres) thus allows recording of a wider range of organisms under a greater variety of conditions than current alternatives. During a dive up to 25 holograms of each geometry may be recorded simultaneously on glass plates by a Q-switched, frequency-doubled Nd-YAG laser, operating at a wavelength of 532 nm and a pulse duration of less than 10 ns to freeze any motion of the organisms. The layout of a lab mock-up of the recording optics is shown in Fig. 8.2. The combination of off-axis reference beam angle (60°), window thickness and window-to-hologram plate spacing during recording has been optimized for replay using a 442 nm HeCd laser (2).

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Fig. 8.2 Optical layout of a mock-up of 'HoloCam', showing the relationship of the in-line and off-axis sample volumes

8.4

The 'HoloScan' replay machine

8.4.1 Background Automated data extraction from holograms requires that the recorded information is digitized and made available for computer analysis, most obviously by illuminating the hologram with the conjugate of the original reference beam used in recording so as to produce the real image of the sample volume through which a suitable imaging system (such as a bare CCD or a videocamera fitted with a microscope objective) may be scanned. As discussed in section 8.2.5, for the replay of in-line holograms one would prefer to traverse the camera along linear, Cartesian axes of movement (here denoted x and y in the plane of the holographic plate and z in depth), whereas for off-axis systems one would prefer to work in a spherical polar frame of reference. Several approaches have been used for in-line replay. The illumination system is generally kept fixed; either the holographic plate is moved along all three axes to project the requisite region of the real image in to a fixed camera [e.g. Bexon et al. (5); Green and Zhao (4)] or the plate may be kept fixed and the camera moved (6). For large sample volumes there may be difficulties ensuring the mechanical stability of a three-axis positioning system, so it is common to separate the transverse and in-depth motions, usually moving the holographic plate in-plane (x, y) and focusing by moving the camera assembly (z) [e.g. Bomnann and Jaenicke, (7)]. This arrangement allows an aperture to be introduced in to the replay beam so that only a limited region of the holographic plate is illuminated, which helps reduce background noise due to scattering in the emulsion.

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Similar considerations apply to the automated scanning of large-volume off-axis holograms, except that the movements should correspond to a polar frame of reference. Kitagaki (8) built a scanning machine in which the hologram was fixed while a pair of cameras moved through the real image on the end of an arm. Since the distance between the holographic plate and the region of interest was 2 to 3 m, a pair of mirrors was used to fold the image beam along the arm. The sampled view was thus defined by the arm position (cp, 6) and the mirror separation (r). Deason (9) and Naon et al. (10) instead rotated the holographic plate (along with the reference beam) so that the camera system only needed to select the appropriate depth plane, either by a mechanical movement (9) or by a change in the focal length of the optics (10). As HoloScan must be able to replay both the in-line and off-axis holograms recorded by HoloCam, it has been decided to keep the holographic plate still and scan the videocamera through the projected real image on a set of three linear stages (up to 1000mm in length); although this will restrict the performance of the replay system near the edges of the off-axis field of view, the increased aberrations of the reconstructed holographic image here will limit the impact on the data gathered. Furthermore, the nature of the work coupled with the overlapping volumes on the camera mean that it is likely that novel organisms or objects requiring manual study will be recorded in two holograms at once, and so the physical machine should be able to be switched between in-line and off-axis modes as quickly and easily as possible. Several replay machine geometries have been considered (11). The approach of keeping the holographic plate and stages and the laser fixed and moving the collimator assembly to change beam angle between in-line and off-axis replay was discarded because of the difficulties expected in maintaining alignment of the collimator with the laser beam during the geometry change. Keeping the laser and collimator fixed and rotating the holographic plate about a vertical axis also requires that the large and very heavy stage/video camera assembly is moved precisely around the plate; the opposite approach of keeping the stages still and instead pivoting an integrated laser/collimator assembly is more viable but still requires rapid and precise movement of a large but delicate assembly. 8.4.2 Physical layout The final system, shown in Fig. 8.3, has two fixed arms, one carrying the stages and videocamera assembly and the other the laser/collimator assembly. The replay beam is reflected on to the holographic plate by a large X/\0 mirror which can easily be moved between two fixed positions for off-axis (46 degree beam angle) and in-line (0 degrees, not shown) replay. There is thus only one relatively small moving part which means that the support frame (MiniTec aluminium extrusion) can be both simple and rigid. A Kimmon 180 mW HeCd laser is mounted vertically above the collimator assembly (a Keplerian telescope composed of a CVI 1000mm focal length achromatic objective and Melles-Griot 10 mm doublet producing a 100 mm diameter beam flat to X/5 over the likely operating temperature range); the mirror pair at the outer end of the laser/collimator arm providing a convenient location for any necessary beam attenuators and shutters.

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Fig. 8.3 HoloScan layout (off-axis configuration)

The long (z) stage has a travel of 1000 mm and a step size of 10 um while the shorter stages have a travel of 200 mm and a step size of 5 um. The Ealing DPS controller and stages are driven from a PC via an RS-232 connection. The standard monochrome signal from the CCD videocamera (JAI CV-M300) is captured by an ITI PC-Vision framegrabber housed in the same PC; this allows both image acquisition and stage movement to be controlled and automated by a single piece of software. For in-line holograms the camera is fitted with a x 10 microscope objective, so that each video frame represents a field of view of 1 mm by 0.8 mm. The larger particles recorded with off-axis holography allow imaging at 1:1, by projecting the real image directly on to the videocamera CCD. One difficulty associated with the wavelength change between recording (532 nm) and replay (442 nm) is that after the emulsion shrinkage associated with photographic processing, the Bragg condition may no longer be fulfilled by the off-axis holograms during replay, resulting in the reconstructed image being too faint to be usefully detected by the current camera. As it is desirable to keep to the processing technique that gives the lowest background noise and best image resolution/geometrical fidelity in the final hologram, alternative solutions to this problem are being explored; notably either running the camera in an integrating mode or controlling the ambient humidity in the plateholder to adjust the emulsion thickness. 8.4.3 Scanning strategy A few typical holographic images of plankton and marine particles are shown in Fig. 8.4. These have been obtained by manually scanning the camera through the whole reconstructed sample volume, locating objects, and focusing by eye. This process is extremely time

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consuming, especially as the oceanological interest lies primarily not in the appearance of any given individual but in the relative distributions of large numbers of organisms. Automation of the analysis step in particle holography is critical to large-scale application, otherwise the data-taking can outstrip the data extraction process. Green and Zhao (4) estimated their automated HPFV analysis system would take twice as long as a human, while that of Bamhart et al. (12) could map out velocity vectors through a 1000 cm3 volume sampled in 1 mm steps in 5-6 hours; but such systems perform minimal analysis of individual objects. Using semi-manual scanning systems, Brown (13) took 'a few hours' for each 150 cm3 sample volume looking at ice crystals in clouds, while in their studies of meteorological fogs Borrmann and Jaenicke (7) required 32 hours for each hologram covering 8 cm3 and 1000 droplets. Improvements to the holocamera allowed Vossing et al. (14) to record ice crystals and snowflakes over a 500 / volume, pushing analysis times up to 70 hours/hologram. Recently, Katz et al. (6) tested a submersible with only an in-line holographic system for plankton studies. The holocamera uses film and can record up to 300 holograms in each dive, but manual analysis then takes two man-weeks for each hologram of the 300 to 2000 cm3 sample volume. The issues with such protracted data extraction are not merely the time scale per se, but that with any manual involvement operator fatigue can result in the introduction of hidden systematic errors in the results.

Fig. 8.4 Holographic reconstructions of (above) a 300 jun long Thalassiosira and a couple of Asterionella, and a piece of floe (below), all at approximately the same scale

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The extraction of species distribution data can conveniently be separated in to two stages: first, the HoloScan machine scans the videocamera through the depth of the sample volume (z) in a series of 0.1 mm steps; when the end of the sample volume is reached the camera is panned sideways before returning so that eventually the entire volume has been covered. For each step, dedicated image pre-processing and tracking software [described in Nebrensky et al., (11)] cleans up the captured image and locates any possible objects in view. These are tracked between steps so that their focal plane can be identified, and the in-focus images are extracted and binarized. In the second stage the binarized images are presented to a neural net (section 8.5) for detailed classification.

8.5

Particle identification and organism classification

Due to the enormous amount of available data and the time necessary for a human operator to perform an analysis, automatic plankton classification is a very challenging and important task. Holographic plankton images present two main problems for the automatic classifier. One is the intrinsic noise of the hologram, which must be dealt with during the pre-processing and binarization of the original images. The second problem (more important from the classification point of view) is that we have two-dimensional images of three-dimensional objects that can be seen randomly from any different point of view and in any orientation. Moreover, plankton is composed of living organisms that can cluster together in different ways, or may have deformations associated with their motion. For this reason the object can change its shape remarkably even among organisms of the same class (see Fig. 8.5).

Fig. 8.5 Binarized images of objects belonging to the same class (Asterionella)

In order to overcome the problems related to rotation in the plane of the image, we decided to use rotation and translation invariant features extracted from the images to train a neural network to classify different organisms. The training algorithm is 'Back Propagation,' adopted for its extreme flexibility and noise resistance. Moreover, as a large database of images will be analysed the feature-extraction method must be fast, so computational time has also been investigated. The following features have been considered: Hu moments (15), pattern spectrum (16), statistical pattern spectrum (17), and high order pattern spectrum (18). We carried out a performance comparison of the methods for extracting the features by evaluating their invariance under rotations and their computational velocity (see Table 8.1). At the end of the tests, we decided to use the first five Hu moments (the last two are too sensitive

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to noise), because even if they may not be the most accurate, the computational time is about 1/30 to 1/300 of the time necessary for other methods. As dimension is an interesting feature for discriminating one species from the other, and Hu moments are shape descriptors invariant under scaling, we decided to add a feature represented by the elongation of the organism (in pixels) along the major inertia axis in order to discriminate objects depending on their size. Another feature that is invariant under rotation is represented by the ratio between elongation along the secondary inertia axis and the elongation along the primary inertia axis. This feature has also been added as it does not need a long computational time and supplies other useful information on object shape. In this way, we have a seven-element vector extracted from the image to be classified by the neural network. A neural network is formed by many processing units (neurons) connected one to another. Computation develops in a parallel way, as many neurons work at the same time; due to the fact that input patterns are evaluated by several processing units, neural nets are noise resistant, and show greater versatility than classical algorithms. Moreover, neural nets have the capacity to generalize, so that a limited number of examples of a given object class become sufficient to allow the recognition of other objects of the same class, thanks to common features. The neural network structure adopted in our experiment is a three layer NN (input, hidden, output) where the first layer (input layer) is formed by seven units, the hidden layer is formed by seven units, and the output layer is formed by five units. It runs a Back Propagation algorithm that has been trained by a 38 pattern training set for 40 000 iterations of learning. The time necessary to train the neural network is 2 h 40 mins (CPU time) by a Pentium II 300 MHz with 128 Mb RAM.

Table 8.1 Features extraction algorithms evaluation Method Hu moments (first five) Pattern spectrum Statistical pattern spectrum High order pattern spectrum

Error due to rotation 5% 12% 10% 3%

Time/time Hu 1 30 100 300

In order to have an effective network, it is necessary to supply a representative training set. The sample images within each class should be neither too similar, to avoid too great a specialization, nor too different, to allow the convergence of the net and to avoid confusion with other classes. Five possible output classes (corresponding to the five output neurons of the network) have been assumed in the available database of test images. There are three different classes of plants (Phytoplankton) - Asterionella, Ceratium, and Thalassiosira; one class of animals (Zooplankton) - Copepods; and a class corresponding to not-living matter or unidentified organisms - Floe. The activation value of a neuron is a signal that depends on the weights of the connections between that neuron and others (depending on network architecture) and on the input vector. The classification is performed by using activation values of output neurons. During training, the weights are optimized in order to classify well some training vectors. In an ideal classifier, the output neuron corresponding to the correct class should have an activation value of 1, while the other neurons should have an activation value of 0. In this sense, we can say that the

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activation value is a measure of 'how well' a given pattern is classified. Usually, to classify patterns the activation values of all the output neurons are compared and the object is classified as belonging to the class corresponding to the output neuron with highest activation value (winning neuron). In our case, however, due to the extreme variability of the target objects, the classification task is not so simple. Some particular shapes are typical of Floc matter and the network can classify them directly. However, Floc may also have a very strange shape, due to the random aggregation of dead organisms and/or inorganic matter. Such objects may not be recognized by the net as Floes. In order to classify these objects as a Floc, a control on the activation values of the output neurons has been added to the classification procedure: Let {P1,..., P5} be the network outputs. Let P.. =Max[Pi]

i = 1,2,3,4,5 , (i* is the winning neuron) and Pj* =Max[Pi]

i=i*;

IfPi* >T1and P.. - P., > T2 the object is recognized by the system as belonging to the class i*, otherwise it is classified as Floc. This means that any time the winning neuron is still characterized by a low activation value or if two neurons posses similar high activation the system classifies the object as Floc. After several tests on classification performances the best values of the threshold have been determined as: T1 = 0. 85 and T2 = 0.15. Results on these thresholds are presented in Table 8.2 on a test set of 100 hand-binarized images. At present classification times are between about one second and two minutes, depending on the size and complexity of the presented object.

Table 8.2 Test results for T1 = 0.85, T2 = 0.15 Class Asterionella Ceratium Thalassiosira Floc Zooplankton

Well classified (%) 95.2 94.4 85.7 100 100

Conclusions This Chapter has covered some of the issues encountered in the implementation of an automated data extraction system for in situ holograms of marine particles. A scanning machine has been designed and built that can conveniently replay both in-line and off-axis holograms, producing high-quality digitized images of the reconstructed particle field. A set of image processing and enhancement routines has been written that can locate the plane of best focus of an object in a series of images and generate a suitable binarized representation for input to a neural net trained to classify objects by shape - here differentiating between several forms of phyto- and zoo-plankton. The combination of the software and replay machine will make it possible to generate the identity and location of every organism within the recorded sample volume without operator intervention.

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So far the image capture, object tracking, and organism classification have been tested separately. However, HoloCam's in-line holograms will cover a volume 100 mm diameter and about 500 mm deep; sampling these at x 10 magnification at 0.1 mm intervals each producing a 500 Kb greyscale image yields an estimate of over 20 Terabytes of potential raw data from each holographic plate. It is this sheer volume of information in each hologram that makes analysis so demanding: it is difficult to store all this data on a single computer, let alone transfer it to another for remote processing. Obviously, most of this data represents empty space and work is currently underway to integrate the scanning and particle extraction stages so that only the relatively small number of binarized images require storage and later analysis. Further options, such as a low resolution prescan to identify regions of interest for detailed study are also under consideration.

Acknowledgements This work was supported by the EC MAST-III initiative (MAS3-CT97-0079).

References (1)

(2)

(3) (4) (5) (6)

(7)

(8)

(9)

Watson, J., Alexander, S., Anderson, S., Craig, G., Hendry, D. C., Hobson, P. R., Lampitt, R. S., Lucas-Leclin, B., Nareid, H., Nebrensky, J. J., Player, M. A., Saw, K., and Tipping, K. 'Development, Construction and Test of a Subsea Holographic Camera ('HoloCam') for Recording Marine Organisms' Proceedings of Oceanology International 2000, 7th-10th March 2000, Brighton, UK, pp.183-192 (2000). Hobson, P. R. and Watson, J. 'Accurate three-dimensional metrology of underwater objects using replayed real images from in-line and off-axis holograms' Measurement Science and Technology 10 pp.1153-1161 (1999). Meng, H. and Hussain, F. 'In-line Recording and Off-axis Viewing Technique for Holographic Particle Velocimetry' Applied Optics 34 (11) pp. 1827-1840 (1995). Green, S. I. and Zhao, Z. 'Reconstructed Double-pulsed Holograms: A System for Efficient Automated Analysis' Applied Optics 33 (5) pp.761-767 (1994). Bexon, R., Gibbs, J., and Bishop, G. D. 'Automatic Assessment of Aerosol Holograms' Journal of Aerosol Science 7 pp. 397-407 (1976). Katz, J., Donaghay, P. L., Zhang, J., King, S., and Russell, K. 'Submersible Holocamera for Detection of Particle Characteristics and Motions in the Ocean' Deep-Sea Research I 46 pp.1455-1481 (1999). Borrmann, S. and Jaenicke, R. 'Application of Microholography for Ground-Based InSitu Measurements in Stratus Cloud Layers: A Case Study' Journal of Atmospheric and Oceanic Technology 10 pp.277-293 (1993). Kitagaki, T. 'One Meter Holographic Bubble Chamber for TEVATRON Neutrino Experiments' Photonics Applied to Nuclear Physics (Nucleophot) 2 pp.99-117. CERN Report 85-10 (1985). Deason, V. A. 'Some Applications of Holography at the Idaho National Engineering Laboratory' Industrial and Commercial Applications of Holography - Proceedings of the SPIE 353 pp.131-137 (1982).

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(10) Naon, R., Bjelkhagen, H., Burnstein, R., and Voyvodic, L. 'A System for Viewing Holograms' Nuclear Instruments and Methods in Physics Research A283 pp.24-36 (1989). (11) Nebrensky, J. J., Craig, G., Hobson, P. R., Lampitt, R. S., Nareid, H., Pescetto, A., Trucco, A., and Watson, J. 'A Data Extraction System for Underwater Particle Holography' in Optical Diagnostics for Industrial Applications, N. A. Halliwell, editor, Proceedings of SPIE vol. 4076 pp. 120-129 (2000). (12) Barnhart, D. H., Adrian, R. J., and Papen, G. C. 'Phase-Conjugate Holographic System for High-Resolution Particle-Image Velocimetry' Applied Optics 33 (30) pp.7159-7170(1994). (13) Brown, P. R. A. 'Use of Holography for Airborne Cloud Physics Measurements' Journal of Atmospheric and Oceanic Technology 6 pp.293-306 (1989). (14) Vossing, H-J., Borrmann, S., and Jaenicke, R. 'In-Line Holography of Cloud Volumes Applied to the Measurement of Raindrops and Snowflakes' Atmospheric Research 49 pp.199-212 (1998) (15) J. Wood 'Invariant Pattern Recognition: a Review' Pattern Recognition 29 pp. 1-17 (1996) (16) Maragos, P. 'Pattern Spectrum and Multiscale Shape Representation' IEEE Transactions on Pattern Analysis and Machine Intelligence 11 pp.701—716 (1989). (17) Foresti, G., Regazzoni, C. S. and Venetsanopoulos, A. N.: 'Statistical Pattern Spectrum for Binary Pattern Recognition,' in Computational Imaging and Vision Mathematical Morphology and its Applications to Image Processing, J. Serra and P. Soille (Eds.), Kluwer Academic Publishers, pp. 185-192 (1994). (18) Z. Xiaoqi and Y. Baozong 'Shape Description and Recognition using the High Order Morphological Pattern Spectrum' Pattern Recognition 28 (9) pp.1333-1340 (1995).

J J Nebrensky and P R Hobson Department of Electronic and Computer Engineering, Brunei University, Uxbridge, UK J Watson and G Craig Department of Engineering, University of Aberdeen, UK G L Foresti, S Gentili, and G G Pieroni Department of Mathematics and Computer Science (DIMI), University of Udine, Italy H Nareid Formerly of Department of Engineering, University of Aberdeen, UK. Now with Axeon Limited, Aberdeen, UK

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9 The Application of LIF to Study the Dispersion of a Surface Film due to Wave Breaking using a Two-camera System T Schlicke, A D Arnott, J M Buick, C A Greated, and N H Thomas

Abstract In this Chapter we report on the application of laser induced fluorescence (LIF) to study the dispersion of a surface layer due to wave breaking. The technical aspects of the experiment are described; in particular we consider the production of a 'wide screen' image using an extended light sheet of length up to 2 m, and a double camera system which was used to obtain the images. The technique employed to produce surface films with a thickness of the order of a few microns is also described. Details are given of the normalization of the LIF images due to intensity variations in the light sheet and the calibration method which was applied.

9.1

Introduction

Surface films in the ocean can be formed in a number of ways ranging from the large-scale pollution which can be produced by, for example, an oil-spill; to small-scale, naturally occurring slicks (1, 2) due to biological processes. It is also possible that surface layers can be formed by marine rains containing high levels of dissolved free amino acids (1, 3), although it is unclear whether this constitutes a source or a sink for the surface film. Naturally occurring slicks are a complex mixture of the organic materials found in the ocean, such as fatty acids, esters, carbohydrates, and hydrocarbons which are mostly the exudes and decay products of various marine life (4). Due to their complex composition, which may be constantly changing due to chemical processes occurring in the film, they are not fully understood in detail.

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Despite this, it has long been established that surface layers, even if only a few molecules thick, can have a significant effect on damping water waves (5, 6); indeed in laboratory experiments it can be necessary to follow a lengthy cleaning procedure to ensure that no film is present (7, 8). Short gravity waves are damped due to the Marangoni Effect (9, 10). When a viscoelastic film covers the water surface, two types of waves can be supported: surface waves and Marangoni waves. The Marangoni waves are produced by the compression and dilation of the surface film due wave motion. The viscous tangential stress at the water surface does not vanish but is balanced by the tangential stress exerted on the surface by surface tension gradients in the film. The tangential force associated with the surface tension gradients provides a restoring force for the Marangoni waves which are strongly damped due to the large velocity gradients in the boundary layer. When the frequency of the surface waves and the Marangoni waves coincide, a resonance occurs which causes a strong dissipation of energy. Significant wave damping has also been observed at longer wavelengths. This has been explained (10) by energy transfer from the longer wavelengths to shorter wavelengths were the energy dissipation occurs by the Marangoni Effect. The damping produced by the surface films has important consequences for synthetic aperture radar (SAR) (11, 12). In this Chapter we are concerned with the development of an LIF system to study the dispersion of such slicks due to wave breaking which has environmental implications concerned with the dispersion after an oil spill or leak and also in determining when a surface can be considered clean with respect to SAR imaging (11, 12). This has recently become increasingly important due to growing awareness of the issues of global climate change and the damage to the environment due to pollution, and hence the increased need to monitor the ocean. An earlier investigation of this problem was performed by Rapp and Melville (13) using a non-fluorescent dye. The application of LIF gives two main advantages: first it is possible to detect lower dye concentrations; and second we obtain quantitative concentration measurements at each point giving a more detailed description of the mixing process.

9.2

Experimental description

9.2.1 Apparatus A plan view of the LIF apparatus is shown in Fig. 9.1 and the details of the illumination system are shown in Fig. 9.2. The wave experiments were performed in a wave flume 9.7 m long and 0.4 m wide with a still water depth of 0.75 m. The breaking wave is generated using the superposition of a number of sinusoidal waves which are generated by the computer controlled wave paddle with frequencies in the range 0.5-1.5 Hz. These waves are focused at a specified distance downstream of the paddle to produce a breaking event which is just inside the measurement region. The extent of the measurement region is determined by two factors: the area of the fluid which can be illuminated by the laser light and the area which is in the field of view of the cameras. It was found that after the wave breaks the surface film is dispersed further horizontally (corresponding to the direction of propagation of the wave) than it is in the vertical direction. For this reason it is necessary to use a measuring region which is greater in the horizontal direction than the vertical. Figure 9.2 shows the illumination system. A laser beam is directed on to an octagonal mirror; each face of the mirror reflects the beam, as shown in Fig. 9.2, producing a pseudo light 'sheet' through the centre of the tank. At the free surface the length of the light 'sheet' is approximately 2 m. A 'wide screen' camera system was set up using two Pulnix TM9701 cameras with 28 mm Nikkon lenses positioned

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3 m from the tank and 1 m apart such that their fields of view overlap slightly as shown in Fig. 9.1. Both cameras were triggered at 20 frames per second and were initialized by the computer controlling the wave paddle. The images were captured using a Coreco Viper Quad frame grabber and stored in RAM. 2000 images were stored for each experiment before each pair of images from the two cameras were combined to give the final 'wide screen' image. The use of the two camera system enables a measurement region of the required shape to be used. If a single camera had been used to image the same area, there would have been a reduction in the resolution and there would have been large areas of the image which contained no information.

Fig. 9.1 Plan view of the experimental set-up

Fig. 9.2 Details of the illumination system

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9.2.2 LIF LIF is a full field, non-intrusive, optical technique for obtaining concentration measurements of a material within a fluid. A photon of frequency v, colliding with an atom will either be absorbed or scattered. The probability of absorption is greatest when the energy of this photon, hv (where h is Planck's constant), matches one of the atom's excitation energies. If the photon is absorbed, its energy is transferred to one of the atom's electrons. The atom is now excited and therefore unstable. In liquids the energy is dissipated by intermolecular collisions, resulting in the emission of a band of photon frequencies. If these photons have frequencies within the visible spectrum, the material will appear to glow.

In the experiments described here we use Rhodamine B, a highly fluorescent dye (14): concentrations of the order of parts per billion can be seen with the human eye. Rhodamine B is soluble in water, producing a pink solution, but when illuminated it glows a bright orange colour. A filter is placed in front of each camera lens to remove the Argon-ion laser light (512.5 nm), leaving only the fluorescence (~ 610 nm)which is captured by the camera system. 9.2.3 Application of the surface film In order to investigate the factors affecting its dispersion, the film should be as uniform as possible. Ideally, the film would lie entirely on the surface of the water and have constant thickness. In addition the application of the film should be repeatable so the initial concentration distribution is not a variable. The method of application that was adopted involved the float shown in Fig. 9.3.

Fig. 9.3 Film applicator

The float consists of an T shaped section of divinycell, a lightweight, high-strength foam material, with a central strip removed. This gap contains Scotch-Brite, a brand of cleaning wool. The Scotch-Brite is soaked in water until saturated, and then the Rhodamine solution is added. A flow of water on to the Scotch-Brite flushes out the Rhodamine solution and spreads it over the surface, see Fig. 9.4. Care must be taken when removing the floats to avoid drips which disturb the film. In the experiments undertaken, the film volume was 5 ml and the Rhodamine B concentration was 1 g per litre of methanol. This was found to be the smallest quantity that did not produce a patchy film. The 5 ml was divided equally between two floats, positioned approximately 1 m apart. This resulted in a film approximately 3 m long with a

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mean thickness of the order of several microns. The surface of the water was skimmed prior to the application of the Rhodamine to remove dust and other particles that would interfere with the spreading of the film.

Fig. 9.4 Application of surface film

9.3

Preparation of LIF images

On completion of the experiment, the image data in the system memory is written to the hard disk. Before information from the images can be extracted, each image must be processed in two ways: to correct for light sheet variations and to obtain the Rhodamine B concentration. Due to the form of the scanning beam system the light 'sheet' is not evenly illuminated, the light intensity is reduced as a function of r, the distance from the rotating mirror. This was observed by taking a normalization image at the end of the experiment. The Rhodamine B which is present in the tank is thoroughly mixed until its concentration is approximately uniform before the normalization picture is taken. Each pixel of the images taken during the experiment is then normalized by its value in the normalization image. This procedure also acts to negate any differences in the sensitivity of the two cameras or of individual elements within the CCD array. Finally, the normalized pixel values are related to the actual concentration of Rhodamine B. To do this an image of a calibration vessel is taken. This vessel is made of perspex, sealed with silicon and contains 12 compartments containing different, but known, concentrations of the rhodamine/methanol stock solution in water; see Fig. 9.5. The image of the vessel is first normalized, as described above. The mean pixel value of each compartment of the calibration vessel is recorded and this is shown in Fig. 9.6. The graph is nearly a straight line through the origin, suggesting that in this range of concentrations, the pixel brightness is directly proportional to the Rhodamine concentration. This calibration technique was applied to the images from both of the cameras. It was noted that Rhodamine solutions of significantly higher concentration fluoresced less brightly, due to

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attenuation of the laser beam. In this way Rhodamine concentrations as low as 0.0005 per cent can be detected.

Fig. 9.5 Calibration vessel

Fig. 9.6 Graph of normalized pixel level plotted against known concentration

9.3.1 Combining the images Once every image captured from both cameras has been corrected for light sheet variation and calibrated, the images can be combined. The cameras were carefully arranged such that there

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was a small horizontal overlap between their fields of view. By inspecting the corresponding image from both cameras, this misalignment can be ascertained and used by a program to combine the images. The heights of the cameras were adjusted such that the MWL was at the same position for both. In most of these experiments, the overlap was approximately 8 pixels, resulting in a combined image size of 1528 (768 + 768 - 8) by 484. The join between the two original images is generally smooth unless waves are passing. Then, the water height appears to contain a discontinuity at the join, due to refraction at the moving water surface. This is not considered to be a problem since this study is more concerned with long-term dispersion. Figure 9.7 shows the two images obtained by the two cameras and the final 'wide screen' image. The final image is obtained by combining the original two images and performing the normalization procedure. If the combined image is inspected closely it is possible to make out the join line, however the effect is very small and is not thought to be important when considering the long term motion of the dye. Figure 9.7 also demonstrates the importance of the normalization technique. There is a clear difference between the intensity observed in Figs 9.7(a) and 9.7(b) which has been removed by the normalization procedure in Fig. 9.7(c).

Fig. 9.7 The LIF images captures by (a) the left camera, (b) the right camera, and (c) the combined 'wide screen' image after normalization

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Optical Methods and Data Processing in Heat and Fluid Flow

Conclusions

LIF has been successfully applied to study the dispersion of a surface layer due to a breaking wave. This has been done using an LIF system which has been modified to allow a 'wide screen' image of the flow to be taken using a double camera system and a wide light sheet. A special technique was also devised for evenly applying a thin film to the water surface with a thickness of a few microns.

Acknowledgements The authors gratefully acknowledge financial support from the Engineering and Physical Sciences Research Council, UK.

References (1)

Carlucci, A. F. and Wolgast, D. M. Microbial populations in surface films: Amino acid dynamics in nearshore and offshore waters off Southern California. Journal of Geophysical Research, 97:5271-5280, 1992. (2) Williams, P. M., Carlucci, A. F., Henrichs, S. M., Van Vleet, E. S., Horrigan, S. G., Reid, F. M. H., and Robertson, K. J. Chemical and microbiological studies of seasurface films in the Southern Gulf of California and off the west coast of Baja California. Marine Chemistry, 19:17-98, 1986. (3) Mopper, K. and Zika, R. G. Free amino acids in marine rains: Evidence for oxidation and potential role in nitrogen cycling. Nature, 325:246—249, 1987. (4) Scott, J. C. and Thomas, N. H. Sea surface slicks - surface chemistry and hydrodynamics in radar remote sensing. In Wind-over Wave Couplings Perspective and Prospects, editors, S. G. Sajjadi, N. H. Thomas, and J. C. R. Hunt. Clarendon Press, Oxford, 1999. (5) Scott, J. C. The historical development of theories of wave-calming using oil. History of Technology, 3:163-186, 1978. (6) Scott, J. C. Oil slicks still the waves. Nature, 340:601-602, 1989. (7) Scott, J. C. The propagation of capillary-gravity waves on a clean water surface. Journal of Fluid Mechanics, 108:127-131, 1981. (8) Fox, J. S. Transport Dynamics of Marangoni Films. PhD thesis, University of Birmingham, 1996. (9) Cini, R. and Lombardini, P. P. Experimental evidence of a maximum in the frequency domain of the ratio of ripple attenuation in monolayered water to that in pure water. Journal of Colloid and Interface Science, 65:125-131, 1981. (10) Alpers, W. and Huhnerfuss, H. The damping of ocean waves by surface films: A new look at an old problem. Journal of Geophysical Research, 94:6251-6265, 1989. (11) Gade, M., Alpers, W., Huhnerfuss, H., and Lange, P. A. Wind wave tank measurements of wave damping and radar cross sections in the presence of monomolecular surface films. Journal of Geophysical Research, 103:3167-3178, 1998. (12) Ochadlick, jr., A. R., Cho, P., and Evans-Morgis, J. Synthetic aperture radar observations of currents colocated with slicks. Journal of Geophysical Research, 97:5325-5330, 1992.

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(13) Rapp, R. J. and Melville, W. K. Laboratory measurements of deep-water breaking waves. Philosophical Transactions of the Royal Society of London, 331:735-800, 1990. (14) Drexhage, K. H. Structure and properties of laser dyes. In Topics in applied physics. Vol I, editor, F. P. Schafer, pages 145-193, Springer, 1973.

T Schlicke, A D Arnott, and C A Greated Department of Physics and Astronomy, JCMB, The University of Edinburgh, UK J M Buick, and N H Thomas CEAC, Aston University, Birmingham, UK N H Thomas CEAC, Aston University, Birmingham, and FRED Limited, Birmingham, UK © With Authors 2002

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10 Thermographic Phosphor Thermometry - Recent Developments for Applications in Gas Turbines J P Feist, A L Heyes, K L Choy, and J R Nicholls

Abstract This Chapter provides a review of the research conducted to date by the authors concerning the development of the thermographic phosphor thermometry technique for application in gas turbine combustors and high-temperature regions of the turbine. In such regions protection of components from excess heating is critical and achieved by a combination of cooling air and protective ceramic coatings. Temperature measurement is an integral part of the development process for cooling and insulation schemes but the application of conventional techniques is problematic. Thermographic phosphors consist of a ceramic host matrix with a lanthanide ion dopant. When illuminated with UV light they exhibit phosphorescence which is temperature dependent by virtue of variations in the relative intensities of distinct emission lines or in the time constant of the exponential decay which occurs once excitation has ceased. The authors have surveyed a range of known phosphors and identified YAG:Dy, YAG:Tb, and Y2C>3:Eu as being appropriate for use in gas turbines. A calibration of the temperature response of these phosphors is presented and shows that overall they have a dynamic range extending to in excess of 1200 °C and allow measurement with a precision which can be better than ±1 °C. The authors have proposed the concept of a smart thermal barrier coating (TBC) consisting of a TBC converted, by a change in composition, to a thermographic phosphor. This would allow temperature measurement on any TBC coated component for development purposes or possibly for in-service condition monitoring. The concept has been demonstrated by the manufacture, and investigation of the properties, of a smart TBC material in powdered form and by the production of smart TBC coatings using two coating techniques. Finally, to demonstrate the applicability of the technique, a small region of the wall of a single sector combustion rig has been coated with a layer of phosphor and the surface temperature measured over an area measuring 8 mm by 8 mm with the combustor operating at a scaled

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maximum load condition. The results demonstrate the utility of the thermographic phosphor thermometry technique and the efficacy of the combustor wall cooling scheme which kept the wall temperature below 600 °C throughout the investigated region. Further work will involve optimization of the smart TBC concept including refining coating composition and establishing the thermo-mechanical properties of the coatings.

Notation

t kB I I0 n1,n2 T T

AE

Time Boltzmann constant Emission intensity Emission intensity at t = 0 Electron pupulation Temperature Emission decay time constant Energy gap

10.1 Introduction The trend in gas turbines is for increasing turbine inlet temperatures due to the increases in overall efficiency that can thereby be obtained. In high temperature regions of gas turbines the free stream temperature already exceeds the limits of the Ni-based alloy from which combustor and turbine components are manufactured. To maintain the structural integrity of these components internal and external cooling is provided using air bled from the compressor while thermal protection is provided by ceramic thermal barrier coating (TBC) layers. New designs will require optimum use of the minimum amount of cooling air and will rely heavily on the insulating properties of TBC's and on their long-term reliability. Design of cooling systems, and the development of predictive design codes, requires knowledge of free stream and surface temperatures and of surface heat flux. The longevity of TBC coatings is also critically dependent on the temperatures and heat fluxes to which they are subjected and in particular to conditions in the region of the interface between the TBC and the underlying metal substrate. Hence a reliable method of measuring temperatures and heat fluxes experienced by components in high temperature regions of gas turbines under realistic operating conditions would be of great utility for design and development. Furthermore, online, in-service measurements of these properties would allow the continued performance of cooling and protective systems to be monitored, enabling timely maintenance interventions and ensuring optimum performance and safety. The application of thermometry techniques in high temperature regions of gas turbines is problematic. Infrared pyrometry is made difficult by signal corruption from flames, variations in surface emissivity, and degradation of optical system cleanliness. Thermocouples may not survive the rigours of the environment, allow temperatures to be measured only at predetermined points, and are difficult to install on rotating components. However, the use of thermographic phosphor thermometry may allow these problems to be overcome. This technique utilizes the temperature dependent luminescent properties of lanthanide doped ceramics such as YAG:Tb, YAG:Dy, or Y2O3:Eu. To measure surface temperature, a thin

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layer of such materials is applied to the surface, illuminated with UV light (typically from a laser) and the subsequent luminescence observed and interrogated. There are a number of features of these 'thermographic phosphors' which make them suitable for the measurement of surface temperatures in gas turbine combustors and high temperature regions of the turbine. The phosphor emission lines are well defined and it is hence possible to distinguish them from broadband background emission using bandpass filters. The luminescent emission is also independent of blackbody radiation from the surface and of the surface emissivity. Furthermore, the phosphors are based on a ceramic host matrix and are hence resistant to high temperatures - the melting point of YAG:Tb, for example, being around 2100 °C. A comprehensive review of thermographic phosphor thermometry and its history stretching back to the 1940s can be found in Allison and Gillies (1996) (1). Herein, we review recent developments (and applications) of the technique by the authors for surface temperature measurements in and around gas turbine combustors. The work has included: calibration of a range of well known phosphors; a review and investigation of coating techniques including a new vapour deposition technique developed in the Materials Department at Imperial College; identification and calibration of a new thermographic phosphor material; development of a new smart thermal barrier coating concept; and an application of the technique for surface temperature measurements in a model aero combustor operating under scaled take-off conditions. More detailed accounts of the various aspects of work reviewed herein can be found in the previous publications of the authors which are referred to at appropriate locations in the text and on which this document is based. The current text also contains a brief review of the physics of thermographic phosphors. This is included for the sake of completeness and since it is particularly relevant to the section where the identification of potential phosphor materials is discussed.

10.2 Phosphor physics There are two commonly used temperature response modes associated with thermographic phosphors. In the first, a pulsed excitation source is used. After each excitation pulse, the phosphors exhibit exponentially decaying emission, the time constant of which is temperature dependent. In the second mode, pulsed or continuous illumination may be used and it is the ratio of the emission intensities of two distinct spectral lines that is temperature dependent. These features may be explained by a brief review of phosphor physics. Thermographic phosphors for high temperature applications consist of a ceramic host material doped with rare-earth ions (Lanthanides) at concentrations typically between 1 per cent and 10 per cent by weight. Excitation of the phosphor by a light source such as a laser promotes electrons, associated with the lanthanide ions, to excited energy levels. Relaxation may then occur by both radiative and non-radiative means. The radiative process causes the luminescence effects, which are observed for temperature measurement, but competes with a phonon quenching process which is non-radiative. The latter become increasingly dominant at high temperatures and it is this which leads to an increase in the decay rate in luminescent emission as temperature increases and allows temperature measurement by the lifetime decay method. The exponential decay in phosphorescence which occurs when illumination of the phosphors ceases can usually be characterized by the decay time constant T of a single exponential function of the form

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where I is the measured intensity of the luminescence and I0 the intensity at time t=0. Decay times of the phosphors employed in high temperature applications are typically below 1ms and the lifetime decay method has been applied to measure temperature on rotating elements such as turbine blades with examples of previous work including Bird et al. (2), Allison et al. (3), and Tobin et al. (4). The lifetime decay method may also be used at low temperatures and an example is the work of Allison & Cates et al. (5) who made measurements below 100 °C. As electrons cascade down through the various energy levels towards their ground state, luminescence tends to be associated with relaxation across large energy gaps where the probability of non-radiate, multi-phonon relaxation is relatively low. If there are closely spaced, adjacent energy levels directly above a large gap relaxation may occur from either and the electron population will be distributed between the levels according to the Boltzmann equation.

Here n2 and n1 are the electron populations of the lower and upper level respectively. AE the energy gap between them, T the temperature in Kelvin, and kB the Boltzmann constant. From this it is apparent that the relative intensities of the emissions associated with relaxation from these two adjacent states, represented by two distinct lines in the emission spectrum, will reflect the electron populations and, hence, be temperature sensitive (6, 7). Experiments have confirmed the applicability of this equation as will be shown below. Successful use of the intensity ratio measurements was demonstrated by Goss (8), Bizzak, and Chyu (9, 10), Ervin (11) et al., and Turley et al. (12). The method has been shown to be applicable for temperature measurements on curved surfaces (11) with the possibility of measuring two-dimensional temperature distributions and of the measurement of heat flux (9, 10). It has also been used to measure the temperature of a reacting surface (8). The lifetime decay mode requires that the time constant of an exponential decay which is of the order of ms in duration be determined. This is typically achieved by recording the decay using a photomultiplier. As a result, this approach is best suited to point measurements. In contrast the intensity ratio technique requires that the emission intensities at two distinct wavelengths be compared. Emission intensity may be recorded as an image using a CCD camera and hence this mode may be utilized for surface temperature measurements. A detailed review of the physics of phosphors can be found in the publications of Hufner (13), Blasse (14), and Wybourne (15).

10.3 Coating techniques An essential component in the successful application of the thermographic phosphor thermometry technique in gas turbines is the production of a reliable phosphor coating, on the surface under investigation, capable of withstanding a hostile, high temperature, environment possibly with combustion. Several methods have been investigated for the fabrication of

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phosphor films. Chemical binders can be used to prepare phosphor paints (2) and produce good quality films for use in development testing but are limited to application at a maximum temperature of around 1100 °C. Other techniques, such as chemical vapour deposition (16) pulsed laser ablation (16) and physical vapour deposition (PVD) [e.g., RF-sputtering and electron-beam deposition (17)] are known to produce more robust films but require complicated and expensive equipment and a controlled atmosphere for their application. In order to investigate vapour deposition coating techniques the authors have collaborated with Dr K L Choy, Department of Materials, Imperial College and Professor J R Nicholls, School of Industrial and Manufacturing Science, Cranfield University. Dr Choy (18) has developed a new coating technique called electrostatic assisted chemical vapour deposition (EACVD). This novel technique has been shown to be capable of depositing a wide range of oxide films in a cost-effective manner. The process involves spraying atomized precursor droplets across an electric field where the droplets undergo combustion and chemical reaction in the vapour phase near the vicinity of the heated substrate. This produces a stable solid film with excellent adhesion on to the substrate in a single production run. The process is capable of producing thin or thick strongly adherent films with well-controlled stoichiometry, crystallinity, and texture and has already been applied to the deposition of simple, multi-component, and doped oxide films. In the current research project, this technique has been used to produce both phosphor coatings and prototype smart thermal barrier coatings both of which will be discussed below. Professor Nicholls has a comprehensive coating facility enabling production standard electron beam physical vapour deposition EB-PVD thermal barrier coatings to be produced. This facility has utilized in further proof of concept research to produce a production standard prototype smart TBC.

10.4 Experimental procedures The experiments described below include calibration of a range of phosphors thought suitable for high temperature applications and of prototype vapour deposited phosphor coatings and smart TBC coatings. In addition a survey of the surface temperature distribution inside a model gas turbine combustor has also been conducted. The lifetime decay characteristics of the various phosphor coatings were determined using the set-up shown in Fig. 10.1. A pulsed YAG:Nd laser (Spectra Physics; Model 201) was used to excite samples, which were housed in a furnace capable of reaching temperatures up to 1200 °C and specially modified to provide optical access to the samples. The laser was operated at 266 nm or 355 nm (with Q-switch), a repetition rate of approximately 16 Hz, and with output energy of about 60 mJ or 100 mJ per pulse for the two wavelengths respectively. As shown in the figure, an external beam dump was incorporated to avoid accidental irradiation of the samples by the 532 nm emission line of the laser, present due to leakage from the harmonic crystal assembly. The beam was steered into the furnace and on to the samples through a synthetic fused silica window of diameter 25 mm. Subsequent luminescence was observed through a second similar window using a standard 50 mm camera lens, which focused an image of the sample on to the entrance slit of a crossed CzerneyTurner spectrometer [Jarrell-Ash MonoSpec (8)]. The systems optical performance was limited by the diameter of the observation window and this was estimated to reduce the

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effective f-number of the collection optics from 3.8 to 12. A photomultiplier was placed at the exit slit of the spectrometer and used to measure the decay lifetime. An analogue to digital converter (PICO; ADC-200; 50 MHz; 8-bit resolution) transferred data simultaneously from the PMT and the power meter to a personal computer. An exponential decay was fitted using custom written software to either a single shot or to the average of a series of pulses. The power meter data was used to monitor irradiation of the sample during testing and for triggering purposes.

Fig. 10.1 Experimental arrangement For the intensity ratio measurements the apparatus was modified to allow the emission spectra of the phosphors to be observed. The PMT was replaced with a linear CCD array (Alton LS2000) which was linked to a grabber card housed in the computer. Spectra were obtained by integrating the response of the phosphor over approximately 0.5 s corresponding to seven laser pulses. The combustion chamber used for the survey of surface temperatures was a single sector of a research combustor designed by Rolls-Royce and of the type typically used in helicopter engines. It uses an airblast atomizer type fuel injector and the walls are cooled by the provision of a thermal barrier coating and an angled effusion cooling flow utilizing about 28 per cent of the total airflow. The combustor and the experimental flow circuit are shown schematically in Fig. 10.15. This rig enables the combustor to be operated at atmospheric pressure with a full range of scaled operating conditions. Optical access to the combustor is provided by two flat optical-quality quartz windows of 5 mm thickness located on either side of the chamber. A film of cooling air is directed over the windows by a row of holes in the dome and this enables them to survive the high temperature environment while keeping them free from wetting and carbon build-up so that unimpeded optical access can be maintained over an extended operating period. For the surface temperature survey the combustor was operated at a scaled take-off condition with an overall air-fuel-ratio of 30 and a preheat

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temperature of 200 °C. At these conditions, previous research (19) has shown that the average gas temperature in the exit plane of the combustor is 1719 K. For surface temperate measurements, three regions within the combustor were coated with samples of Y2O3:Eu, YAG:Tb, and YAG:Dy respectively using the paint technique and with the assistance of the Thermal Paint Laboratory of Rolls-Royce Derby. The painted regions were located on the inside surface of the outer wall of the combustor in-line with the axis of the fuel injector and just downstream of the primary dilution holes as shown in Fig. 10.16.

10.5 Results and discussion 10.5.1 Calibration of standard phosphors Y2O3:Eu, YAG:Tb, and YAG:Dy are well known thermographic phosphors selected by the authors as being suitable for use in gas turbine combustors. All three were studied under the lifetime decay mode with YAG:Dy also studied under the intensity ratio mode. For the results discussed herein, all the coatings were produced by the paint technique with the paint produced by mixing commercially available phosphor powder (Phosphor Technology) of particle sizes between 1 mm and 10 mm into a high-temperature binder. The resulting slurry was sprayed on to a Ni-alloy substrate and oven cured for several hours at temperatures up to 1000 °C. The thickness of the coatings were between 14 mm and 50 mm. Figure 10.2 shows the lifetime decay characteristics over a temperature range extending from 500-1200 °C for the main emission lines of each phosphor. Each measurement point corresponds to the average time constant derived from 10 individual laser pulses. From the figure it can be seen that the indicated dynamic range of Y2C>3:Eu extends from about 500-750 °C. However, previous workers (1) have reported dynamic response for this phosphor at temperatures above 800 °C. The shortfall in response was found to be due to limited frequency response in the PMT amplifier system which restricted the minimum lifetime decay time constant detectable to around 100 ms. The system has subsequently been modified and can now resolve decay time constants down to 3 ms although the measurements with Y2C>3:Eu are yet to be repeated.

Fig. 10.2 Lifetime decay response characteristics of selected phosphors

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From the figure it can be seen that YAG:Tb and YAG:Dy both have a dynamic range greater than that for Y2O3:Eu and in the case of YAG:Dy it extends to temperatures in excess of 1200 °C (which was the maximum temperature that could be obtained with the furnace used in the current investigation). The standard error was calculated for each measurement point but no error bars have been shown on thegraph. This is because, in practise, they are too small to be represented demonstrating the excellent repeatability of the results. As an example, at around 1100 °C the YAG:Dy curve reaches a maximum gradient of approximately -1.2 ms/°C. At this value, the standard error in the measured time constant implies a temperature uncertainty of +0.2 °C although it should be recognized that this represents the region of the distribution where errors are smallest. The emission lines for YAG:Dy and YAG:Tb are in the blue-green part of the spectrum and at these wavelengths blackbody radiation is weaker than at the red end i.e. at the emission wavelength of Y2O3:Eu. This gives a better signal-to-noise ratio and, hence, these phosphors are better suited for applications in combustion chambers. Of the two, YAG:Tb shows the wider dynamic range, its dynamic range extends to lower temperatures and the response curve has the steepest gradient implying greater accuracy but YAG:Dy has a dynamic range extending to higher temperatures and indeed to temperatures in excess of 1200 °C. For the intensity ratio method the experimental arrangement was changed, as described previously, and spectra were obtained by integrating the response of the phosphor (YAG:Dy) over approximately 0.5 s, corresponding to seven laser pulses. Figure 10.3 shows the spectrum of the phosphor scaled using the 493 nm emission line as a reference. From the figure, the increase with temperature of the relative intensity of the 455 nm line with respect to 493 nm line can clearly be seen. The intensities of the 493 nm and the 455 nm line were calculated by integration of the lines over a region of width approximately 1 nm. An Arrhenius plot (Fig. 10.4) presents the ratio versus the temperature and confirms an underlying Boltzmann distribution. Based on the linear region of the response function the error in temperature measurements made using the intensity ratio technique are currently estimated to be around ±5 per cent although it is expected that this value can be reduced with improvements to the instrumentation including the use of a gated CCD detector.

Fig. 10.3 Scaled spectrum of YAG:Dy illustrating the increasing emission lines around 455 nm

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Fig. 10.4 Arrhenius plot of the intensity ratio for YAG:Dy

From the results obtained with YAG:Dy it can be seen how this phosphor could be used to obtain surface temperature distributions. Two CCD cameras can be used to record images of the coated surface where each camera views the surface through a band pass filter one centred on the 455 nm line and the other centred on the 493 run line. In this way intensities from corresponding pixels in the two recorded images could be used to form the emission intensity ratio and hence obtain the temperature. This principal has been demonstrated by Goss (8) who measured a one-dimensional temperature distribution on a reacting surface. From the calibration data YAG:Dy was the phosphor chosen by the authors as being best suited to measurements in combustors and high-pressure turbines. This phosphor shows sensitivity in both the lifetime decay and intensity ratio modes and may, therefore, be used for measurement of surface temperature distributions and for high-accuracy point measurements using the lifetime decay method. YAG:Dy has the widest dynamic range of the phosphors considered (if both response modes are considered) and its dynamic range extends to the highest temperature corresponding to values which may be expected for turbine entry temperatures in future generations of gas turbines. Excitation of YAG:Dy is at 355 nm, as opposed to 266 nm for the other phosphors considered, and at this wavelength transmission of laser pulses down optical fibres is considerably easier which bodes well for temperature measurement in enclosed rigs. The emission from YAG:Dy is at a relatively short wavelength (493 nm as opposed to 543 nm and 611 nm for YAG:Tb and Y2O3:Eu respectively) and, since here interference from black body radiation diminishes, the prospects for surface temperature measurement on high temperature components are thought to be good. 10.5.2 New phosphors At the outset of the current research project, YAG:Dy was the only thermographic phosphor widely known to have an intensity ratio response. However, having established the energy level arrangement necessary to produce this type of response, the authors reviewed the energy level diagrams of a range of rare earth ions [Carnell et al. (20)] and identified, as shown in Fig. 10. 5, that a similar feature is exhibited by samarium (Sm). A calibration was conducted of a powdered sample of Y2O2S:Sm and revealed as expected the intensity ratio response. The results are shown in Figs 10.6 and 10.7 and discussed in greater detail in Feist and Heyes (6).

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The results show that ¥2O2S:Sm can be used for intensity ratio measurements up to 1100 K and for lifetime decay measurements between 900 K and at least 1400 K. For the intensity ratio measurements the uncertainty was about +1 per cent while for the lifetime decay measurements it was about +0.1 per cent.

Fig. 10.5 Schematic of the energy level diagram for free ions of Dy and Sm

Fig. 10.6 Arrhenius plot of the intensity ratio for Y2O3S:Sm

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Fig. 10.7 Lifetime decay characteristics of Y2O3S:Sm Although this phosphor has smaller dynamic range than YAG:Dy it has the advantage of a much shorter lifetime decay constant, being approximately two orders of magnitude quicker. It may therefore be better suited to applications on moving components such as turbine blades and indeed refinement of the phosphor composition or implementation in a different host may see the dynamic range increase. 10.5.3 Smart thermal barrier coatings One problem of the thermographic phosphor method, as described above, is that even though applied phosphor layers are typically very thin (between 5 and 50 mm thick), they constitute an intrusion by providing additional thermal resistance and perhaps changing the surface emissivity so that temperatures measured with them may not reflect the true value at the surface. On TBC coated surfaces this problem could be overcome by locally modifying the composition of the TBC so that it acts as a thermographic phosphor. This concept, of a 'smart' TBC with both insulating and sensory properties was first proposed by Choy, Feist, and Heyes (21). Such a coating would be useful in the development of cooling/coating systems but might also be useful for condition monitoring during coating production or inservice temperature monitoring of turbine components. A smart TBC might consist of a single doped layer but could also be of a multi-lamina construction with each layer containing a different dopant. With this arrangement temperatures at discreet points in the coating could be measured to enable heat flux to be calculated or the temperature in critical regions, such as the interface between the coating and substrate, to be determined. TBC's typically consist of a thin bond coat and an insulating top layer with the entire coating of the order of 250 mm thick. The most common material currently used for the topcoat is Yttrium-stabilized-Zirconia (YSZ; ZrO2-8wt%Y2O3) due to its phase stability and low thermal expansion coefficient at temperatures typical of gas turbine operating conditions (22, 23). To date, experiments conducted to prove the concept of a smart TBC have included: production and testing of YSZ doped with europium (YSZ:Eu) in powdered form, production of a YSZ:Eu coating using the EACVD technique, and production of a YSZ:Dy coating using the EBPVD technique.

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10.5.3.1 YSZ:Eu powder To establish if lanthanide doped YSZ could act as a thermographic phosphor, a material sample was obtained from a commercial source where it was manufactured in the manner described in Dexpert-Ghys et al. (15). The TBC systems providing the highest thermomechanical resistance are those in which the yttria content is in the range 6-9 wt.% (25). To comply with these requirements and ensure that the spectroscopic results were compatible with those of Dexpert-Ghys the sample was prepared with composition 90ZrO2-9YO1.5— lEuO1.5 (YSZ:Eu). The lifetime decay of the YSZ:Eu powder was then determined using the set-up shown in Fig. 10.1. The powder was contained in a cuvette made of fused silica placed within the furnace on a solid ceramic stand to position it in line with the window. Phosphorescence was only observed with an excitation wavelength of 266 nm and with irradiation at 355 nm no luminescence could be detected. This is due to the absorption properties of the YSZ bulk material (26) is a result similar to those previously reported with phosphors like Y2C>3:Eu where excitation at 266 nm produced phosphorescence while irradiation at 355 nm did not (1, 27). For this material, the lifetime decay studies were focused on the strongest emission lines which were at 607 and 591 nm. A single exponential fit function was found to be a good approximation to the results and Fig. 10.8 presents the decay constant t versus temperature for both emission lines. The decay time constant values which have been observed are similar in magnitude to those seen with other common thermographic phosphors and vary from 1950 ms at 70 °C to 5.5 ms at 800 °C. The uncertainty in the decay time constant varies with temperature. However, at the high temperature end of the range the uncertainty in the temperature is better than +0.1 per cent and is consistent with the best precision reported by Allison and Gillies (1) for common thermographic phosphors. These results established the feasibility of converting the material commonly used to produce TBC's to a thermographic phosphor. The next step was then to see if effective phosphor coatings could be produced.

Fig. 10.8 Lifetime decay response of YSZ:Eu

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10.5.3.2 Smart coatings using EACVD The first smart TBC coatings were produced using the new EACVD technique developed in the Materials Department at Imperial College (28). A detailed description of the technique can be found in Choy et al. (12) and herein only a brief summary of the coating conditions is presented. To produce the coatings a precursor solution of 8 wt.% partially stabilized Y2O3-ZrO2 was prepared by dissolving stoichiometric yttrium and zirconium alkoxides into alcohol solvent (e.g. butanol). The concentration of the precursor solution was 0.05 M. For the deposition of Eu-doped YSZ, europium acetate was added to the above mentioned precursor solution and the doping level was 1 mol%. YSZ and Eu-doped YSZ coatings were deposited on Ni-based superalloy (Nimonic 80) substrates. The substrates were ultrasonically cleaned in acetone prior to deposition. The substrates were coated with either a single layer of Eu-doped YSZ or a double layered coating consisting of a Eu-doped YSZ layer and a YSZ top coat. After deposition, the coated substrates were cured at 1050 °C for 2 hours. The cross-sections of the coating layers were examined using a Jeol T220A scanning electron microscope (SEM) and the crystal structure of the coating was determined using a Philips PW1710 x-ray diffraction spectrometer with Cu Ka radiation. SEM examination of the cross-section of the coated samples revealed that for the two-layer coating the average thickness of the YSZ top coat was 50 mm and that the microstructure had typical columnar-like growth features (Fig. 10.9, Layer A) which is desirable for the application of YSZ as a thermal barrier coating for gas turbine components. The Eu-doped YSZ layer was approximately 15 mm thick for both the single and double layered coating (Fig. 10.9, Layer B). The SEM revealed loose contact between the columns which is also advantageous since it provides a release mechanism for the thermal stress resulting from the difference in thermal expansion rates between the metal substrate and the ceramic coating.

Fig. 10.9 Scanning electron micrograph cross section through the YSZ:Eu coating. A: YSZ, B: YSZ:Eu, C: Ni-based superalloy substrate (Nimonic 80)

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The thermoluminescent properties of the coatings were studied using the instrumentation already described with the laser operated at 266 nm. The emission spectrum of the single YSZ:Eu coating was similar to that for the powder sample. With coating a line at 606 nm was used to investigate the lifetime decay response over a temperature range between room temperature and 1100 K and the results are presented in Fig. 10.10. Three curves are plotted, two for the single coating and one for the dual layer coating. The similarity between the three curves indicates not only the repeatability of the results but also that reliable results may be obtained from a subsurface layer. In each case the lifetime decay constant decreases slightly from room temperature to about 650 K and then decreases rapidly up to a temperature of 1100 K. The upper temperature limit is set by the limited bandwidth of the photomultiplier which restricts the minimum lifetime decay that can be measured to 3 ms. The standard error on the individual data points of each set was estimated to be less than 1 per cent which, although higher than that found for the powder sample, indicates a consistently high level of precision.

Fig. 10.10 Lifetime decay response of the single and double layered EACVD smart TBC coatings The results of this stage of the research showed that it is possible to produce a smart thermal barrier coating incorporating thermal insulation and temperature sensing properties. Further work is required to optimize the coating composition and coating parameters and to characterize the thermal and mechanical properties of the new coating materials so that they may be brought up to a production standard. In pursuing this, the next stage of the research was to produce YSZ:Dy coating using the EBPVD technique. Using this technique enabled a coating of a standard similar to commercial TBC's to be produced. 10.5.3.3 Smart coatings by EB-PVD Dysprosium doped YSZ EB-PVD thermal barrier coatings were produced at Cranfield University using a custom designed electron beam, physical vapour deposition system, that can evaporate ceramic from a 38 mm diameter rod-fed source. Each source rod is 38 mm diameter x 150 mm long, made from ZrO2-7wt%Y2O3. To incorporate the dysprosium 4 x 5 mm diameter holes were drilled into a rod longitudinally and equi-spaced around the rod centre. Each of these

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holes was filled with dysprosia (Dy2O3) compacted and then sealed with a ZrO2-7wt%Y2O3 plug. The area fraction of dysprosia was approximately 10 per cent of the total melt area. The EB-PVD TBC, with dysprosia doping, was evaporated at a chamber pressure of 1 x 10-2 torr and substrate temperature of 1000 °C in a 10%Ar-90%O2 environment. The source to substrate distance was 160 mm. At a gun power of 5 kW a deposition rate of 3 mm/min was achieved resulting in a coating approximately 180 urn thick, after 1 hour of deposition. The substrates were aluminized C263 coupons (approximately 100 mm by 25 mm and 2 mm thick), which were rotated through the vapour flux at 20 r/min. Figure 10.11 is a typical micrograph of an EB-PVD coating produced by the manufacturing route, using the Cranfield coater, but is not a micrograph of the dysprosia doped coating.

Fig. 10.11 Microstructure of an EB-PVD TBC, showing the strain tolerant columnar microstructure

A luminescence spectrum was recorded for the YSZ:Dy coating at room temperature following an excitation pulse at 355 nm and compared with similarly recorded spectrum for YAG:Dy. The spectra over the wavelength range 430-510 nm are presented in Fig. 10.12 which also includes the mercury emission lines used for wavelength calibration. Differences in the spectra are due to changes in the crystal field to which the dysprosium ions are exposed in the two different hosts but most of the main emission lines still remain. However, the emission from the 493 nm line is more intense relative to the 480 nm line in the YAG host than in YSZ and in YSZ the line splitting below 480 nm is not as pronounced as it is with the YAG crystal. The different Dysprosium dopant levels might account for this effect since the higher concentration of Dysprosium in YSZ could broaden the energy levels due to additional overlapping of energy states. This can be seen in particular for higher temperatures when the lines broaden into a continuum-like emission spectrum. Further work is required to optimize the Dy dopant level to reduce the line merging and maximize signal intensity and temperature response.

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Fig. 10.12 Spectra of dysprosium doped YSZ and YAG in comparison. Hg lines were utilized for calibration purposes and to ensure wavelength consistency Temperature dependent emission spectra of the YSZ:Dy coating are shown in Fig. 10.13 over a temperature range from 334 K to 744 K and normalized according to the 482 nm emission line. To form intensity ratios the spectra were integrated as described above and according to the intervals indicated in the figure. The relative increase in intensity for the band below 470 nm is similar to that observed previously with YAG:Dy. At higher temperatures the lines broaden and emission lines merge together making it difficult to distinguish between lines and creating a continuum-like background. One effect of this can be seen in Fig. 10.14 where the intensity ratio of two-line pairs are presented as a function of temperature and where it can be seen that the response levels out at temperatures above 1000 K. The two ratios plotted are between the 455 nm and 493 nm lines, as was done previously for YAG:Dy, and the 455 nm and 482 nm lines. Due to the strength of the 482 nm line relative to the 493 nm line the second ratio shows a better temperature response with no levelling out in the temperature range studied. The results indicate that the dynamic range suitable for temperature measurement extends from room temperature up to at least 950 K with luminescence still being observed at higher temperatures.

Fig. 10.13 Scaled spectra of YSZ:Dy at different temperatures presenting the wavelength ranges used for ratio determination

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Fig. 10.14 Intensity ratios for different emission lines with temperature

Fig. 10.15 Schematic representation of the combustor rig

Random uncertainty was investigated by comparing exposures recorded at the same temperature and a typical value for the uncertainty in the temperature was +1.6 per cent. There is scope for improvement in this uncertainty by using enhanced instrumentation such as a gated CCD detector to minimize the background noise and improve signal-to-noise ratio. 10.5.3.4 Combustor measurements In parallel with the coating development activities, experiments to apply the thermographic phosphor technique were conducted and consisted of the measurement of surface temperatures inside a single sector research combustor operating at a scaled take-off condition. Three regions of the combustor wall were coated with Y2O3:Eu, YAG:Tb, and YAG:Dy respectively using the paint technique. Initial experiments showed that the wall temperature in the coated region was outside the dynamic range of both YAG:Tb and

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YAG:Dy. Therefore, temperature measurements were made only with the Y2O3 :Eu and, since it is the only response mode of this phosphor, only with the lifetime decay technique. The area investigated, shown in Fig. 10.16, was approximately 8 mm by 8 mm and point measurements were recorded at 0.5 mm spacing.

Fig. 10.16 Combustor rig showing measurement region

The results of the survey of combustor wall temperatures are presented in Fig. 10.17 which shows a continuous temperature distribution derived from measurement points spaced at 0.5 mm intervals. The cold spots observed in the temperature distribution correspond to the exits of the angled effusion cooling holes where cooling air at the preheat temperature (200 °C) emerges to form an insulating layer on the inside surface of the combustor. There is no evidence, in the distribution, of cold streaks on the surface which might be expected due to an attached jet of cold air issuing from the cooling holes. This implies that a uniform cooling layer exists on the combustor wall due to coalescence of the jets from the many cooling holes. It is certainly apparent from this data that the cooling scheme is very effective since the wall temperature is maintained below 600 °C everywhere on the studied portion of the surface when the free stream gas temperature can be expected to be in excess of 1500 °C.

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Fig. 10.17 Combustor wall temperature distribution These results clearly demonstrate that the phosphor thermometry technique can be used to measure surface temperatures in the harsh environment of a combustion chamber and in the presence of illumination from flames.

10.6 Concluding remarks The thermo-luminescent properties of selected phosphors (Y2O3:Eu, YAG:Tb, YAG:Dy) have been studied using laser irradiation over a temperature range from room temperature to 1200 °C. Both the lifetime decay and intensity ratio response modes have been considered and all three phosphors have been shown to be suitable for high temperature measurements. However, YAG:Tb and particularly YAG:Dy are preferred for applications in gas turbine combustors since the emission lines of these phosphors are further from the infrared end of the spectrum than those for Y2O3:Eu and since they have dynamic ranges extending to higher temperatures. YAG:Dy is particularly favoured since it is the only phosphor known to exhibit both response modes and since its dynamic range extends to more than 1200 °C. The concept of a smart TBC has been proposed consisting of YSZ doped with lanthanide ions so as to convert the TBC material to a thermographic phosphor. The concept has been demonstrated first by the manufacture of YSZ:Eu in powdered form. This material was successfully shown to behave as a thermographic phosphor with a dynamic range extending to 1400 K and with measurement precision comparable with that of the standard phosphors already tested. In the next stage a YSZ:Eu coating was laid down using the EACVD

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technique. The coating showed properties consistent with a high quality TBC and again behaved as a thermographic phosphor with a dynamic range extending to at least 1100 K and measurement precision of +1 per cent. These experiments also demonstrated that subsurface temperature measurements can be made using a doped sub-layer. This important result demonstrated the potential for the measurement of heat flux and of critical temperatures such as at the TBC/bond coat interface. In the final proof of concept experiment a smart TBC was produced using the EB-PVD technique. This technique is suitable for the manufacture of production quality TBC's. The coating produced was doped with dysprosium and was shown to behave as a thermographic phosphor responding according to the intensity ratio mode a with a dynamic range extending to 950 K and a measurement precision of ±1.6 per cent. These experiments have conclusively demonstrated the feasibility and potential of smart TBC's with properties similar to those of thermographic phosphors. Further work is now required in this area to optimize coating compositions and coating parameters to fully exploit the measurement capabilities and establish the thermo-mechanical of smart TBC's. A preliminary investigation of the surface temperature distribution inside a model gas turbine combustor has been carried out. The surface temperature was found to be outside the dynamic range of both YAG:Tb and YAG:Dy and hence the only phosphor used in the investigation was Y2C"3:Eu and the only response mode the lifetime decay mode. The results indicate the existence of a continuous cooling air layer on the inside surface of the combustor keeping the surface temperature below 600 °C throughput the investigated region.

References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

Allison, S. A. and Gillies, G. T. (1997) Rev.Sci.Instrum.S6 (t), July p. 2615. Bird, C., Mutton, J. E., Shepherd, R., Smith, M. D. W., and Watson, H. M. L. Internal Report, Rolls-Royce plc, Derby. Allison, S. W., Beshears, D. L., Cates, M. R., Noel, B. W., and Turley, W. D. (1997) Mechanical Engineering, January. Tobin, K. W. Jr., Beshears, D. L., Turley, W. D., Lewis, W. III, and Noel, B.W. (1991) Fibre Optic and Laser Sensors, Vol. 1585, IX p. 23. Allison, S. W., Cates, M. R., Noel, B. W., and Gillies, G. T. (1988) IEEE Trans. Instrum. Measurem., Vol. 37, No. 4, p. 637. Feist, J. P. and Heyes, A. L. (2000) Meas. Sci. Technol. 11 1-6. Kusama, H., Sovers, O. J., and Yoshioka, T. (1976) Japanese Journal of Applied Physics, Vol. 15, No. 12, pp. 2349-3258, December. Goss, L. P., Smith, A. A., and Post, M. E. (1989) Rev.Sci.Instrum. 60 (12), pp. 37023706. Bizzak, D. J. and Chyuk, M. K. (1995) Int.J. Heat Mass Transfer, 38, No.2, 267-274. Chyu, M. K. and Bizzak, D. J. (1994) Transactions of the ASME, Vol. 116, 264, Febr. Ervin, J., Murawski, C., Macarthur, C., Chyu, M., and Bizzak, D. (1995) Experimental Thermal and Fluid Science, 11, 387-394. Turley, W. D., Borella, H. M., Noel, B. W., Beasley, A., Sartory, W. K., and Cates, M. R. (1989) Los Alamos Report, LA-11408-MS, UC-000, January. Hufner, S. (1978) Optical Spectra of Transparent Rare Earth Compounds, Academic Press.

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(14) Blasse, G. (1979) Chemistry and Physics of R-activated Phosphors, Handbook on the Physics and Chemistry of Rare-Earths, Eds by Gschneidner, K. A., Jr., Eyring, L., North Holland Publishing Company. (15) Wybourne, B. G. (1965) Spectroscopic Properties of Rare Earths' , J.Wiley Sons, Inc. (16) Hirata, G. A., McKittrick, J., AvalosBorja, M., Siqueiros, J. M., and Devlin, D. (1997) Appl. Phy. Sci., 114, 509. (17) Alaruri, S., McFarland, D., Brewington, A., Thomas, M., and Sallee, N. Optics and Lasers in Engineering, 22, 17 (1995). (18) Choy, K. L. and Bai, W. British Patent. 9525505.5 (1995). (19) Heyes, A. L., Jelercic D., and Whitelaw J. W. (1999) 14th Int. Symp. on Airbreathing Engines, Florence, Sept. (20) Carnall, W. T., Fields, P. R., and Rajnak, K. (1968) The Journal of Chemical Physics, 49 (10) 4424-4442. (21) Choy, K. L., Feist J. P., and Heyes, A. L., British Patent Application No.: 9823749.8 (22) Jones, R. I. (1996) Metallurgical and Ceramic Protective Coatings (Ed. K.H. Stern), pp. 194-235 (Chapman and Hall, London). (23) Pettit, F. S. and Goward, G. W. Coatings for High Temperature Applications, (Ed Lang, E.), Applied Science Publishers Ltd., (1983) pp. 341-355 (24) Dexbert-Ghys, J., Faucher, M., and Caro, P. (1984). J. Solid State Chem., 54, 179192. (25) Lelait, L., Alperine, S., Diot, C., and Mevrel, M. Materials Science and Engineering, A121 (1989) pp. 475-482. (26) Preusser, S., Stimmig, U., and Wippermann, K. Electrochimica Acta, Vol. 39, No.8/9, (1994) pp. 1273-1280. (27) Choy, K. L., Feist, J. P., Heyes, A. L., and Su, B. (1999) J. Mater. Res., Vol. 14, No. 7, Jul. (28) Choy, K. L., Feist, J. P., Heyes, A. L., and Mei, J. (2000) Surface Engineering, Vol. 6, No. 6, pp. 469-472.

Bibliography Choy, K. L. (1998) Materials World, 6, 144. Gupta, A. K. (1997) Energy Convers. Mgmt. Vol. 38, No. 10-13, pp. 1311-1318. J P Feist and A L Heyes Department of Mechanical Engineering, Imperial College of Science Technology and Medicine, London, UK K L Choy Department of Materials, Imperial College of Science, Technology and Medicine, London, UK J R Nicholls School of Industrial and Manufacturing Science, Cranfield University, UK © With Authors 2002

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11 Improved Liquid Crystal Thermography by using True-colour Image Processing Technology M Wierzbowski, M Ciofalo, and J Stasiek

Abstract Liquid crystal thermography combined with image processing techniques used for experimental work at the Department of Heat Technology are described. The experimental procedure covers full-field flow patterns in classic heat exchanger elements (flat plate with fine-tubes in-line, staggered, and with vortex generators) describing local heattransfer coefficient and Nusselt numbers on the surfaces. Also the dependence of average heat-transfer and pressure drop on Reynolds number and geometrical parameters was investigated.

11.1 Introduction The main features of performance required of a heater are high heat-transfer rates, lowpressure losses and low sensitivity to fouling. These depend crucially on the geometrical design of the heat-transfer elements. However, despite the importance of the problem, the various classic and compact designs have been developed, mainly on an empirical basis. Although both operating and laboratory data are available, they do not cover the full range of shapes, sizes, and operating conditions (in particular Reynolds number) that would be required for an optimization study. Moreover, data are generally available in the form of overall performance (average heat-transfer coefficient) and the phenomena that determine these, including flow patterns and transition to turbulence, are not fully understood. Authors at Technical University of Gdansk have carried out a comprehensive experimental and predictive research program. The main purposes of the study are:

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• to determine the flow patterns in the classic (in-line, staggered) heat exchanger and the distribution of the local heat-transfer coefficient on the heat exchange surface; • to investigate the dependence of average heat-transfer on Reynolds number and geometrical parameters.

11.2 Experimental arrangement and measurement The experimental study was carried out using an open low-speed wind tunnel consisting of entrance section with fan and heater, large settling chambers, and then mapping and working sections. Air is drawn through the tunnel using a fan able to give Reynolds numbers between 500 and 10 000 and the heaters can provide an air temperature (Tf) between 25 and 80 °C. The major construction material of the wind tunnel is perspex. Local and mean velocity are measured using conventional Pitot tubes and DISA hot-wire velocity probes. The alternative effects of constant wall temperature and constant heat flux boundary conditions are obtained using a water bath (Fig. 11.1).

Fig. 11.1 Experimental stand

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Fig. 11.1 Experimental stand (cont)

Photographs of liquid crystal patterns are taken using a RGB video camera and a true-colour image processing technique. Usually several isotherms (each corresponding to a different heat flux) are taken by RGB video camera to record the local Nusselt contours under an oblique Reynolds number. The locations of each isotherm and colour (adjusted to each Nusselt number) are digitized following a projection of the false colour image on a digitizing image respectively (this particular method can be called 'image combination technique' - ICT).

11.3 Calibration of liquid crystals Before the execution of the visualization experiment characteristics of combination of the liquid crystals, light source, together with optic and camera system, should be recognized. Both the colour-play interval and the event temperature for a liquid crystal can be selected by adjusting its composition; materials are available with event temperatures from -30 to 115 °C and with colour play band from 0.5-20 °C. However, combinations of event temperature and colour-play bandwidths of 1 °C or less will be called 'narrow band' materials. Those whose bandwidth excess 5 °C will be called 'wide band'. The types of material specified for a given task depends on the type of image interpretation technique to be used. The absolute uncertainty in surface temperature measurement appears to be about 0.5 °C using liquid crystals, for both visual and image processed interpretations, hence the relative uncertainty of the measurement is estimated to be about ±0.1 to ±0.2 °C because the specimen was calibrated in situ with the lighting system arranged just as it was during the application.

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The liquid crystal used in this experiment is Thermochromic Liquid Crystal purchased from Hallcreast Limited. It has an event temperature range of 35-40 °C. The isotherms of the red colour band (tR = 35 °C), and yellow-green band (to = 35.5 °C), are recorded by photographing the liquid crystal layer with colour film and RGB image processing system (Fig. 11.2).

Fig. 11.2 Scheme of digital image processing system

11.4 Results Local heat-transfer measurements have been carried out for Reynolds number based on the centerline inlet velocity along the main flow direction. Local heat-transfer coefficient and Nusselt number maps, derived from local wall-temperature distributions as indicated by LC's for different Reynolds number, are reported in Fig. 11.3(a). The liquid crystal colour temperature 35.5 °C was below the air temperature 45 °C for these experiments. False colour isotherm representation of local Nusselt number was made automatically by GLAB software of Data Translation Inc. [Fig. 11.3(a)] contrary to images on Figs 11.3(b) and 11.3(c). The experiment has been prepared for compact tube heat exchangers elements, both in-line and staggered. Air flows around the tubes and heat-transfer between the fluid and the tubes is determined by the flow structure. Results obtained during the experiment are recorded using the digital image processing system described earlier.

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Fig. 11.3 Scheme of steps leading to vectorization of obtained pictures: (a) RGB image; (b) inverse of gray-scaled image prepared for vectorization process; (c) vectorized version of experimental image The geometry shown on Fig. 11.3(a) can be modified by adding different shape obstacles called vortex generators. They can be installed in a different position. Presence of these vortex generators causes changes of local heat-transfer distribution which can be clearly viewed in (Fig. 11.4).

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Fig. 11.4 Photographs taken by digital image processing system during the experiment for a different positioning of vortex generator behind the tube

11.5 Conclusions The use of liquid crystals combined with image and data processing has been successfully applied to different geometry researches to provide quantitative heat-transfer data suitable for performance purposes. Figures 11.5 and 11.6 of friction factor versus Reynolds number show the comparison of the different layout of the test section (in-line or staggered). It is easy to find out that in both applications insert of wings angled 60 degrees causes less pressure drop than the other positioning.

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Fig. 11.5 Friction factor relation for in-line arrangement of working section

Fig. 11.6 Friction factor relation for staggered arrangement of working section

It is clear to find hydraulic jump at the transitional region of Reynolds number 1000-2500 both for 30 and 45 degrees positioned winglets.

Acknowledgement Much of the work reported here was sponsored by State Committee of Research Science grant No. KBN - 8T10B01814.

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References (1) (2)

(3)

Moffat, R. J. Experimental Heat Transfer. 9th International Heat Transfer Conference, Jerusalem, Israel, 1991. Jones, T. V., Wang, Z., and Ireland, P. T. The use of liquid crystals in aerodynamic and heat transfer experiment. Optical Methods and Data Processing in Heat and Fluid Flow, London, UK, 1992. Wierzbowski, M., Mikielewicz, D., Ciofalo, M., and Stasiek, J. Heat transfer modeling using thermo chromic liquid crystals and true-colour image processing. 17th Thermodynamics Conference, Cracow, Poland, 1999.

M Wierzbowski and J Stasiek Department of Heat Technology, Technical University of Gdansk, Poland M Ciofalo Department of Nuclear Engineering, University of Palermo, Italy

12 Development of an Optical Measuring Technique for the Study of Acoustical Phenomena J M Buick, J A Cosgrove, D M Campbell, and C A Greated

Abstract We consider the application of optical methods to the measurement of acoustical phenomena. In particular we consider the suitability of applying measuring techniques that rely on the flow being seeded with small particles. This is a common practice when acoustic fields are not present, however, interactions between the seeding particles and the sound waves can change both the motion of the particles and any acoustically generated flow. The novel measuring technique of fluorescent dye velocimetry (FDV) is proposed which does not require the flow to be seeded. The accuracy and applicability of the new technique is then assessed using simulated images.

12.1 Introduction Optical measuring techniques are now widely applied to flow measurement in a range of applications in fluid mechanics. One significant advantage that optical measuring techniques have compared to other methods is that they are considered to be non-intrusive; that is, there is no probe or measuring device inserted in the fluid and so the procedure of making a measurement does not alter the system in any way. However, many techniques, such as particle image velocimetry (PIV) and laser Doppler anemometry (LDA), rely on small seeding particles being present in the fluid. Provided these particles follow the flow in a reliable manner they are not considered to influence the fluid motion. This is generally true, however, when an acoustical field is present a number of problems can arise. Firstly, the action of the sound waves can move the particles at a different speed from the surrounding fluid thus preventing an accurate velocity measurement from being taken. Secondly, the particles can

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alter the acoustic field. This means that any flow that is derived from the acoustic field will be changed by the addition of the seeding particles. The first of these problems has already been addressed (1) by considering the radiation stress acting on the seeding particles. A suitable seeding particle is then selected so that its deviation from the fluid motion is negligible. This will be described briefly in Section 12.2. In the remainder of the Chapter we propose and evaluate a new optical technique: fluorescent dye velocimetry. This uses a fluorescent dye which diffuses at a molecular level and so there are no seeding particles to interact with the sound field.

12.2 Radiation stress Consider an acoustical wave propagating through a fluid which contains a number of suspended particles. If there is a mismatch between the acoustic impedance of the fluid and a suspended particle, then the wave will experience a strong reflection at the particle. This causes a large spatial change in the energy density and a large radiation pressure. The effect of this radiation force on a seeding particle has already been investigated (1). If the density of the seeding particle, p1, is similar to the density of the fluid, po, such that A—> 1, where A = p0 /p1 and X0 = kR « 1, where R is the radius of the particle and k is the wavenumber of a plane progressive wave, then the radiation stress is given by (2, 3)

where A is given by

a) is the angular frequency, and / is the probe intensity. Now, the motion of a spherical seeding particle moving under the action of the radiation stress force Frs and Stoke's drag force -6nmRv, where m is the fluid viscosity and v is the velocity of the particle relative to the fluid, is described by

where m is the particles mass. Following (1) the particle terminal velocity can be found by integrating and letting t —> 8, which gives

The magnitude of the terminal velocity depends on the radius of the particle as v8 8 R5. Thus, the value of v8 is reduced rapidly with decreasing R and so the seeding particles will follow the flow provided R is small enough that v8 is negligible with respect to the fluid velocity.

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12.3 Fluorescent dye velocimetry In this section we consider a new optical technique FDV for velocity measurement in acoustic flows. 12.3.1 Motivation If the seeding particles are selected according to the criteria outlined in Section 12.2, it is clear that the particles will follow the flow to a desired degree of accuracy. The acoustic field, however, will still experience a small reflection at each particle; the cumulative effect of which could still be large. When measuring flows such as acoustic streaming which is produced solely by the attenuation of the acoustic field, any change in the acoustic field due to the seeding particles will inevitably change the fluid motion. It is therefore important to ensure that the measured (seeded) flow is not significantly different from the unseeded flow. Here we consider a new measuring technique in which a dye is added to the fluid rather than seeding particles. The dye diffuses on a molecular level and so will not alter the acoustic field. 12.3.2 Image capture Figure 12.1 shows a typical experimental set up for obtaining FDV images. Fluorescent dye, for example Rhodamine B, is introduced in to the flow upstream of the measurement region. This must be done in such a way that any velocity with which the dye is injected in to the flow has dissipated by the time the dye reached the measurement region. It is also important that the dye has mixed sufficiently so that it occupies a significant fraction of the measurement region, but not to the extent that it is evenly mixed.

Fig. 12.1 Experimental setup for obtaining FDV images

The measurement region is illuminated with a laser sheet at the frequency at which the dye fluoresces. The fluorescence is then captured by a CCD camera through an optical filter which removes the laser light and transmits the fluorescent light. A pair of images taken at a short

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time separation are required for the analysis. Methods for inserting the dye, producing the light sheet, and obtaining the images will vary depending on the application. Illumination and image capture techniques widely applied in PIV (4, 5) can also be applied to FDV. 12.3.3 Simulated images To test the effectiveness of the new technique a number of simulated images were created. To do this a pair of images must be generated in which the dye configuration in the second image has been shifted slightly with respect to its position in the first; corresponding to two images of a real dye system taken with a short time separation. This was done using a random walk routine on a 6400 by 6400 grid. Starting at the centre of the grid a series of 5000 points were generated using a random walk with a Gaussian distribution with mean 0.0 and standard deviation of 128 points. If the random point lies outside the grid, the sequence is continued and future points which lie on the grid are considered. The sequence of points was then jointed by a series of straight lines. Two sets of random walks are shown in Fig. 12.2. An intensity level, L, at each point on the grid was then calculated by fitting a Gaussian about each line according to the formula L = 3.0exp(-d2 /30 2 ), where d is the shortest distance between the grid point and the lines. The final intensity for each grid point was the sum of the contributions to L from each of the random walk lines. The grid was then divided in to a 640 by 640 array of pixels, each pixel containing 10 by 10 grid points. The pixel intensity was found by summing the intensity level of each of the grid points making up the pixel and taking the integer part. This takes account of the averaging effect of the CCD camera. If the pixel intensity is greater than 255 it is set to a threshold value of 255 as would happen to an image taken by a CCD camera with 256 grey levels. The simulated FDV image produced for two of the random walks are shown in Figs 12.3(a) and 12.3(b). Using this system it is possible to impose any desired shift on the original grid to a resolution of 1 grid point, corresponding to 0.1 pixel. If the shift corresponds to an integral number of pixels, then the pixel intensities will be the same in the two images, the only difference will be in the position. If, however, a non-integral pixel shift is applied, then the two images will be different due to the averaging procedure.

Fig. 12.2 Random walks

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Fig. 12.3 Simulated FDV images for two of the random seeds considered

12.3.4 Analysis of Images The next stage of the process is to obtain velocity information from the FDV images. Each velocity vector was found by considering an interrogation region consisting of a 64 by 64 pixel area of the simulated image. A cross-correlation routine was applied to the original and shifted image and the peak value in the correlation plane determined. As with all digital correlations this introduces a bias error due to the windowing method. This bias (5, 6) is in the form of the correlation of two square window functions and can be divided out of the computed correlation to give the correct correlation values from which a peak can be identified. This peak gives the displacement between the two simulated images to integer accuracy. As in PIV, the position of the peak in the correlation plane was determined to sub-pixel accuracy using a three-point estimator (6). Different estimators can be applied depending on the shape of the peak, in the results presented here a Gaussian fit was applied and was found be satisfactory. We note that, as the shift imposed on the second image is increased, the area of overlap between the two images is reduced. To overcome this a second phase of the analysis can be performed. The original interrogation region of the first image is correlated with an interrogation region from the second image which is displaced by an amount (-m,-n) from its position in the initial analysis, where m and n are the closest integers to the two-component displacement obtained from the first phase of the analysis. If the second phase gives a real displacement (using the Gaussian fit estimator) which is (x',y'), then the final displacement is given as (x,y) = (m,n) + (x',yr). This second phase of analysis is known as reinterrogation. Since we are dealing here with simulated images we are content to consider the measured displacement and compare it to the known shift, thus enabling us to consider the accuracy of the technique. When real images are analysed the velocity is obtained by dividing the displacement by the time separation of the images. 12.3.5 Results In this section we present results obtained using the simulated images and consider aspects of the analysis. A total of five random seeds were used to produce five pairs of FDV images, each

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640 by 640 pixels. Interrogation regions were selected every 32 pixels (note that there is an overlap between adjacent regions) giving 19 x 19 interrogation regions per image pair. The maximum number of data points obtained for each shift is therefore 5 x 19 x 19 = 1805. In practice the number of data points obtained was slightly smaller since, for example, it is not always possible to find a suitable region for reinterrogation if the original is at the edge of the image. 12.3.5.1 Reinterrogation Figure 12.4 shows the percentage of displacements which were calculated correctly within a tolerance of 0.5 pixels, with and without reinterrogation, for shifts of 0.0, 1.0, 3.0, 6.0, 9.0, and 12.0 pixels along the x-axis. In each case there was no shift along the y-axis. For both techniques the number of correct measurements is approximately 100 per cent for a shift of 0.0 pixels, that is when the two images are identical. For larger shifts the reinterrogation method shows a significant improvement giving an accuracy of over 85 per cent compared to an accuracy of about 20 per cent when there is no reinterrogation. The reason for this can be seen by comparing Figs 12.5(a) and 12.5(b) which show the spread of measured displacements when the second image is shifted by 3.0 pixels. The results are plotted in the form of a histogram with box widths of 0.1 pixels. Figure 12.5(a) shows the results when reinterrogation is not applied. The distribution is approximately Gaussian in shape with the maximum occurring between 2.95 and 3.05 pixels, however, there is a slight bias towards lower values. In particular, there are a few measured values less than 2.5 but none greater than 3.5. The results in Fig. 12.5(b) are for reinterrogation. All interrogation regions which gave an initial measurement between 2.5 and 3.5 [Fig. 12.5(a) shows that this is a significant percentage of all the results] are reinterrogated by moving the second image by -3 pixels so that it coincides with the first image. From Fig. 12.4 we can see that approximately 100 per cent of the interrogation regions give the correct displacement (to within ±0.05 pixels) when the two images coincide. Thus virtually all the results which originally lie between 2.5 and 3.5 are seen to give the correct answer (to within +0.05 pixels) after reinterrogation. In Fig.l2.5(a) we also see a few results which predict a displacement of slightly less then 2.5 pixels. There are a similar number of results in Fig. 12.5(b) with a displacement slightly less than 2.5; although the reinterrogated process will change the measured displacement it does not appear to significantly improve it in this region. The peak at a displacement of 3.0 pixels in Fig. 12.5(b) is about 85 per cent. The other 15 per cent of results include those below 2.5 pixels which are shown in the histogram and also about 10 per cent of the results predict a displacement close to 0. These are not shown in the histogram and are mainly due to regions where the mean intensity is low. This will be discussed in the next section.

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Figure 12.4 Percentage of displacements which were calculated correctly to within a tolerance of 0.05 pixels - the thin and thick lines were obtained with and without reinterrogation respectively

Fig. 12.5 Histograms showing the spread of measured displacement for a shift of 3.0 pixels, without (a) and with (b) reinterrogation

12.3.5.2 Intensity levels The results presented above have shown that it is possible to use FDV to obtain a reasonable measurement of the displacement between images. We have seen that with reinterrogation we can obtain an answer correct to +0.05 pixels from about 85 per cent of interrogation regions. It is important to have a method for identifying regions which are likely to give a good or a poor measurement. One factor which is likely to be important here is the average intensity of the interrogation region. Some of the interrogation regions will contain no dye, in which case it will not be possible to obtain any velocity information for that portion of the image. However, even regions with only a small amount of dye, see for example Fig. 12.6(a), can provide

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velocity information. Similarly, it is possible that there will be interrogation regions where all the pixel values will be 255 and again it will not be possible to obtain any information. In fact, such a region was not found for any of the random seeds used. Even in regions with a high mean density there were small regions with lower intensities, see Fig. 12.6(b).

Fig. 12.6 Two interrogation regions taken from Fig. 3(b)

To investigate this the measured displacement is plotted in Fig. 12.7 as a function of the mean intensity level in the interrogation region of the first image. The results are represented by dots, since the majority of them lie close to a displacement of 3.0 pixels they form an almost continuous line at this value. Also shown in Fig. 12.7 are the running mean calculated over twenty points and the corresponding running standard deviation. As the average intensity decreases from about 50 to zero bits per pixel, the deviation between the running mean and the shift value of 3.0 is seen to increase with many of the measured values significantly different from the shift value. It is noted that, in this region, the analysis tends to pick displacements which are smaller in magnitude than the known shift. In this region there is also a significant increase in the running standard deviation as the mean intensity approaches zero. A similar but much less significant effect is also noticed at high intensities as the mean intensity approached 255 bits per pixel. For the remaining intensities the running mean is close to the shift value and the corresponding standard deviation is small.

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Fig. 12.7 The measured displacement plotted as a function of the mean intensity of interrogation region. Also shown are running averages and running standard deviation calculated using 20 sequential points

Figure 12.8 shows AM, the difference between the mean and the shift value, and a, the standard deviation, found from all the results that have a mean density greater than 50 bits per pixel. The results are shown for shifts of 1.0, 3.0, 6.0, 9.0, and 12.0 pixels, with and without reinterrogation. The results suggest that provided the mean intensity is greater than 50 bits per pixel and we apply the reinterrogation algorithm we can obtain reliable displacement (and hence velocity) measurements for the integer shifts considered here. For each shift the mean measured displacement differs from the known shift by less then 0.1 pixels and the standard deviation is less then 0.02 pixels.

12.4 Conclusion The application of optical measuring techniques to fluid flows associated with acoustical phenomena has been considered. In particular, consequences of introducing seeding particles in to the flow are identified. The novel measuring technique fluorescent dye velocimetry has been proposed. This involves introducing a fluorescent dye in to the fluid. This dye diffuses at a molecular level and does not have the draw backs associated with traditional seeding particles. The new technique is then assessed using simulated images and it is seen that under certain circumstances FDV is capable of producing satisfactory results.

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Fig. 12.8 The mean and standard deviation for different image shifts with (thin) and without (thick) reinterrogation

References (1)

(2) (3) (4) (5) (6)

Cosgrove, J. A., Buick, J. M., Campbell, D. M., and Greated, C. A. PIV applied to acoustical phenomena. In Proceedings of the 8th International Conference on Sound and Vibration, pages 479-486, Hong Kong, China, 2001. King, L. V. On the acoustic radiation pressure on spheres. Proceedings of the Royal Society of London A, 147:212-240, 1934. Doinikov, A. A. Acoustic radiation pressure on a rigid sphere in a viscous fluid. Proceedings of the Royal Society of London A, 447:447-466, 1994. Adrian, R. J. Particle-imaging techniques for experimental fluid mechanics. Annual Review of Fluid Mechanics, 23:261-304, 1991. Raffel, M., Wilert, C., and Kompenhans., J. Particle Image Velocimetry - A Practical Guide. Springer, 1998. Westerweel, J. Digital Particle Image Velocimetry - Theory and Applications. PhD thesis, Delft University, 1993.

J M Buick, J A Cosgrove, D M Campbell, and C A Greated Department of Physics and Astronomy, JCMB, The University of Edinburgh, UK © With Authors 2002

13 A Study of the Flow Structure in the Near-wall Region of a Complex-shaped Channel using Liquid Crystals G M Zharkova, V N Kovrizhina, and V M Khachaturyan

Abstract In the present work, results are reported of a study of the structure of a forced convective flow in a complex-shaped channel performed using cholesteric liquid crystals (LC). The model under study was a channel formed by two corrugated surfaces pressed closely together. To measure the temperature fields on the model surface, a thermochromic LC coating was used. To visualize the shear-stress distribution over the model surface the texture transition method based on the use of LC insensitive to temperature was used. The obtained data permit reconstruction of the vortex flow pattern in the near-wall region.

13.1 Introduction The problem of heat-transfer intensification in various apparatus is a most important one from a practical viewpoint (1, 2). To make a proper choice of the method for raising the heattransfer rate (or that of the configuration of the heat-exchanging surface for a particular application), it is necessary to possess exact information as to the flow structure and conditions in the channel. In many cases, most suitable surfaces have an intricate shape for which analytical design is impossible. The complexity and great diversity of the possible shape of heat-exchanging surfaces and channels formed by these surfaces require direct experimental studies of the flow structure in such channels to be carried out. The heat-transfer rate, hydraulic resistance, and efficiency of any surface of interest all are determined by the flow structure in the channel. A knowledge of the distribution over model surfaces of such quantities as shear stress and temperature allows one to reconstruct the panoramic pattern of the flow and make an optimal choice of model geometry.

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In the present work, results are reported of a study of the structure of a forced convective flow in a complex-shaped channel performed using cholesteric liquid crystals (LC). These crystals are known to readily respond to the combined action of temperature and shear stress (3). By the proper choice of coating, eliminating other possible influences, one can visualize solely the shear-stress distribution. On the other hand, the use of thermochromic coatings on the base of LC encapsulated in to a polymeric matrix (4) makes it possible to exclude the effect due to the shear stress. In this manner, the LC method allows one to easily determine both local and integral heat-transfer characteristics, which substantially cuts the time required for performing experiments (5).

13.2 Experimental setup and model Experiments were carried out in a small subsonic wind tunnel with a closed working section of dimensions 15 x 80 x 250 mm. The model under study was a channel formed by two corrugated surfaces pressed closely together. One of the surfaces was made of an organic glass, and the other of a 0.15 mm-thick stainless steel. The angle between the plate ribs forming the surfaces was 90 degrees. The main geometry parameters of the plates are the following: pitch p = 10.5 mm and the internal height of corrugation h = 6.3 mm. Figures 13.1 and 13.2 show a model and a diagram of one elementary cell of the channel.

Fig. 13.1 The test section of the wind tunnel with the model (a) 1-corrugated plate, 2-copper contacting tie. The inlet (b) and the outlet (c) of the channel

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Fig. 13.2 Diagram of an elementary cell of the channel under study

As it was known, in the range of Reynolds number studied in this work, turbulent flows in channels of intricate shapes possess a complex three-dimensional vortex flow structure. At the channel inlet, the incoming stream is divided into a large number of elementary jets, each of which change in direction and motion, and the flow core in each jet gets disturbed. As a result, the structure of the turbulent flow in the channel is determined by the flows in elementary channels (grooves) along the ribs of lower and upper corrugated plates and their interaction in the mixing zone. The type and details of the flow structure and, hence, the heat-transfer rate and hydraulic losses depend on both the flow structure and geometric parameters.

13.3 Experimental procedure and results 13.3.1 Study of temperature fields To measure the temperature fields, thermochromic LC coatings were used with the temperature range of selective reflection of light AT = 3 and 5 °C. The experiments were carried out under conditions of constant heat-flux density on the wall. In this case, the isotherms are simultaneously the curves of constant heat-transfer coefficient. The LC film was glued on to the outer side of the corrugated surface, and, therefore, on the images recorded by camera through the transparent window, the rib top on the outer surface corresponds to the valley on the flow side. The optical response of the LC coating to the temperature field (colour images of the model surface) was recorded by a camera or immediately grabbed in to a PC. Then, using calibration dependencies 'hue-temperature', the colour pattern was converted in to the temperature field. Figure 13.3 shows the map of isothermal regions in colour representation. In this figure one can see periodic cellular structure, where areas with the higher surface temperature are consistent with lower heat exchange (separation zones, for example). Highly sensitive LC displayed small local temperature gradients. The temperature is gradually increased from the inlet to the outlet of the channel. For more detail, Fig. 13.4 shows the plot of temperature

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variation on one of the ribs in its central part. The heat-transfer rate on the lee side is seen to be less than that on the windward side, which results in a higher lee-side temperature. Besides, interchanging sections with reduced and elevated temperatures are observed along the contact line of the corrugated plates, which is indicative of the probable presence of local twodimensional flow detachment regions formed in the flow over ribs, three-dimensional vortex structures, etc. At the same time, an increase in temperature along the rib length and downstream is observed.

Fig. 13.3 Temperature distribution on the rib

Fig. 13.4 Temperature variation along the rib length close to valley on the lee (t1) and windward (t3) sides, and on the crest (t2)

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To gain more information about the flow structure, we performed visualization of the shearstress distribution over the channel wall using LC coatings insensitive to temperature but readily responding to shear stress. 13.3.2 Visualization of the shear-stress distribution by the method of texture transition in cholesteric LC To visualize the shear-stress distribution over the model surface, we used the texture transition method based on the use of LC insensitive to temperature (in the given temperature range) but sensitive to shear stress. On spraying the LC coating on to the model surface, we obtained the so-called focal-conic texture. In this texture, the LC molecules are arranged at random, without any predominant direction of molecule alignment. On exposing such an LC film to white light, the surface appears colourless because of light scattering, see Fig. 13.5. However, under the action of a shear stress, a transition from the focal-conic to the so-called Granjean (planar) texture occurs in which the LC molecules are aligned along the direction of shear. Under these conditions, the LC film selectively reflects the incident light and looks coloured.

Fig. 13.5 Image of the surface covered with an LC coating prior to experiment

The LC coating was sprayed on to the model surface painted black. After a certain time of model flow (equal to 5 min in our experiments), the test section of the wind tunnel was dismounted. The non-reversible colour pattern obtained on the corrugated surface was recorded with a camera. The colourless, black regions correspond to the zones of zero shear stress or, more accurately, of stresses lower than a certain threshold value. The regions with shear stresses higher then the threshold look coloured due to texture transition. Figure 13.6 shows the distribution of shear stress over the windward rib side. On this side, quite a uniform distribution of shear stress throughout the whole corrugated plate is observed. Two specific features are observed, the black region near the valley that becomes narrower when going from channel inlet to outlet and a coloured region on the remaining area of this part of the rib. The flow wake in the coloured (lighter) region resembles a fur-tree branch with the axis lying close to the rib crest.

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Fig. 13.6 Distribution of shear stress over the model surface.U8 View on the windward side

Fig. 13.7 Distribution of shear stress over the model surface. View on the lee side

Figure 13.7 shows the distribution of shear stress on the lee rib side. The white dashed lines show the rib direction of the opposite plate. A three-dimensional cellular structure is clearly seen which varies both along the test section and along the length of the rib (elementary channel). A rise in the shear-stress level from inlet to outlet is clearly seen. Just behind the crest of the rib, a semicircular wake of a vortex brought about by the interaction between the streams flowing in the upper and lower elementary channels is observed in each elementary cell. Closer to the valley, a wavy wake of vortices localized in this region is seen. These two regions are separated from each other by a narrow black strip with zero shear-stress level. Near the rib ends, within one or two elementary cells, edge effects caused by flow deflection are observed, see Fig. 13.7.

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The data permits a reconstruction of the vortex flow pattern in the near-wall region shown in Fig. 13.8. In this figure the development of both plates with a qualitative sketch of the shear stress pattern is presented.

Fig. 13.8 Sketch of the vortex flow structure (not to scale). 1, 2, 3-regions with low shear stresses, 4-cell vortex, 5-longitudinal vortex

Conclusions Measurements of temperature and shear-stress fields indicate that regions of low or zero shear stress correlates well with regions of decreased heat transfer. The combined use of LC coatings sensitive either to temperature or shear stress permits easy determination of the flow-structure features on a complex-shaped model. The obtained data can be used for optimization of the geometry of heat-exchanging surfaces.

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References (1) (2)

(3) (4) (5)

Kays, W. M. and London, A. L. Compact Heat Exchangers. McGraw-Hill, New York, 1984. Stasiek, J., Collins, M. W., Ciofallo, V., and Chew, P. E. Investigation of flow and heat transfer in corrugated passages-I. Experimental results. Int. J. Heat Mass Transfer, Vol.39. No.l. 1996, P.149-164. de Gennes, P. G. The Physics of Liquid Crystals. Clarendon Press, Oxford, 1974. Zharkova, G. M. Visualization of temperature fields by method of LC. Experimental Heat Transfer. Vol.4. 1991 P.85-94. Jones, T. Liquid Crystals in Aerodynamic and Heat Transfer Testing. Int. Seminar on Optical Method and Data Processing in Heat and Fluid flow. Proc., London, 1992. P.51-66.

G M Zharkova, V N Kovrizhina, and V M Khachaturyan Institute of Theoretical and Applied Mechanics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia

14 Spatio-temporal Reconstruction of the Unsteady Wake of Axi-symmetric Bluff Bodies via Time-recording DPIV C Brucker

Introduction The present study was undertaken with the objective of a more detailed quantitative analysis of the evolution of the flow field in the wake of axi-symmetric bluff bodies like spheroids or bubbles. Much data exist for the wake of nominally two-dimensional bodies like the wake of a cylinder. On the other hand, the wakes of axi-symmetric bodies exhibit grave differences in the shape and dynamic of the vortices being shed in comparison to the two-dimensional case. The wake structures are basically three-dimensional and unsteady which makes the measurement and interpretation difficult. Up to now, experiments providing detailed instantaneous flow field data in the wake do not exist. Therefore, our current day knowledge about the vortical structures contained in the wake is still based mainly on earlier flow visualization studies. A pair of attached vortices was observed by Nakamura (1) for sphere wakes in the range 210 < Re < 270. For higher Reynolds-numbers up to Re = 490, the flow visualization experiments revealed a shedding of hairpin-like vortex structures (2-4). The wake appears as a chain of hairpin-vortices with the heads pointing always to the same side. However, any flow pattern visualized with dye-injection technique always depends on the way, how, and where the dye is released in to the flow. Recent numerical simulations of the sphere wake flow by Johnson and Patel demonstrated that most of the flow visualization results have overlooked vortex structures in the wake which are induced within the fluid and are not connected with the base of the bluff body (5). These occur as oppositely oriented hairpin-vortices in between the one-sided chain of hairpin-vortices seen in the flow pictures from Nakamura etc. This further highlights that detailed chronological flow field measurements are necessary in order to provide the velocity and vorticity information for the entire bubble wake.

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In our work, we investigate the structure and dynamics of the streamwise vortices in the wake by chronological high-speed PIV recordings which were captured in radial cross-sections downstream of the axi-symmetric bluff bodies. A digital high-speed camera records the flow and the DPIV results are analyzed by spatio-temporal reconstruction techniques of the streamwise vorticity in the wake.

14.1 Experimental set-up Our investigation focused on the spatial structure and dynamics of the streamwise vortices in the wake of axi-symmetric bodies. The flow field in the wake was recorded with a digital high-speed camera and evaluated frame-by-frame with the method of Digital-Particle-ImageVelocimetry (DPIV). As test-objects we used a solid sphere and a cylindrical rod with an elliptical nose and a sharp trailing edge, which was aligned with its axis along the flow axis (Fig. 14.1). In contrast to the sphere, this cylinder has a defined separation edge and the boundary layer thickness is controlled by the length of the cylinder. This allows us to study the effect of the boundary layer thickness on the stability of the wake.

Fig. 14.1 Shape of the two axi-symmetric bluff bodies used in this study

The experiments were carried out in a vertical water channel shown in Fig. 14.2. The test section of transparent acrylic plates has a cross-section of 10 x 10 cm square and is 1.2 m high. The bluff bodies with a diameter of 3 cm were held in the vertical position by a thin hollow tube of 2 mm diameter in the center of the channel. The boundary layer fluid along the tube in the part upstream of the fixation of the body is sucked through tiny holes in to the tube to elliminate its effect on the flow around the body. Flow is driven by the constant head of the water in the settling chamber at the top of the channel and is controlled by a valve at the righthand side bottom end of the test section. Upstream of the inlet in to the test section, a contraction zone with a reduction factor of 5 is placed to provide a uniform flow with low turbulence level. Demineralized water was used as test liquid. Small tracer particles (Vestosint, mean diameter of 30 mm, p = 1.02 g/cm3) were added to the fluid upstream in the water basin and were mixed homogenously.

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The DPIV set-up was arranged to measure the velocity field in a horizontal cross-section, downstream of the body (see the optical arrangement in Fig. 14.2) in order to obtain the streamwise component of the vorticity vector from the results. This is of importance for the understanding of the arrangement of the hairpin vortices and their legs which should leave a distinct pattern of the streamwise vorticity in the wake. The beam of a continuous Ar+ laser was expanded with a rotating polygonal mirror to form an intense virtual light-sheet in a horizontal cross-section of the channel. A digital high-speed video camera (Weinberger GmbH, Germany, resolution: 512 x 512 pixel; maximum frame rate: 1000 Hz; maximum number of frames: 2048) was used to record the flow, synchronized with the polygonal mirror so that each frame corresponds to a single sweep of the laser beam.

Fig. 14.2 Experimental set-up

Within the horizontal light-sheet plane (x-y plane) (see the co-ordinate system in the channel as defined in Fig. 14.2) the in-plane velocity components were obtained from crosscorrelation of successive images in small interrogation windows with a size of 32 x 32 pixel.

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The results represent a two-dimensional data set, in the form of velocity vectors v(x, y) on a grid with 31 x 31 equidistant nodes over a squared cross-section area of 5 x 5 cm2, see Fig. 14.3. The streamwise vorticity component was determined out of the velocity field by calculating the gradients in the 3 x 3 neighbourhood of each node using central difference schemes.

Fig. 14.3 Example of DPIV result in a horizontal plane in the wake of the sphere (left: velocity field and sectional streamlines; middle: vorticity field; right: angular orientation of the wake)

14.2 Results Before we discuss the results by means of the spatio-temporal reconstruction images, it is important to understand the reconstruction procedure. Therefore, the dynamic of the wake of the bluff bodies is first shown by a sequence of DPIV results, see Fig. 14.4, for which we explain the reconstruction in detail.

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Fig. 14.4 Evolution of the streamwise vorticity distribution in a cross-section in the wake of a sphere (Re = 500)

From such a time sequence the spatio-temporal reconstruction of the vorticity field is created as follows. The vorticity values in the horizontal plane were taken, and stacked plane by plane vertically in a data cube over the complete sequence of DPIV results in the recorded period. The resulting data matrix can be displayed as an iso-surface giving the spatio-temporal evolution of the component of streamwise vorticity at a stationary location within the wake. To reduce the high-frequency part and enhance the global pattern in the reconstruction image the surfaces were finally smoothed by a moving average technique. The following figure displays such a spatio-temporal reconstruction of the sphere wake for a Reynolds-number of Re = 500, in addition to the graphical presentation of the temporal profiles of the amount and direction of the radial velocity component at the centerline as well as the variation of the amount of streamwise vorticity in the vortex cores.

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Fig. 14.5 Variation of amount and orientation of induced cross-flow in the sphere wake and spatio-temporal reconstruction of the streamwise vorticity distribution (Re = 500)

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Fig. 14.6 Variation of amount and orientation of induced cross-flow in the cylinder wake and spatio-temporal reconstruction of the streamwise vorticity distribution (Re = 500)

Both examples at the same Reynolds-number let us clearly recognize the periodic shedding of alternately opposite oriented vortex pairs within nearly the same plane. Comparing our results with the figures published by Johnson and Patel one can see that the pairs represent parts of the actual three-dimensional hairpin-like vortices being shed in the wake, namely the

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streamwisely oriented parts of the legs. A remarkable feature of our results in agreement with the numerical results is the fact that the one-sided chain of vortex pairs is interconnected with counter-oriented vortex pairs of seemingly lower strength. A measure of the circulation of the hairpin-like vortices is the streamwise vorticity in the core of the legs which we calculated out of our data. Surprisingly, it turns out that the amount of streamwise vorticity in the legs does not reach the same maximum values in the 'zig' and 'zag' parts of the cycle. This obvious asymmetry (different amount of circulation) of the hairpin-vortices gives a strong indication that the bluff body experiences a net-lift force as found by Johnson and Patel (5), too. In addition to a periodic fluctuating lift force, they observed a small offset of the average lift, indicating a stationary part of the lift force. The calculated vortical structure was found to show not only the typical sequence of shed hairpin-vortices but additional oppositely oriented induced hairpin-vortices, which do not have the same circulation as the shed ones. These previously unrevealed induced structures - as they describe - are generated by the interaction of the near wake flow and the outer flow and are based on a different mechanism to the shed hairpins. It is clearly demonstrated in Fig. 29 of their article, that the amount of streamwise vorticity of the shed hairpin-vortices is larger than that of the induced ones which agrees with our observation of different strength of the hairpin-vortices. A comparison of the wake of the sphere with the cylinder shows that the boundary layer thickness has a strong influence on the stability and coherence of the flow. In comparison to the sphere, the wake of the axially oriented cylinder (with a larger thickness of the boundary layer) is approximately periodic in time with a stable orientation of the vortices. Recognize the strong coherence of left-right alternation which demonstrates the near periodic creation and discharge of the vortices. The transition of the wake is shifted overall to higher Reynoldsnumbers than that reported for the sphere wake flow, see e.g. Nakamura et al. An interesting feature can be deduced from the variation of the strength of the vortices over time. There is a slight low-frequency modulation of the overall strength of the vortices which appears as a small peak in the spectrum at Sr = 0.015 in addition to the peak at Sr = 0.168 representing the primary periodic shedding. This low-frequency modulation was also observed by Schwarz et al. in their numerical simulation. In comparison, the sphere wake exhibits slight random variations of the orientation of the vortices. Similar to the case of the cylinder wake, there is a low-frequency modulation of the strength of the vortices which can also be found in the spatio-temporal reconstruction image. The vortices seem to be shed in 'pockets' of 3 to 4 hairpin-vortices, interrupted by phases with only low action. In one of these phases the shedding is completely suppressed which is marked as phase 'A' in Fig. 14.5. This clearly shows that there is an interaction of instabilities at different characteristic frequencies which was not observed in experiments before. Note that the peak of the low-frequency modulation compared to the peak of the primary shedding in the power spectrum for the sphere wake flow is higher than the peak for the cylinder wake and is shifted to a higher Strouhal-number of Sr = 0.05. The primary peak at Sr = 0.187 for the sphere wake falls well within the range of values reported by Sakamoto and Haniu (6) and Tomboulides et al. (7). Besides these comments, the results revealed much more interesting features which will be published elsewhere due to the limited space herein. Figure 14.7 gives an example of the transition of the sphere wake with increasing Reynoldsnumbers. For Re = 280 the wake is continuous and consists clearly of a pair of attached streamwise vortices. A slight waviness in the size and strength of the vortices can be seen. For Re = 400, hairpin-vortices are periodically shed in the wake as described above. Finally, in the case when Re = 700 the wake structure allows us to recognize phases, in which - besides an

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irregular orientation of the hairpin-vortices - the shedding is partly suppressed and the structure is twisted as a whole to a double-helical vortex structure. Such a phase is shown in Fig. 14.7 at the right-hand side. One can see the beginning and end of this phase by means of the open legs at the top and bottom end of the structure, which mark the generation of the shed hairpin with opposite circulation.

Fig. 14.7 Spatio-temporal reconstruction of streamwise vorticity distribution in the wake of a sphere for three different Reynolds-numbers

14.3 Conclusion The instability of the wake of an axi-symmetric bluff body originates from the absolute instability against helical waves as demonstrated by Monkewitz (8) for a family of general axi-symmetric wake profiles. With a global stability analysis of the steady sphere wake, Natarajan and Acrivos (9) could show, that the first instability appears for a non-fluctuating helical wave with an azimuthal wave-number of k = 1 (this means that the wave-length fits once within the circumference), i.e. the wake becomes asymmetric but remains steady. Beyond this first critical Reynolds-number, the flow becomes unstable against fluctuating helical waves with k = 1, and the flow becomes unsteady. In a numerical study using DNS

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and a Fourier-description in the azimuthal direction, Schwarz et al. (10) studied the wake of a streamwise oriented cylindrical rod with an elliptic nose and a plane base with a sharp trailing edge. In particular, they studied the amplification and spectra for different modes of helical waves. They proved that the dominant amplified waves in the wake of the simplified bluff body are helical waves with an azimuthal wave-number of k = 1. At a Reynolds-numbers of Re = 400 the wake was steady and axi-symmetric for the chosen geometry. By imposing a small initial disturbance, they could prove for Re = 500 the amplification of two counterrotating helical waves. These long waves were of very low frequency and had a Strouhalnumber of Sr = 0.015, and were observed to result in the flow pattern as two streamwise vortex filaments. After nonlinear saturation the counter-rotating waves reached the same amplitude and phase velocity which yielded a perfect planar oscillations of the wake (this can be proved by simple additive superposition of both waves). For a higher Reynolds-number when Re = 700, the helical waves were further amplified and additional modes were generated by nonlinear interaction, leading to an intermittent discharge of vortex loops at a Strouhal-number of Sr - 0.133. In between the periods with regular shedding, phases of domination of one of the helical waves were observed, in which the orientation of the loops changed or the shedding was completely suppressed and the wake was twisted. With the measurement technique described here, we could prove for the first time the observations of Schwarz et al. from quantitative experimental data and confirmed that the wake instability results in similar flow pattern for different axi-symmetric objects. However, the boundary layer thickness influences the critical Reynolds-number at which the wake transforms. Our results show, that a larger boundary thickness shifts the critical Reynolds-number to higher values.

References (1) (2) (3)

I. Nakamura, 'Steady wake behind a sphere', Phys. of Fluids 19(1), 5-8 (1976). E. Achenbach, 'Vortex shedding from spheres', J. Fluid Mech. 62, 209-221 (1974). P. Hirsch, 'Uber die Bewegung von Kugeln in ruhenden Flussigkeiten', Z. Angew. Math. Mechan. 3, 93 (1923). (4) W. Willmarth, N. E. Hawk, and R. L. Harvey, 'Steady and unsteady motions and wakes of freely falling disks', Phys. of Fluids 7, 197-208 (1964). (5) T. A. Johnson and V. C. Patel, 'Flow past a sphere up to a Reynolds Number of 300', J. Fluid Mech. 378, 19-70 (1999). (6) H. Sakamoto and H. Haniu, 'The formation mechanism and shedding frequency of vortices from a sphere in uniform shear flow', JFM 287, 151-171 (1995). (7) A. Tomboulides, S. A. Orszag, and G. E. Karniadakis 'Direct and large-eddy simulations of axisymmetric wakes', AIAA paper no. 93-0546 (1993). (8) A. P. Monkewitz, 'A note on the vortex shedding from axisymmetric bluff bodies', J. Fluid Mech. 192, 561. (1988). (9) R. Natarajan and A. Acrivos, 'The instability of the steady flow past spheres and disks', J. Fluid Mech. 254, 323-344 (1993). (10) V. Schwarz, H. Bestek, and H. Fasel, 'Numerical simulation of nonlinear waves in the wake of an axisymmetric bluff body', AIAA paper no. 94-2285 (1994).

Christoph Brucker Aerodynamisches Institut der RWTH Aachen, Germany © With Author 2002

15 The Measurement of the Velocity Field around a Ship Hull Model in a Towing Tank using PIV Method J Dekowski, M Kocik, J Podlihski, J Wasilewski, J Mizeraczyk, L Wilczyhski, J Kanar, and J Stasiek

Abstract This Chapter concerns experimental investigation of the velocity field in the flow around a ship hull model using particle image velocimetry (PIV). The results of measurements of the velocity field, i.e. vector graphs and streamline patterns obtained during ship hull model tests in a towing tank are presented. As far as the sensitivity and accuracy of measurements are concerned the experiments proved that the PIV method perfectly met the requirements specific for ship hull model testing. The flow patterns obtained with the use of the PIV technique serve as the valuable source for further flow analysis as well as for numerical algorithms verification.

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Introduction

Despite the fruitful and rapid development of analytical models and their numerical implementation, the experimental investigation of velocity field still serves as the ultimate source of data for the analysis of the flow around a ship hull model. It is generally accepted that the complexity of the flow around a ship hull results from the high value of the Reynolds number, the presence of a free surface and propeller-hull interactions substantially restrains the application of computational fluid dynamics (CFD) methods. Moreover, continuously increasing performance requirements concerning newly built ships, especially their size (block co-efficient) and cruising speed, forces designers to consider more complex phenomena occurring in the flow around a hull.

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Different experimental methods for the velocity field investigation have found their application in the field of ship hull model testing. The simplest technique named paint test method consists in visualization of streak lines on the ship hull model surface. It provides qualitative data concerning the flow in the closest vicinity of the hull and enables the recognition of flow separation areas. Another method of flow visualization is the so-called thread method. It consists of attaching thin threads to the ends of tiny stiff 'riding booms' protruding from the hull surface and recording their orientation in the flow during hull motion. This qualitative method provides the data concerning the direction of the flow in the ship hull surrounding. However, the length of the stiff elements to which threads are attached limits the information about the flow character. A more sophisticated method consists of using a wake rake probe or any other set of probes. Usually the wake rake probe is a set of tiny five-hole Pitot probes. Although the application of the wake rake probe is limited to the afterbody region of the flow, it provides threedimensional data concerning velocity field. Usually the wake rake probe is used for determination of velocity field in the propeller region of operation but it is also possible to examine other regions of the flow with a similar device operating on the same principle. Except for the above-mentioned, two laser-based methods should be mentioned: LDA (laser doppler anemometry) and PIV (particle image velocimetry). The first one enables threedimensional local velocity field identification, i.e. a 'single' measurement provides data concerning the velocity field limited to the single point of the flow domain. In order to obtain complete information about the flow, measurement has to be repeated as many times, as many control points have been assumed within the flow domain. This feature excludes the application of LDA method for velocity field investigation in a towing tank, because of limited experiment time. In contrast to this, PIV method provides much more global information about the flow. It enables you to obtain, relatively quickly, two-dimensionalvelocity field pattern in the specified flow section. Therefore, its application in a towing tank seemed to be promising. Moreover, the experimental methods of velocity field investigation can be divided into two groups: • invasive, i.e. interrupting theflowfield; • non-invasive, i.e. indifferent to the flow. The PIV method belongs to the second group, if certain requirements are met. The basic advantages of PIV method are as follows: • the whole assumed flow domain section can be scanned promptly and simultaneously; • the method is non-invasive, i.e. the interaction of seeded particles with the velocity field is negligible (if seeding particles are small enough); • the results of measurements are available 'on-line' during the experiment. The following section contains the description of the test stand, presentation of obtained experimental results, and their discussion.

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The study presented in this Chapter was carried out within the frame of the project entitled 'Investigation of the mechanisms of vortex structures generation in the flow around modern cargo ship hull'. The project consisted of experimental investigation and numerical analysis of the flow around ship hull model, which was carried out with the use of various techniques and methods. One of the experimental techniques used during the investigation was the PIV method. The aim of model tests performed with the use of the PIV method was to determine the velocity field in the flow around a ship hull model. The goals of this investigation were as follows: • assessment of the usefulness of the PIV method for measuring the velocity field in relatively extensive flow domain as towing tank; • providing the experimental data for verification of the numerical simulations results; • examining the influence of hull shape modifications as well as screw propeller operation on the character of the flow. All the experiments described below were carried out in a small towing tank at Gdansk Ship Model Basin of Ship Design and Research Centre (CTO). The dimensions of the towing tank are as follows: length 60 m, width 7 m, depth 3.5 m. Three different hull models of different types of ships were selected for experimental investigation. Their basic parameters are presented in Table 15.1. Table 15.1 Basic ship model data Scale Ship model type/dimensions A: LNG ship 32,8 90 B: Tanker 50 C: Container ship LPP*- Length between perpendiculars

LPP*[m] 2,97 3,6 3,28

Beam [m] 0,51 0,63 0,56

Draught [m] 0,19 0,23 0,19

Until it is not indicated otherwise, the models were towed with the same constant velocity v = 1.5 m/s. The measurements were carried out with the use of standard PIV equipment, which consisted of the following components: • twin second harmonic Nd:YAG laser system (y = 532 nm, pulse energy 50 mJ); • Dantec PIV 1100 image processor; • Kodak Mega Plus ES 1.0 CCD camera. The limitation of the method is that the results obtained contain only two-dimensional information concerning the velocity field. Thus the method can be applied successfully when one of the velocity components can be neglected. In the case of the flow around the ship hull the longitudinal velocity component is dominating (except the behind stern region). As the project was focused on investigation of the mechanisms of the large-scale vortices generation in the flow around the ship hull, the measurement sections were set perpendicularly to the

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longitudinal symmetry plane of the hull. The schematics of location of measurement sections with respect to the hull model are presented in Figs 15.1 and 15.2.

Fig. 15.1 The schematics of longitudinal location of the flow field sections (numbered 1-18) with respect to the hull within which PIV measurements were carried out

Fig. 15.2 Location of the flow field sections (1 and 2) with respect to the hull transverse section

Figures 15.3 and 15.4 present the details of the PIV set-up. The set-up was situated aside the towing tank. Figure 15.3 presents the model 'A' attached to the towing carriage as it passes the PIV set-up. A view of the PIV set-up, just before the model enters it, is presented in Fig. 15.4. In the bottom part of Fig. 15.4 one can notice a laser sheet illuminating seeding particles dispersed in the tank water. The laser beam optical guiding system as well as the submerged CCD camera is also visible. The tank water in the measuring area is seeded before every single measurement.

Fig. 15.3 Model 'A' under a towing carriage passing the PIV set-up

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Fig. 15.4 Laser sheet illuminating measurement area with seedling particles dispersed in tank water

15.3

PIV Method

Measurement of flow field with PIV method is based on calculating of displacement of seeding particles between two images, which were captured with camera, in known time interval. Seeding is brought into the measurement area. Measurement area is lit with 'laser sheet'. Two images, with seeding particles lit by the laser, are captured. Captured images are divided in to interrogation areas. For every interrogation area, the average particle displacement between two images is calculated with a numerical procedure based on cross-correlation. Velocity is calculated with the following formula:

where: V - velocity of the fluid in interrogation area; S - average displacement of particles of the seeding between two images; t - time interval between capturing two images. Measurement cycle can be divided into following steps: • • • • •

seeding selection; illumination of the measurement area; image capture; analysis of the captured images; display of the results.

The following sections contain descriptions of each step.

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15.3.1 Seeding selection To make a PIV measurement, fluid has to contain particles that scatter laser light. These particles, which can be either natural for the flow or injected, are called seeding. Size and material of the seeding depend on specific measurement conditions. Seeding particles should be small enough to follow the flow, but also large enough to effectively scatter laser light. It is very important to get proper seeding density. It is known that seeding density should be about 7-10 particles per the smallest interrogation area used (1, 2). Seeding optimization depends on illumination intensity (scattering ability of particles). 15.3.2 Illumination of the measurement area In the PIV method the measurement area has to be lit by laser beam shaped in so called 'laser sheet'. Twin double-frequency Nd:YAG laser is a typical laser used in the PIV method. In the measurements described here, two Nd: YAG lasers were used with pulse energy of 50 mj each. Beams from the lasers are collimated and transmitted to the measuring area with a special optical system. To obtain 'laser sheet' from laser beam the cylindrical telescope was used. The thickness of obtained laser sheet was about of 3-5 mm. 15.3.3 Image capture Every single PIV measurement consists of two images of light scattered by seeding particles from the volume of 'laser sheet' captured with digital CCD camera Kodak ES 1.0. This camera enables measurements at relatively high velocities dependent on size of measurement area. Maximum resolution for this camera is 1008 x 1018 pixels. Dimensions of pixels of the recorded image depend on dimensions of the measurement area. Proper setting of time between capturing two images is crucial for measurement accuracy. This parameter value should be chosen so that each seeding particle will pass approximately one quarter of the interrogation area. Proper setting of time between two images often proves difficult for flow fields with large velocity gradients. Minimum time between capturing two images for Kodak ES 1.0 camera is 2 ms. The example of a pattern of seeded particles in the flow around screw propeller model illuminated with laser sheet and recorded by digital camera is presented in Fig. 15.5.

Fig. 15.5 Example of seeded particles image recorded by digital camera in the flow around screw propeller model

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15.3.4 Analysis of the captured images After capturing two images the numerical methods (correlation analysis) have to be applied to get velocity field. The purpose of the correlation analysis is to derive mean displacement of seeding particles between area from first image and area from second image corresponding to area from first image. First the captured images are divided into rectangular interrogation areas dimensions, which are from 8 up to 128 pixels. This rectangular fragment of image is represented with image brightness function, f(m,n) for area from first image and g(m,n) for area from second captured image corresponding to area from first image. Discrete crosscorrelation function is described with formula:

Co-ordinates of correlation function represent displacement between image brightness function of first and second image. If seeding particles displacement will be close to each other for particles in one interrogation area then correlation function has high values for these displacements. Correlation not corresponding to particle displacements becomes measurement noise. Noise is increasing with the increase of particles, which have been only on one image (they go in to or go out from interrogation area in time between capturing two images). With high signal to noise ratio one can take the co-ordinates of the correlation function maximum as the mean seeding particle displacement in this interrogation area. To speed up calculations it is convenient to use one of the properties of Fourier transform (FFT):

Where S(u,v), F(u,v), G(u,v) are Fourier transforms of functions s(m,n), f(m,n), g(m,n). With the FFT algorithm it is possible to transform the image brightness functions f(m,n) and g(m,n). Next one can calculate transform correlation function S(u,v) with the formula mentioned above. Finally with the inverse Fourier transform one can get correlation function s(m,n). 15.3.5 Display of the results As a results of analysis of images flow velocity fields and seeding density distribution in measurement area are obtained. Additionally, it is also possible to calculate from obtained results vorticity fields, streamlines pattern, and for statistical calculations standard derivation distribution. Results prepared in this way describe the investigated flow very well.

15.4

Discussion of the results

As it has been stated above the most meaningful aim of the experimental research carried out within the frame of the project was to provide the information concerning velocity field in the flow around the ship hull model towed in the tank. Among others the PIV method has been used for this purpose. As the research project was focused on the investigation of the mechanisms of vortex generation in the flow around the ship hull, thus it was necessary to

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measure transverse components (with respect to the hull symmetry plane - see Figs 15.1 and 15.2) of the velocity field. Therefore, the application of PIV method, which by principle serves as the source of two-dimensional data concerning the velocity field in a flow, seemed to be perfectly suited to this task. Moreover, it is advantageous to use the PIV technique when both velocity components are of the same order of magnitude. In the case of flow around the ship hull model, the longitudinal velocity component is dominating and additionally both of the transverse components are of the same order. The application of the PIV technique in such a large liquid volume as the towing tank brought numerous difficulties and required solving different practical problems. In order to carry out the experiments it was necessary to: • • • •

paint the ship model side with black dull paint; introduce carefully the seeding particles into the water before each model run; reduce the ambient light intensity; wait until the tank water became calm between two successive model runs.

According to the aim of the experiments, explained in the previous section, the results can be divided into three groups: • investigation of the influence of the ship hull geometry on the transverse velocity field maps - three different types of the ship hulls were examined; • investigation of the so called 'scale effect' on the way of examination of the velocity field in the flow around two differently scaled models of the same hull; • examination of the influence of the screw propeller as well as bilge keels and turbulence stimulators on the flow around the ship hull model. The following sections contain the description of the results referring to the above classification. 15.4.1 Three different hull models - investigation of the velocity field Three selected types of the hulls, i.e.: A - LNG ship, B - tanker, and C - container differ from each other significantly. Therefore, it has been expected that the images of the velocity field sections will follow those differences. The results of measurements are gathered in Table 15.2. The letter and the number denote the images displayed in the Table. The letter refers to the model type and the number corresponds with the location of the section with respect to the hull (see Fig. 15.1). Each column of the table refers to the same model, and each row to the same flow field section. The table contains the results referring to the section denoted by T in Fig. 15.2. The various lengths of the models and the distance between the successive sections are the same, therefore, the total number of sections is different for each model. The images displayed in Table 15.2 present streamline patterns. The differences can be easily recognized in each row of the Table. As the container ship hull is the most slender one the velocity field seems to be the smoothest. The differences between the flow cases can be also recognized within the wake region.

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Table 15.2 Comparison of streamline patterns for three different types of hulls

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The traces of vortices forming below the bottom of the ship hull can be found in the diagrams B3 and A5. In all of three examined flow cases vortices can also be easily found in the wake field. Nevertheless the hull outline is not marked in the pictures; it can be concluded that the use of the PIV method in the closest vicinity of the hull surface is limited. The density of seeding particles within the boundary layer region of the flow was insufficient to record the reliable scattered signal. 15.4.2 Two different model scales The LNG ship hull model was selected for testing in two different scales, i.e.: 1:32.8 and 1:16.4. The hull model manufactured at the 1:16.4 scale is denoted by A'. The model A' was towed with the constant velocity 2.1 m/s. The comparison of the velocity field obtained for both scales is presented in Table 15.3. The left column of the table refers to the model A and the right one to the model A'. Each row of the table contains the results of measurement corresponding to the section located identically with respect to the hull in both cases. The notation used in Table 15.3 is similar to the one used in Table 15.2. The diagrams in Table 15.3 present the two-dimensional velocity vector maps.

Table 15.3 Comparison of the velocity fields measured in the flow around the LNG ship model tested in two different scales

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The influence of the model size on the character of the flow can be noticed starting from diagram A9. The Froude number in both flow cases was the same and was equal to Fn = 0.28 therefore, the wave pattern generated by both hulls was similar. Consequently the flow around the bow part of the hull is almost identical (see Table 15.3 diagrams A1-A8 and A'l-A'8) The identity of the Froude number does not coincide with the identity of Reynolds number. Thus the differences between velocity fields can be noticed in the midship region, where the boundary layer is already developed and the influence of the waves generated by the moving hull is not so significant. Some differences between velocity fields referring to both scales can be also noticed in the stern part of the flow region where the viscous effects are dominating. 15.4.3 Screw propeller, turbulence stimulators, bilge keels The hull model A was also tested equipped with the screw propeller. The propeller was the four bladed B-Wageningen series model with diameter equal to 120 mm. Its number of revolutions was equal to 500 r/min. The measurements of the velocity field in the operating propeller vicinity were carried out in order to examine the capabilities of the PIV system. The examples of the streamline maps are presented in Table 15.4. The notation used in the table is the same as in previous tables. The left column (diagrams denoted by the letter A) refers to the 'bare' hull and the right one (diagrams denoted by letters AP) refers to the hull equipped with the propeller. The numbers of diagrams correspond to the numbers of successive sections of the flow domain. The difference between the velocity field is distinct. The influence of the screw propeller on the velocity field within the wake can be easily recognized.

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Table 15.4 Comparison of streamlines behind the 'bare' hull (left column) and hull with operating propeller (right column)

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In order to assess the sensitivity of the PIV method to the hull modifications were introduced and the additional measurements were carried out. The model B was equipped with the turbulence stimulator located behind the bow and the model C was equipped with a pair of bilge keels. The modifications of the hulls are presented in Figs 15.6 and 15.7.

Fig. 15.6 Model B on the water with the sand-strip turbulence stimulator behind the bow

Fig. 15.7 Bilge keels assembled on the side of the hull model C

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The influence of turbulence stimulator on the velocity field is presented in Table 15.5. The left column presents the results of the velocity measurements obtained for the hull without the turbulence stimulator (diagrams denoted by letter B) and the right column contain the results for the hull with the stimulator assembled (diagrams denoted by letters BTS). The numbers of frames refer to the successive sections of the flow domain.

Table 15.5 The influence of turbulence stimulator on the character of the flow around the ship hull model

The sand strip turbulence stimulator was located on the hull surface just behind the section No. 4. Nevertheless it is impossible to determine the velocity field in the closest vicinity of the hull surface the modification of the flow due to the turbulence stimulator can be deduced from the comparison of the frames B5 and BTS5.

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The last cluster of the PIV measurement results is presented in Table 15.6. In this case the side of the hull model C was equipped with the pair of bilge keels (see Fig. 15.7). The purpose of placing bilge keels on the hull model surface was only to examine the sensitivity of the PIV method. In practice the bilge keels are assembled in different locations. The analysis of the results leads to the conclusion that the PIV technique enables recognition of the influence of ship hull appendages on the velocity field. Two cases are compared: the bare hull model (the left column - the diagrams denoted by the letter C) and the bilge keels equipped model (the right column - the diagrams denoted by the letters CBK). The numbers correspond with the successive sections of the flow domain. Table 15.6 The influence of bilge keels on the flow around the ship hull model

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The influence of the bilge keels on the streamline patterns is recognizable starting from the frame No. 9. Thus the PIV measurement technique proves its suitability for the precise measurements of the velocity field in the flow around the ship hull model in the towing tank. The bottom bilge keel reverses locally the flow and generates vortices (see frames CBK9 and CBK11).

15.5

Conclusions

PIV measurements of the velocity field in the flow around the ship hull model in the towing tank were far more difficult then PIV measurements in cavitation or wind tunnel. The most difficult problem was selection and proper distribution of the seeding in measurement area because of the large size of the towing tank. It was impossible to inject seeding properly in the whole tank. Also large quantities of seeding had dissipated laser light scattered on seeding particles in the measurement area before they reached the camera. Only proper quantity of seeding in selected volume gives reliable results. PIV measurement technique provides two-dimensional information concerning velocity field in the flow. The PIV method enables precise recognition of the velocity field especially when both velocity components are of the similar order of magnitude. Therefore the application of PIV technique for determination of the transverse velocity field in the flow around ship hull model towed in the tank seems to be very promising. Basing on the results of PIV measurements in the towing tank it can be concluded that:

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• PIV technique provided reasonable and repeatable data concerning velocity field; • the sensitivity of the PIV method meets the requirements of ship model testing practice; • PIV method was successfully applied for measurements of velocity field in the flow around operating screw propeller model; • the form of the PIV results enables easy comparison with the numerical calculations.

References (1) (2)

Tukker, J. et al. Wake flow measurements in towing tanks with PIV, 9th International Symposium on Flow Visualisation, Glasgow 2000. Westerweel, J. What is PIV?, Delft University of Technology 1998.

J Dekowski, M Kocik, J Podlihski, J Wasilewski, and J Mizeraczyk Centre of Plasma and Laser Engineering, Institute of Fluid Flow Machinery, Polish Academy of Sciences, Gdansk, Poland L Wilczyhski and J Kanar Ship Design and Research Centre, CTO, Gdansk, Poland J Stasiek Faculty of Mechanical Engineering, Technical University of Gdansk, Poland

16 Velocity Measurements in Impinging Turbulent Jets using Digital Particle Image Velocimetry M Fairweather, G K Margrave, and T C Williams

Abstract Two-dimensional measurements of mean and fluctuating velocities, and shear stresses, within the wall jet region of impinging turbulent air and water jets have been obtained using a Digital Particle Image Velocimetry (DPIV) technique. The data gathered demonstrate that DPIV can be used to derive useful information in this type of flow, of value in improving understanding of what is a practically important flow configuration. The present air jet results, considered in detail, are in good agreement with earlier measurements in similar flows gathered using laser Doppler anemometry, although significant differences do occur with previous hot wire anemometer measurements.

16.1 Introduction The impingement of a jet on a solid flat surface is a flow configuration of interest in many engineering applications, including cooling, heating, and drying processes, vertical take-off and landing aircraft, and ventilation during mining and tunnelling activities. It is also encountered in evaluations of the consequences of accidental releases of flammable material performed as part of safety and risk assessments on chemical and process plant, and transportation systems. The orthogonally impinging jet has been the subject of renewed interest lately, not only because of its practical importance, but also because it represents an important test case for the development and validation of mathematical models of turbulent flow. This work has concerned, in particular, the provision of detailed experimental data for the velocity fields encountered in such flows (1), as well as the development of second-moment (2, 3) and non-

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linear eddy viscosity (4) turbulence closures to more accurately account for the influence of pressure reflections from the solid surface in damping velocity fluctuations normal to the wall. All this work has resulted in a greater understanding of the characteristics of impinging turbulent jets, and in improved methods for predicting reliably their velocity fields. Data for the velocity and concentration fields in the flow formed by the normal impingement of a methane jet on a flat surface have also been reported, and predictions obtained using a second-moment turbulence closure, modified to include wall reflection effects, demonstrated to be in reasonable agreement with these results (5). However, at large distances from the solid surface, in the outer regions of the radial wall jet, predictions of the latter approach were found to underestimate both mean and fluctuating concentrations. These discrepancies were attributed to differences between experimental and computational boundary conditions and, in particular, the fact that downward firing jet studied experimentally may have given rise to a large recirculation zone which could re-entrain methane from the outer to the initial regions of the wall jet. The existence of such a recirculation zone has important implications for ventilation, tunnelling, and safety applications, since the re-entrainment of jet fluid from the outer to the initial regions of the wall jet could result in the build-up of unwanted flammable material or dust. The presence of such a recirculation zone has not been observed in many earlier studies, primarily since these studies focussed on the near field and stagnation regions of impinging jets, where wall reflection effects are most significant, and also because most measurements have been made using techniques which are not able to discriminate between vertical and radial velocity components. Profiles of mean radial velocity obtained in the wall jet region of an impinging axisymmetric air jet studied by Bradshaw and Love (6) do, however, indicate that such velocities become negative outside the wall jet region, and the existence of a recirculation zone is clearly visible in the predictions made by Childs and Nixon (7) for the jet studied by the latter authors. Evidence that such zones also occur in plane turbulent impinging jet flows is documented by Looney and Walsh (8). In addition, the majority of earlier studies of impinging jets gathered data by means of probe techniques, for example, using either pitot tubes (6, 9) or more recently hot wire anemometry (1, 10). The intrusive nature of these devices does, however, mean that they are prone to error. In addition, both probe techniques and laser Doppler anemometry (LDA), as used by Dianat et al. (5), are, by their nature, necessarily restrictive in terms of the extent of the data sets that can be gathered. The present work was therefore undertaken to resolve the uncertainties concerning the existence of recirculation zones raised by earlier studies, to provide detailed experimental data for use in the formulation and validation of mathematical models of turbulent flows, and to improve understanding of what is a practically important flow configuration. The majority of the work reported herein was carried out using impinging air jets, although data gathered from impinging water jets is also included for comparison purposes.

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16.2 Experimental work The air jet studied experimentally issued from a circular cross-sectioned pipe with an internal diameter, d, of 13.3 mm. The pipe was 50 d in length and air was supplied to the pipe from a compressor, with flow rates metered using mass flow controllers. The average exit velocity from the pipe was 20 m s-1 which gave rise to a fully developed turbulent flow with Re= 18 800. The air jet impinged vertically on to a flat, smooth steel plate 1.3 m x 1.1 m in length situated above the release pipe. For the velocity measurement, the tracer particles employed were mm-sized olive oil droplets, generated using an air-assisted atomizer. These droplets were injected into the release flow pipe 3 m before the exit to ensure uniform seeding. The jet and seeder were also run for 20 minutes before the start of each experiment to build up a high concentration of seed particles in the ambient air to allow the measurements to extend far from the impinged surface, and to minimize velocity bias. The water jet studied was a fully submerged impinging jet issuing from a circular crosssection pipe with an internal diameter of 13.3 mm. The water jet was contained within a 450 mm cubic tank and impinged vertically down at the centre of the lower surface. The pipe was 50 d in length and was supplied from a pump which re-circulated water from the main tank through a secondary overflow tank, with the water flow rates metered using a turbine flow meter. The mean exit velocity of the release was 1.77 m s" , giving a release Reynolds number of 23 000. Tracer particles of 7 to 12 mm and a relative density of 1.05 were employed in this study. For both experiments, the pipe exit was located 26.6 mm from the impingement surface (h/d = 2) and the rigs were mounted on a hydraulic table to allow vertical positioning relative to the measurement locations. Two-dimensional velocity field measurements were made using Digital Particle Image Velocimetry (DPIV). This technique provides rapid access to two-dimensional velocity fields by imaging the displacement of tracer particles in a flow field using high-resolution digital cameras. For these studies the DPIV system was configured for cross-correlation analysis. With this technique two consecutive images of a particle image field are captured with a predetermined time interval and the images cross-correlated to determine the particle displacements and thereby the particle velocities. The illumination source for the measurements was a pair of Nd:YAG lasers. These lasers were configured to generate 200 mJ pulses with a pulse width of 10 ns at a rate of 10 Hz. The beams from the two lasers were configured to intersect at the point of interest in the flow and focussed using a 1.5 m focal length spherical lens. The beams were formed into a laser sheet using a 100 mm focal length cylindrical lens. In the measurement region the sheet was uniform over an area of 50 mm square, and was approximately 1 mm thick. Imaging of the particles was accomplished using a Kodak ES1.0 Digital CCD camera. This provides images with 1000 x 1000 pixels and 8 bits depth. This camera was operated in a double exposure mode that allowed the recording of two consecutive images on the single CCD. The camera was interfaced to a PC using a frame grabber that allowed recording of the double images at the 10 Hz repetition rate of the laser. The imaged field was varied from 18 x 18 mm to 6 x 6 mm, providing a maximum resolution in the flow field of 6 mm.

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A control system provided synchronization of the image capture. This unit used a 10 Hz clock synchronization pulse to the computer, which triggered the image capture, and two trigger pulses to fire the lasers. The separation between the pulses was varied from 15 ms to 500 ms, depending on the local flow velocity, in order to control the particle displacement on the image. For the cross-correlation analysis the interrogation region was maintained at 64 x 64 pixels and the pulse separation varied to achieve 30 per cent to 50 per cent displacement. In the air jet, instantaneous velocity profiles were captured at radial displacements of 5, 10, 20, and 30 diameters from the stagnation point. At each measurement location typically 800 image pairs were recorded for each region of the velocity profile. At 20 d in the air jet study, this corresponded to a total of 4800 individual image pairs required to build the complete profile through the wall jet region. From the large number of individual velocity vectors recorded at each measurement location it was then possible to calculate time-averaged mean and fluctuating velocities, as well as local shear stresses. For comparison purposes, similar measurements in the water jet were made through the wall jet boundary layer at a radial distance of five diameters from the stagnation point.

16.3 Results and discussion Instantaneous laser images of the complete air jet flow, obtained by forming the light from an Nd:YAG laser into a planar sheet and recording flow images using the CCD camera, demonstrated high levels of intermittency in the outer regions of the wall jet region of the flow, and the presence of large-scale structures. These structures in turn caused atmospheric air to penetrate the wall jet and reach the surface of the plate very early on in the wall jet's development. Further experiments, in which the ambient air was seeded, demonstrated that the free portion of the jet flow generated entrainment lines that were at approximately 45 degrees to the vertical which led to the establishment of a large, low velocity recirculation zone of a size the same order as the impinged plate and with a time period of = 8 s. This carried material from the periphery of the wall jet back to its initial regions, causing low levels of jet material to persist to large distances from the surface. Mean radial velocity profiles through the wall jet region of the impinging air jet at four distances from the stagnation point, r, of 5, 10, 20, and 30 d are given in Fig. 16.1. In this figure, U represents the mean radial velocity, Vb the bulk mean vertical velocity from the pipe, and y the distance from the solid surface. The development of the wall jet region is clearly illustrated in these figures, as is the influence of the recirculation zone on flow velocities outside the wall boundary layer. Mean radial velocity data within the wall jet peak at small distances from the surface, with maximum values (Um) steadily decaying as the boundary layer mixes with ambient fluid. Peak values also increase in distance from the surface as the boundary layer grows in thickness and the influence of the solid wall increases. In addition, the influence of the recirculation zone is clearly seen at the outer limits of the wall jet region where mean radial velocities become negative. These velocities are relatively small compared to those encountered within the wall jet region itself, and show peak values which move progressively away from the surface with increasing radial distance. Maximum negative values are encountered close to the stagnation point where entrainment velocities into the jet are high. Data gathered at larger distances from the surface indicate that, at r/d = 10 for example, the recirculation zone extends to approximately y/d = 8.5. The presence of the

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recirculation zone also causes jet fluid from the outer regions of the wall jet to be re-entrained into its initial regions. This in turn causes jet fluid to persist to large distances from the surface and may, in some situations such as for releases of flammable material, allow ignition to occur at greater distances from the surface than might otherwise be anticipated.

Fig. 16.1 Mean radial velocity through the wall jet region of the impinging air jet at four radial locations

The large number of individual velocity vectors recorded at each measurement location within the air jet not only allowed mean radial and vertical (V) velocities (relative to the plate) to be determined, but also permitted the derivation of root mean square (r.m.s.) fluctuating horizontal (u) and vertical (v) velocities, and the shear stress (uv). Typical values, obtained at r/d= 10, are shown in Figs 16.2 and 16.3. The profile of mean vertical velocity within the air jet (Fig. 16.2) again illustrates the influence of the recirculation zone, with values of this velocity component becoming negative outside the wall jet region which, at this location, extends to approximately y/d = 1.2. Data gathered at larger distances from the surface again indicated that the recirculation zone extended to approximately y/d = 8.5. Data for r.m.s. fluctuating vertical (Fig. 16.2) and horizontal (Fig. 16.3) velocities are in qualitative agreement with earlier findings (1, 5, 10) with both data sets showing peaks within the wall jet region. Shear stress data (Fig. 16.3) is similarly in accord with earlier results, with values close to the surface being slightly negative, but then becoming positive as distance through the wall jet increases. Beyond the limit of the wall jet, at y/dx = 1.2, profiles of v'and v'asymptote to small, but non-zero, values due to the presence of the recirculation zone, with the uv profile also decreasing from its maximum within the wall jet region to effectively zero at y/d = 2.

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Fig. 16.2 Mean and r.m.s. fluctuating vertical velocities through the wall jet region of the impinging air jet at r/d = 10

Fig. 16.3 R.m.s. fluctuating radial velocity and shear stress through the wall jet region of the impinging air jet at r/d=W

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Mean and r.m.s. fluctuating radial, and vertical velocities through the wall jet regions of the impinging air and water jets at r/d = 5 are compared in Fig. 16.4, where results have been non-dimensionalized by the peak mean radial velocity, Um. These data show excellent agreement between measurements taken in the two flows, with the location of velocity peaks and the widths of the wall jet regions being in close accord. Some differences do occur, particularly in terms of the r.m.s. fluctuating vertical velocity, although this is to be anticipated since wall jet profiles for fluctuating quantities had not reached self-similarity by this measurement location (discussed further below). Values of the shear stress, not shown in Fig. 16.4, differed significantly at this location in the flow for the same reason. Overall, these results demonstrate that the structure and development of the two jets remains similar, despite the differing nature of the two flows. Importantly, however, data obtained for the water jet do not exhibit any influence of a recirculation zone outside the wall jet region, with mean radial velocities remaining positive at all positions through the wall jet. Preliminary data obtained at other radial locations and flow visualization results confirmed this finding. Given the similarity in the exit Reynolds numbers of the two flows, it is therefore evident that the occurrence of such recirculation zones is independent of Re and a function of local flow velocities, with the higher velocities encountered in the air jet triggering the onset of such zones.

Fig. 16.4 Mean and r.m.s. fluctuating radial and vertical velocities through the wall jet region of the impinging air and water jets at r/d = 5 (open circles - water jet, solid circles - air jet)

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The analysis of instantaneous velocity vector images to derive time-averaged mean and fluctuating velocities, and shear stresses, was carried out using purpose written software. In calculating such values, it was obviously important to use a sufficient number of vector images to ensure that statistically meaningful time-averaged data were derived. As an example, the influence of the number of vector images employed in determining r.m.s. fluctuating vertical velocity data through the wall jet region of the impinging air jet at r/d =10 is illustrated in Fig. 16.5. Results are given for 200, 600, and 1200 image pairs used to calculate time-averaged information at each measurement point. The number of images employed obviously has a critical effect on the derived time-averaged profile, with data determined using 200 images clearly exhibiting a great deal of scatter since the number of images used was insufficient to provide statistically meaningful r.m.s. values. The scatter is particularly large close to the surface and within the wall jet region at y/d < 1.2 where high r.m.s. values are encountered, and the influence of the wall and large mean radial velocities is greatest. The amount of scatter was reduced when 600 images were employed, although it was only with 1200 images and above that derived profiles became independent of the number of images used.

Fig. 16.5 Influence of number of velocity vector images on time-averaged r.m.s fluctuating vertical velocity data through the wall jet region of the impinging air jet at r/d = 10

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Comparison of the present data with earlier results, obtained using hot wire (10) and laser Doppler anemometry (5), is shown in Figs 16.6 and 16.7. These figures give, respectively, profiles of normalized mean radial and r.m.s. fluctuating vertical velocities, with the results having been non-dimensionalized by the peak mean radial velocity, Um, and the appropriate velocity half-width, 6. The lines used to represent the results of Dianat et al. (5) and Poreh et al. (10) are fits which include data from a number of measurement stations at similar radial locations (r/d > 9) to those examined in the present work. For the air jet, the present results are shown for all four radial measurement locations in Fig. 16.6, but exclude those from r/d = 5 in Fig. 16.7 since self-similarity of the r.m.s. profile had not been achieved by this measurement station. Measurements at r/d = 5 in the water jet are included in Fig. 16.6 for comparison purposes.

Fig. 16.6 Profiles of normalized mean radial velocity within the wall jet region

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Fig. 16.7 Profiles of normalized r.m.s. fluctuating velocity normal to the surface within the wall jet region of the impinging air jet

Agreement between the present air jet data and that of Dianat et al. (5) is in general good for both mean and r.m.s. fluctuating velocities, with the scatter observed in the present results being similar to that found in the earlier (5) LDA data. In particular, agreement for the mean velocity profiles is excellent over most of the range considered by Dianat et al. (5), with the original data of the latter authors again indicating that negative velocities did occur outside the wall jet region (10). The present non-dimensionalized air jet profiles of r.m.s. fluctuating velocity are also in good agreement with earlier results (5), both qualitatively and quantitatively, with peak values in particular being in close accord. There is, however, a slight underestimation by the DPIV measurements of both mean and r.m.s. values obtained using LDA at locations just outside the wall jet region at y/§> 1.0. Mean velocity data obtained for

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the water jet, at r/d = 5, is in reasonable agreement with the air jet results in the inner regions of the boundary layer, although some differences do occur very close to the impinged surface. These differences are most likely caused by a lack of self-similarity in the water jet measurements. Differences also occur towards the outer regions of the wall jet where close agreement with the results of Dianat et al. (5) is apparent. This divergence between air and water jet data is inevitable given the absence of any recirculation zone in the latter flow. Agreement with the data of Dianat et al. (5) may also be considered fortuitous given the fact that the original data of the latter authors indicates negative velocities at larger y/y. In contrast, agreement with the hot wire data of Poreh et al. (10) is less satisfactory. Mean velocities are again in reasonable agreement over the bulk of the wall jet region, but towards the outer edge of the boundary layer significant deviations begin to occur. The original data of Poreh et al. (10) do in fact indicate that values asymptote to zero in this region rather than becoming negative. This may be due to the absence of any external recirculation zone in the impinging jet configuration examined by these authors, or because hot wire anemometers are unable to discriminate between vertical and radial velocity components. Towards the outer edge of the wall jet region, therefore, and in the presence of a significant V velocity component, hot wires would record values of (U2 + K 2 ) 0 ' 5 IVb which must asymptote to zero. In addition, the present results overestimate peak values of the r.m.s. fluctuating velocity obtained by Poreh et al. (10), with the peak observed by the latter authors also occurring much closer to the impinged surface. Again, these differences may be due to the intrusive nature of the measurement technique employed by these authors, or the directional ambiguities encountered in using probe techniques in highly turbulent flows. Overall, therefore, good agreement is observed between data obtained in the present DPIV study and the earlier LDA measurements of Dianat et al. (5) particularly given the widely different conditions for which the data of the latter authors were obtained (i.e. methane jet firing vertically down, d = 10.8 mm, h/d = 12.82, V/,= 152 m s-1). Significant differences do, however, occur between data gathered using both of the latter laser diagnostic techniques and the intrusive probe measurements of Poreh et al. (10), calling into question the accuracy of probe measurement techniques.

16.4 Conclusions An experimental study of turbulent air and water jets that impinge orthogonally on to a flat surface has been described. This study was undertaken to resolve the uncertainties concerning the existence of recirculation zones raised by earlier work, to provide detailed experimental data for use in the formulation and validation of mathematical models of turbulent flows, and to improve understanding of what is a practically important flow configuration. The major conclusions of this work are: •

Useful data for mean and fluctuating velocities, and shear stresses, can be obtained using the DPIV technique, provided that a sufficient number of instantaneous velocity vector images are employed to ensure that statistically meaningful time-averaged data are derived.

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•

The present DPIV data are in good agreement with earlier results obtained in impinging jet flows using laser Doppler anemometry. Significant differences do, however, occur between data gathered using both of the latter laser diagnostic techniques and earlier hot wire anemometer measurements, calling into question the accuracy of probe measurement techniques.

•

Velocity field data obtained for the impinging air jet clearly demonstrate the existence of a large, low velocity recirculation zone in the impinging jet flow that carries material from the periphery of the wall jet back to its initial regions. In contrast, this feature is absent from measurements obtained in a water jet with similar exit Reynolds number. Given the similarity in the exit Reynolds numbers of the two flows, it would therefore appear that the occurrence of such recirculation zones is a function of local flow velocities.

References (1)

Cooper, D., Jackson, D. C., Launder, B. E., and Liao, G. X., 1993, 'Impinging Jet Studies for Turbulence Model Assessment - I. Flow-Field Experiments', Int. J. Heat Mass Transfer, Vol. 36, pp. 2675-2684. (2) Craft, T. J., Graham, L. J. W., and Launder, B. E., 1993, 'Impinging Jet Studies for Turbulence Model Assessment - II. An Examination of the Performance of Four Turbulence Models', Int. J. Heat Mass Transfer, Vol. 36, pp. 2685-2697. (3) Dianat, M., Fairweather, M., and Jones, W. P., 1996, 'Reynolds Stress Closure Applied to Axisymmetric, Impinging Turbulent Jets', Theoret. Comput. Fluid Dynamics, Vol. 8, pp. 435^147. (4) Craft, T. J., Launder, B. E., and Suga, K., 'A Non-Linear Eddy Viscosity Model Including Sensitivity to Stress Anisotropy', Tenth Symposium on Turbulent Shear Flows, Pennsylvania State University, 14-16 August 1995. (5) Dianat, M., Fairweather, M., and Jones, W. P., 'Predictions of the Concentration Field of an Impacting Turbulent Jet', Tenth Symposium on Turbulent Shear Flows, Pennsylvania State University, 14-16 August 1995. (6) Bradshaw, P. and Love, E. M., 'The Normal Impingement of a Circular Air Jet on a Flat Surface', A.R.C. R&M3205, 1959. (7) Childs, R. E., and Nixon, D., 'Simulation of Impinging Turbulent Jets', AIAA 23rd Aerospace Sciences Meeting, Reno, 14-17 January 1985. (8) Looney, M. K., and Walsh, J. J., 1984, 'Mean-Flow, and Turbulent Characteristics of Free, and Impinging Jet Flows', J. FluidMech., Vol. 147, pp. 397^29. (9) Beltaos, S., and Rajaratnam, N., 1974, 'Impinging Circular Turbulent Jets', ASCE J. Hydraulics Div., Vol. 100, pp. 1313-1328. (10) Poreh, M., Tsuei, Y. G., and Cermak, J. E., 1967, 'Investigation of a Turbulent Radial Wall Jet', Trans. ASMEJ. Appl. Mech., Vol. 34, pp. 457^163.

M Fairweather University of Leeds, UK G K Margrave and T C Williams Loughborough University, UK

17 Application of Particle Image Velocimetry to Helicopter Vortex Interactions R B Green and C J Doolan

Abstract This Chapter presents flowfield measurements of a simulated helicopter tail rotor/main rotor vortex interaction using a particle image velocimetry (PFV) technique with a transverse vortex generator in a wind tunnel. The generated vortex passes a fixed blade, which has its axis nominally orthogonal to the vortex. The PFV technique uses two PFV systems that allow individual vortices to be tracked as they pass over the blade, and this helps to isolate the details of the blade-vortex interaction. Close to the blade surface a significant radial flow out of the vortex core was observed which led to distortion of the vortex, hi addition a ridge of oppositely signed vorticity to the vortex was observed which is thought to be the result of the interaction of the vortex and blade boundary layer.

17.1 Introduction Blade vortex interactions (BVI) are a source of unwanted noise and vibrations in rotorcraft (1). hi this fluid dynamic event, the tip vortex generated from a main rotor blade can interact with either a following main rotor blade or travel downstream and collide with a tail rotor blade. In a tail rotor interaction, the vortex approaches the blade with their axes nominally orthogonal (Fig. 17.1) and the vortex is eventually cut by the blade. There has been much recent interest in the orthogonal interaction. This has included analytical (2, 3) numerical (4, 5) and experimental (5-9) studies which have shown that the vortex dynamics is controlled by the impingement of core axial flow on to the blade surface. Water flow visualization using pulsed laser induced fluorescence (5) shows vortex core bulging and thinning due to the impingement of the core flow on the blade. Surface pressure measurements (8, 9) indicate an

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increase in pressure where the core flow is directed towards the blade and a decrease when this flow is away from the blade.

Fig. 17.1 Schematic diagram of sense of orthogonal BVI and laser sheet alignment for PIV Although some flow visualization in air has been performed (7, 10) there have been no previous quantitative studies of the flowfield during an orthogonal vortex interaction. This is primarily due to the fundamental problems of probe interference and vortex wandering. Particle image velocimetry can provide a global, non-intrusive measurement system, which prevents the problems of probe-based techniques. Accurate flowfield measurements of the orthogonal vortex interaction are important for the better understanding of vortex dynamics and blade aerodynamics and also for the proper validation of computational fluid dynamics codes. This Chapter describes a PIV technique applied to the orthogonal blade-vortex interaction problem. To help account for the effects of vortex meander two PIV systems are used together, so that the effect of the interaction on the same vortex can be studied.

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17.2 Experimental method 17.2.1 Blade vortex interaction experiment The experiments were conducted in the Glasgow University 1.15 m * 0.85 m closed return low-speed wind tunnel. A unique vortex generator (9, 11) was placed in the contraction of the wind tunnel for the production of convecting three-dimensional vortices in the working section. The vortex generator is essentially a rotor rig, aligned in the horizontal plane, with a single blade of 0.75 m radius and 0.1 m chord with a NACA 0015 cross-section. During rotation, the blade pitch was varied using a spring-loaded pitch link running on a cylindrical cam, which created a useful vortex in the working section. A symmetrical blade of NACA 0015 profile and 152.4 mm chord (c) was placed vertically in the working section so that the convecting vortices were allowed to interact orthogonally with it. The leading edge of the blade was positioned between 12 and 13 chord lengths downsteam of the rotor rig centreline. The blade position was varied during the test in order to obtain interaction data at different chordwise locations. 17.2.2 PIV system 17.2.2.1 Illumination and photography The PIV system at Glasgow is a fully digital system and consists of commercially available, off-the-shelf components. Illumination is provided by a pair of Spectra-Physics GCR-130-10 frequency-doubled, double-pulsed Nd:YAG lasers running at a nominal repetition rate of 10 Hz. The pulse separation was variable between 20 (is and 250 us. For the present application the beams were combined using a Brewster plate beam combiner to produce two co-linear or nearly co-linear beams. After the beam combiner one beam was horizontally polarized and the other was vertically polarized. The beams were delivered in to the wind tunnel using a beam shaping telescope and cylindrical lens. The flow patterns in the wind tunnel were photographed using two 8-bit Kodak Megaplus ES1.0 digital cameras of 1 k x 1 k resolution operating in triggered double-exposure mode. Each camera was dedicated to its own laser, with which it was synchronized. Image capture was performed using two National Instruments PCI 1424 digital frame grabbers, and the cameras, frame grabbers, and laser synchronization system were programmed in Lab VIEW on a PC. The flow in the wind tunnel was seeded with a Shell 'Ondina' oil mist produced by a smoke generator, and the data sheets supplied by the manufacturer indicated a smoke particle size of 2 urn. It was observed that the polarization sense of the light reflected off the smoke was conserved. The experiment was designed so that each camera detected the flow pattern illuminated by its own laser. Therefore to prevent the light from one laser contaminating the image recorded by the other camera, polarizing filters were placed in front of each camera lens. Each filter was rotated to allow light from the appropriate laser through, which at the same time blocked out the light from the other laser. In principle, therefore, two vector maps of the same region or two closely neighbouring regions of the flow could be obtained with, in principal, a time separation in the range 20 us < At < 0.09 s. For the present application this feature allows the interaction details of the vortex to be isolated. 17.2.2.2 Analysis and validation system The analysis employed uses a basic cross-correlation and sub-pixel interrogation scheme. The Forward/Reverse Tile Test (FRTT) vector validation described by (12) is used to improve the quality of the vector map. This method basically treats the initial analysis as an estimate,

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which is then compared with two additional analyses at positions either side of the original; these two additional positions are displaced relative to the original in the positive and negative sense of the estimated vector. The three results are then compared, and a decision is made to the validity of the original estimate. Once the complete vector map has been produced a vector map validation scheme based upon that of (13) is used. A synthetic image analysis suggested an improved vector map quality, especially in areas of poor seeding. The analysis scheme was programmed in MATLAB and ran on either a UNIX or PC platform.

17.3 Results The significant features of the orthogonal BVI will be presented, and the vortex evolution as the vortex convects from the leading edge to the trailing edge will be described. For the BVI tests the wind tunnel was set at a speed U of 20 ms"1 and the rotor was set at a nominal speed of 600 r/min. The inter-pulse separation of each laser was 50 (is, although the delay between each laser was set as necessary. The rotor and laser system were synchronized so that the laser pulses illuminated the vortex as it passed through the required location(s) in the wind tunnel. To reveal the phenomena during the BVI the flow field was interrogated with the laser sheets at two positions away from the blade centre-line; 30 mm (0.2 c) and 15 mm (0.1 c). For purposes of brevity, figures will only be shown for those cases where interesting phenomena were observed. 17.3.1 Isolated vortex flow field Prior to the blade-vortex interaction study the vortex flow field on its own was measured using the PIV system. The PFV data showed that the vortex core size was some 15 mm and the vortex strength was 0.58 mV. These values agree well with an earlier hot-wire survey of the flow field (9). Another useful result from this aspect of the study is that the degree of wander of the vortex was observed to be some 4 cm in both the streamwise and cross-stream directions. This underlines the difficulty of using single point probe-based techniques in this type of unsteady flow. 17.3.2 BVI flow field: laser sheet at 0.2 c from blade centre-line By varying the delay between the two lasers and by moving the blade along the wind tunnel working section a complete picture of the BVI with the vortex at various positions over the blade chord was built up. At this distance from the blade centre-line the flow field (velocity and vorticity) revealed no significant structural changes of the vortex during the interaction along the entire chord length of the blade. 17.3.3 BVI flow field: laser sheet at 0.1 c from blade centre-line The BVI flow field was built up in a similar way to above. Note that at the maximum blade thickness position the flow visualization was at a distance of only 0.02 cmm from the blade surface. Figure 17.2 shows the vortex just ahead of and just behind the leading edge of the blade. This figure was obtained using the PIV system in its dual camera/laser configuration, and therefore Figs 17.2(a) and 17.2(b) show the same vortex separated in time by 1.7 ms (equivalent to 0.22 chord lengths). The leading edge is indicated on each frame, and the mean u-component has been removed to highlight the vortex flow field. In Fig. 17.2(a), just prior to interaction, the velocity vectors show the appearance expected of an isolated vortex flow; the centre of the vortex is clearly visible, and all the vectors appear to be normal to a line drawn to the vortex centre. In Fig. 17.2(b) the centre of the vortex is still clear although the velocity

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vectors have a significant component in the radial direction away from the centre of the vortex; the appearance of Fig. 17.2(b) is that the flow is spiralling outwards from the vortex core. This behaviour was repeatable, and is a genuine feature of the orthogonal BVI at this laser sheet location.

Fig. 17.2 Velocity map of orthogonal blade-vortex interaction; vortex passing leading edge. The mean u-component has been removed. Laser sheet at 0.1 c from the blade centre-line, and the blade leading edge is indicated by the dashed line. Note that the position of the y-origin is arbitrary. Key: n maximum magnitude velocity vector

Figure 17.3 shows the vortex over the mid-chord, with the light sheet 0.1 c from the blade centre-line. The velocity vectors in this figure reveal the spiraling first observed as the vortex passed the leading edge, except that the effect appears to be stronger and a larger area of the flow field is affected. Finally, Fig. 17.4 shows the vortex in the trailing edge region of the blade. Again the spiral appearance is observed in the vortex flow field, although the effect seems to be weaker than in Fig. 17.3.

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Fig. 17.3 Velocity vector map of the orthogonal BVI; vortex over mid-cord. The y-origin is arbitrary. Laser sheet at 0.1 c from blade centre-line. The mean u-component has been clarity. Key: n maximum magnitude velocity vector, 11 mm"1

Fig. 17.4 Velocity vector map of the orthogonal BVI; vortex in trailing edge region. The y-origin is arbitrary. The blade trailing edge is shown by the dashed line. Laser sheet at 0.1 c from blade centre-line. The mean u-component has been removed for clarity. Key: n maximum magnitude velocity vector, 8.5 ms"'

17.4 Discussion The visualization of the flow at 0.2 c from the blade centre-line showed no noticeable effect of the interaction upon the vortex structure. However, the visualization at 0.1 c revealed a spiralling motion outward from the vortex core, which was present as soon as the vortex passed the leading edge.

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The spiral effect observed in Figs 17.2(b), 17.3, and 17.4 may be shown to be the superposition of a strong radially outward flow, caused by a source type flow, upon the vortex flow field. This may be achieved by isolating the radial and circumferential velocity vectors from Figs 17.2, 17.3, and 17.4 for a polar co-ordinate system with its origin at the vortex centre. Alternatively the isolated u- and v-components from Figs 17.2, 17.3, and 17.4 produce patterns that can readily be shown from simple potential flow theory to be the result of a superposition of a source and a vortex. Clearly the flows in this case are much more physically complicated, but for illustrative purposes the analogy is valid. From the isolated u- and vcomponents the 'source' strength relative to the vortex strength can be calculated. For Fig. 17.2(b) the ratio of source to vortex strength is approximately 0.44, while in Figs 17.3 and 17.4 the ratio is 1 and 0.29 respectively. A comparison of the peak radial and circumferential velocities also yields useful data. In Fig. 17.2(b) the peak radial velocity was observed to be the same as the peak circumferential velocity at 7.5 ms"1, while in Fig. 17.3, however, the peak radial velocity is much greater than the peak circumferential velocity (10.5 ms"1 compared to 7.5 ms"1). Figure 17.4 revealed a weaker radial flow velocity (5.5 ms"1 compared to 7.5 ms-1). This behaviour was typical of all cases observed at these chordwise locations. The hot-wire measurements described by Doolan et al. (9) of the isolated vortex revealed a strong axial velocity component (this axial flow is typical of helicopter blade trailing vortices). For the side of the blade under investigation, the axial flow is towards the blade, and the hot-wire measurements indicate a velocity magnitude of approximately 25 per cent of the free stream speed, or 5ms-1. It is therefore reasonable to argue that the radial outflow is due to the stagnation of the axial velocity at the blade surface, which must be relieved by a radial outflow from the vortex core, which effectively generates a source component. The smaller radial velocity observed in the trailing edge region would then be the result of the surface of the blade receding away from the direction of the axial flow; the receding blade surface velocity, based upon the assumption that the vortex convects at a nominally free stream value, reduces the isolated vortex axial velocity by approximately 2 ms-1. Note that the term 'radial' has been applied loosely. The authors acknowledge the highly three-dimensional nature of the flow field; strictly speaking the radial component discussed is in a plane parallel to the local plane of the blade, and the true alignment of the vortex axis is not generally orthogonal to this plane. Therefore there is a component of the velocity that has not been measured. There are plans to extend the study using a stereoscopic system that will account for this, and will more fully complete the physical description of the phenomena. The above radial outflow ought to provide a considerable convective transport mechanism for the vorticity (ca), which will tend to expand the vortex rapidly. The isolated vortex (not shown) has a well-defined vortex core, and the vorticity contours form slightly skewed concentric circles with a clear vortex centre. Figure 17.5 shows the non-dimensional vorticity contours ^jy from Fig. 17.3. It is clearly seen that the vortex core is diffuse, with no clear central vorticity peak. A surprising observation, however, is the weak ridge of negative vorticity above the vortex. Great care was taken in the analysis of the images to ensure that this feature was not an artefact; the vector validation parameters were examined, and the images were re-analysed using different tile sizes and positions. The positive vorticity was observed when the vortex was in the mid-chord region. With the laser sheet at 0.1 c from the blade centre-line, the maximum thickness point of the aerofoil surface was only some 0.02 c away from the laser sheet. Using turbulent flat plate correlations, calculations indicated a boundary layer thickness of some 0.01 c at the maximum blade thickness position. The positive vorticity may therefore be a result of vortex/boundary layer interaction. It is not clear what the details of this are, and further testing is necessary.

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Fig. 17.5 Non-dimensional vorticity contours from Fig. 17.3. Vorticity co non-dimensionalized as toc/U. Light sheet at 0.1 c from blade centre-line. The y-origin is arbitrary

17.5 Conclusions A study of the orthogonal blade-vortex interaction has been performed using PIV. The PIV method was refined to allow velocity maps of the same region to be obtained in rapid succession. This feature allowed the effect of the blade upon the same vortex during the interaction to be studied. At a large distance from the blade, the effect of the interaction upon the vortex structure is weak. However, close to the blade surface the interaction causes a significant radial flow from the vortex core outwards which diffuses the vorticity. The strength of this source flow was estimated and was found to vary along the blade chord, with a maximum value in the midchord region.

17.6 Acknowledgements The work was carried out with the assistance of the University of Glasgow, the Royal Society, and the EPSRC.

References (1) (2) (3)

Sheridan, P. F. and Smith, R. P. (1980) 'Interactional Aerodynamics - a New Challenge to Helicopter Technology'. Journal of American Helicopter Society, 25:3-21. Howe, M. S. (1989) 'On Unsteady Surface Forces, and Sound Produced by the Normal Chopping of a Rectilinear Vortex'. Journal of Fluid Mechanics, 206:131-153. Marshall, J. S. (1994) 'Vortex Cutting by a Blade, Part 1: General Theory and a Simple Solution'. AIAA Journal, 32(6): 1145-1150.

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(4) (5) (6) (7) (8) (9) (10)

(11) (12)

(13)

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Lee, J. A., Burggraf, O. R., and Conlisk, A. T. (1998) 'On the Impulsive Blocking of a Vortex Jet'. Journal of Fluid Mechanics, 369:301-331. Marshall, J. S. and Krishnamoorthy, K. (1997) 'On the Instantaneous Cutting of a Columnar Vortex with Non-Zero Axial Flow'. Journal of Fluid Mechanics, 351:41—74. Ahmadi. A. R. (1986) 'An Experimental Investigation of Blade-Vortex Interaction at Normal Incidence'. AIAA Journal, 23(l):47-55. Johnston, R. T. and Sullivan, J. P. (1998) Unsteady Wing Surface Pressures in the Wake of a Propeller. In AIAA Paper No: 92-0277. Doolan, C. J., Colon, F. N., and Galbraith, R. A. McD. (1999) 'Three-Dimensional Vortex Interactions with a Stationary Blade'. The Aeronautical Journal, 103; 579-587. Doolan, C. J., Coton, F. N., and Galbraith, R. A. McD. (2001) 'Surface Pressure Measurements of the Orthogonal Vortex Interaction'. AIAA Journal, 39; 88-95. Gary, C. M. (1987) 'An Experimental Investigation of the Chopping of Helicopter Main Rotor Tip Vortices by the Tail Rotor. Part ii: High Speed Photographic Study'. Contractor Report 177457, NASA, September 1987. Copland, C. M. (1998) 'Methods of Generating Vortices in Wind Tunnels'. PhD Thesis, Department of Aerospace Engineering, University of Glasgow, 1998. Green, R. B., Doolan, C. J., and Gannon, R. M. (2000) 'Measurements of the orthogonal blade-vortex interaction using a PIV technique' Experiments in Fluids, 29; 369-379. Nogueira, J., Lecuona, A., and Rodriguez, P. A. (1997) 'Data Validation, False Vectors Correction and Derived Magnitudes Calculation on PIV Data' Meas. Sci. Technol., 8: 1493-1501.

R B Green and C J Doolan Department of Aerospace Engineering, University of Glasgow, UK

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18 Recognition of Two-phase Flow Patterns with the use of Dynamic Image Analysis R Ulbrich, M Krotkiewicz, N Szmolke, S Anweiler, M Masiukiewicz, and D Zaja_c

Abstract For many years interest in multiphase flow has been increasing in heat science and flow mechanics. Multiphase flow appears in many technique domains. It is a very common phenomenon in chemical, agricultural, food, cooling and power industries, and environmental or processing engineering. The techniques described here are as valid in gas-liquid as in gassolid flow. The recognition of two-phase flow patterns is essential in process engineering. This kind of flow pattern is the most important criterion that enables correct calculation of processes of the heat and mass transfer. Researches of fluidization are very significant from the practical point of view, because they enable, for example, minimization of energy consumption in pneumatic transport or of selection and keeping proper parameters in fluidic power boilers.

18.1 Introduction For the proper running of most apparatus, in which two-phase flow appears, generation of strictly specified two-phase flow pattern is required. Investigation of the process in industrial conditions requires the use of an objective method of identification, which is based on measurable features, which are characteristic for the specified two-phase flow. As far as single-phase flow is concerned, definition of distribution of velocity is enough but in twophase flow, the definition of phase concentration is also very important (1). There has been significant research in to the recognition of phase concentration and various measurement techniques have been developed.

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Most of these measurement methods are based on a visual estimation of the flow structure, which is performed by hand using a comparison scale to draw conclusions. Unfortunately, the results of this approach are very subjective. In this connection it is necessary to develop a universal, precise, quick, and cheap method of measurement. Digital processing image methods have this advantage. In comparison to commonly used methods of recognition of two-phase flow structures the distinguishing marks of digital methods are fast time of response, repeating results, and elimination of the subjective human factor for direct measurements. This method can be used in a range of hostile environments where human intervention is not possible (2).

18.2 Basic concept The research work aimed at developing techniques for the recognition of two-phase flow patterns with the use of dynamic image analysis. In the case of gas-liquid flow the continuous phase was coloured methyl alcohol and the dispersed phase was air (this choice was made because of combination of air and alcohol gives better contrast between two phases then air and water). In the case of gas-solid flow the continuous phase was air, and the solid phase were spheres of 2.5 mm diameter and 800 kg/m3 density. These two kinds of flow have been realized in flat channels with rectangular profile. Proper regulation of intensity of airflow (in both cases) and proper dosing of liquid and solid phases allows realization of each type of flow. The first step was to record each individual flow pattern using a video camera, in this case a SVHS C JVC GR-S 707 video camera. The structures obtained were recorded with the shutter speed of 1/1000 s. The acquisitioned object was illuminated with a halogen spotlight. A light dispersion filter was placed, between the lamp and the channel. After recording the dynamic changes of the flow on videotape, digitalization was started to change the analogue form of the image (magnetic record) to a digital form. With the use of a frame grabber video card and ADOBE PREMIERE* software on a PC computer in WINDOWS® 98 environment, this step in the image processing was realized. Image preprocessing included several improvements such as contrast and brightness adjustment for each realization of flow structures. In addition the colour image was converted in to monochromatic. After the internal MPEG compression was made by the video card, realizations were saved as AVI files. The recorded sequences have 24-bit depth of colours, and thanks to a low compression coefficient, they were very good quality. The main part of the digital image processing research was realized with the use of our own software. In this program recorded changes of phase concentration distribution were put under analysis. The program allows the flow structure analysed at any one part of the film and any time. By using a suitable size of zone - probe (there is possibility of adjusting the area of the probe and choosing between single and multiple probe) - analysis of the most interesting part of flow was possible. The result of this analysis was a two- or three-dimensional matrix of data (it depends on the probe used), which represents the sequence of grey level, changes. In the area of the probe, the mean grey level counting, have been made. Obtained values of grey level were included in range of 0-255, where 0 is white and 255 is black. A zero value can be treated as 100 per cent gas-phase (but fully transparent). On the other hand a value of 255 is a pure solid phase (black) in the fluidization process. The liquid phase has much lower grey level values than solid phase (3).

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The dynamics of change, while the two-phase flow persists, has been signified as the function of grey level rate in time domain. The unit was frames. Knowing the frequency rate of the video camera recording, which is 25 Hz, this rate was converted in to seconds and the new received unit was 1 frame = 0.04 s. The results of the analysis have been transformed in to graphical form.

18.3 Gas-liquid flow 18.3.1 Flow pattern classification and probes characteristics Figure 18.1 represents classified individual structures of the gas-liquid flow, Fig. 18.2 shows the sizes and situations of the probes used. Realizations were the subject of an examination with four types of zone probes - the superficial probe [Fig. 18.2(a)], the elongated probe [Fig. 18.2(b)], the spot probe [Fig. 18.2(c)], and the multiple probe [Fig. 18.2(d)]. Each of these probes allows different kinds of flow pattern to be obtained. In this case characteristics for each probe has been made.

Fig. 18.1 Representative frames for each type of flow (3)

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Fig. 18.2 View of the used zone probes (3)

The size of the elongated and spot probes have been adjusted in order to get the object like a bubble that appears and disappears in the area of the probe during the movement from one frame to another. The distance that the bubble passes through is ten pixels long. That is why the width of these probes is set up on ten pixels. With the superficial probe the largest area of the channel has been analysed. Considering this probe, each acquired graph has the mean gray level placed in the lowest level among all other types of probes for the structure taken. This level was chosen as a basis from which positive deviation took place (upwards in gray level values). The individual feature of this probe is that it has the lowest, among all other, amplitude of realizations obtained. The other characteristic is that it has the lowest sensitivity. It is the result of the biggest averaging area. The big measurement area implies that the time of being single objects is long enough to collect many in this area. The time of acquisition of such objects was quite long. The objects, which flow in and out from the examined area, are hardly seen on the graph. Only big amounts of these objects or one big object can be seen. In case of small objects or big ones, consisting of many smaller ones (agglomerates), when their amount flowing in to the examined area is bigger than the amount flowing out, then the impulse is increasing. It is increasing until the time, when the proportions of the flow in and flow out are inverted. Then the impulse is decreasing. In the case of big homogeneous objects, such as plugs and bubbles the situation inverts. To sum up, this probe gives a general view dealing with the specific flow structures. It describes changes of grey level value with global characteristics rather than local ones. This means that it is much easier to notice the presence of areas with greater density of gas phase in the form of various objects such as single bubbles or agglomerates than single objects. The elongated probe, because of its similar shape but smaller area, is a more sensitive version of superficial probe. The increased sensitivity results from the fact that objects in the time of passing from one frame to another appear in the area of this probe and then disappear. The frames obtained by this probe feature greater amplitude in comparison to the superficial probe. The explanation of this fact is that the proportion of the area of the single object, like a bubble, to the area of the whole probe is much greater than in the case of the superficial probe. That is why the object is better visible on the frame. Thanks to the length of the probe, which is ten pixels, the density of the impulses on the graph is much higher, which means that even single objects, such as bubbles, are shown. Due to the much smaller length of the

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described probe, shorter time of acquisition of the object implies the reduced width of its impulse, hi spite of the reduced length of this probe, the fact that its width remains unchanged allows the conclusion of the character of the examined structure. That is why the graphs of this and the previous probe cover each other (3). The spot probe covers the smallest area among all others. In this connection, the relation of the area of the examined object with the area of the probe is the highest and is almost unity (this happens when a whole object covers the area of the probe). It influences the highest amplitudes most. In other words, the spot probe has the highest sensitivity. Because it has the same length as the elongated probe, the width of the generated impulse, in both cases, is the same. Additionally the grey level value obtained by the probe for a specific object corresponds with its real value. The reason of this is the fact that the probe does not average this value with the values of the other objects, which falls in to the probe area (the probe's size is slightly larger then the smallest possible object). On the basis of the value obtained we can conclude the type of examined object, but we can not conclude the type of flow structure. Three examples of the results are shown on the Fig. 18.3.

Fig. 18.3 Examples of the courses obtained by used probes (3)

18.3.1.1 Type of objects and their effect on the results On the received courses the following kinds of objects have been observed: • single bubbles; • agglomerate constructed with single bubbles;

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• big bubble;

• plug; • foam. The important feature related to the two-phase flow is the interfacial surface. The refraction of light is occurring on it. Because of this, the interfacial surface is recorded as a part of the image with the greatest grey level value. The fact that the objects (gas structures) are in reality convex, causes that observed line of the interfacial surface to be thick (and that is why it is dark). Single bubbles are usually of small size. Because of this, their edges are close to each other. This is why these objects have a thick border that causes their darker colour from the enclosed liquid. The form of the graphical representation of the objects is the peek, which goes upwards - in the direction of ascending grey level, values. The superficial probe doesn't detect the single bubble - we have no clear signals on the trace recorded by this probe because the value, which represents this object, is getting lost among the averaged background and values of the other objects. The elongated probe gives the response in form of peeks with 0.7-1.5 amplitude in grey level scale. Only for SBl-flow is it possible to give the amplitude for the single bubbles. In the case of other kinds of flow, we are then counting the larger quantity of the objects crossing the area of the probe in one moment. The spot probe gives the clearest peeks, whose amplitude is placed in the 4-14 range of the grey level scale. It always gives the right information about the single bubble. Agglomerates are conglomerations of the single bubbles, and therefore their view has a form of a larger object, but with the similar grey level value, as for the single bubble. Also the view of the impulse is similar - which is oriented upwards on the ascending grey level values. Agglomerates, treated as a single object, are properly represented on the traces of the superficial and elongated probes. The result of its analysis is an increasing impulse. The signal strength depends on the size of the agglomerate and its interfacial surface. The interfacial surface depends on the packing density of single bubbles. The spot probe gives us the view of the agglomerate that depends on which part of the agglomerate crosses the probe area. If the interfacial surface is crossing the area of the probe, the impulse increases, and if the body of the big bubble crosses the probe area, the impulse decreases. Big bubble and the plug are big and homogenous objects, so their interfacial surface has no influence on the character of impulse. That is why the graphic representation on the signal is the clear peek directed downwards on the grey level values. The superficial probe records those objects as a wide decreasing impulse with big amplitude. The signal of these objects obtained by the elongated and spot probe is similar. They are narrow peeks, directed downwards, but they differ in amplitude: the elongated probe gives the impulses of amplitude from 8-10, and the spot probe from 25-30 in grey level value scale. The above description and analysis of particular impulses on the given records, are just samples of recognition of the particular patterns, based on such characteristics as amplitude, frequency, width, and kind of impulse, which is shown below in Table 18.1. To show other characteristics, which can be used to describe individual structure, composition of courses given by three probes has been made for each concerned flow type.

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Table 18.1 Composition of the amplitudes for each individual flow structures (5) Flow structure

SB 1

Superficial probe Elongated probe 0.7-1.5 Spot probe 3-10

SB2

DB1

DB2

BB

P

F

A

1-2 3-9 5-19

1-3.5 3-10 5-32

1-4

0.7-1.5 3-10

1-1.5 3-7 5-19

5-15

7-25 5-35

5^0

10-50

10-40 10-50 15-60

The first conclusion that can be drawn from these compositions was the order of appearance of the traces: the lowest was the trace obtained by the superficial probe, next was the elongated probe trace, and the highest was the trace recorded by the spot probe. This configuration was characteristic for each of the structures concerned. Although they reveal other interesting characteristic these traces draw aside while the velocity of flow increases, and after the extreme values of the flow velocity have been reached these traces are brought together. This allowed the making of a juxtaposition of the mean grey level values for each realization, related to each individual structure, segregated by increasing velocity of the mixtures flow (see Fig. 18.4). After the analysis of juxtaposition on Fig. 18.4 confirmation has been made. For the spot probe we noticed that from the SB1- to P-structure, the graphs curve increases minimally which is caused by the sequence appearance of the greater number of the single bubbles and/or agglomerates. Their high grey level value is compensated by big bubbles, which appear in the next structures (their low grey level value). This is why the curve has a small slope in the examined range.

Fig. 18.4 Juxtaposition of the mean gray level values for each realization (5)

From P- to F2-structure we now have very high velocity of flow. It is the reason that the big bubbles have irregular shape and therefore superficial surface is bigger. It is also the reason of the larger part of agglomerates and big quantity of small bubbles, which are often rotating backwards (it causes them to remain longer in the probe area). This is why the curve in this area has a violent increase. After the next increase intensity of flow and creating A-structure, disappearing of objects such as single bubble, agglomerate, or foam has been observed.

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However in the centre there persists a bright gas core, which is periodically drowned (because of immovability of the liquid phase). Now gas is creating a film of liquid on the channel walls. From the analysis of grey level value, we can speak about the replacement of the dispersed phase in to a continuous phase - and now low values are averaged by high values. This result is the decreasing slope of the curve (3). The elongated probe better shows the amount of small objects such as agglomerates or single bubbles. This can be seen on DB2- and BB-structures where the curve gently increases. Also increased amounts of those objects in type P-structure is accented clearly. Whereas in case of F-structure we can see that breakdown and violent decreasing of curve until the A-structure took place. Together with stream volume velocity of gas, more and more fine bubbles appear - dark objects. In case of SB1- and SB2-structure the image is similar, because the recording is made in chosen channel fragments. For SB2-structure, bubbles flow in three rows and the recording is made only in one of them, which equals the results with SB2. From DB1- to BB-structure we have progressive increasing of single bubbles and agglomerates. All of those objects are characterized by a greater optical density than the liquid. This happens because of a thicker interfacial surface line. Therefore the curve increases until big bubbles appear. When, in the flow structure, big bright bubbles begin to dominate, the compensation of dark small objects starts, and graph curve decreases a bit. This happen in the BB- to F-structure range. For the Fstructure, where long and big bright objects appear in the probe area, a considerable drop of grey level value follows. For the A-structure, where there are long periods of gas core appearance, the curve on the graph falls violently (brightening). Crossing of the spot probe curve and superficial and elongated probe curves in case of Astructure, comes from the spot probe characteristics. This probe is situated in the middle of the channel, and during the recording it detects only gas core or flooding moment. It does not record (and in reality does not average) the influence of the liquid film that remains on the channel walls. Superficial and elongated probes as insignificant blackening record this liquid film. 18.3.2 Gas-solid flow In the case of the gas-solid flow, except the digital image processing and analysis method, the analysis of random signals has been used. Theoretically, these two methods can work together for the best results in two-phase flow pattern recognition. Therefore the research in this area was oriented to verify if the connection of these two methods would give a rational effect, hi practice many of the physical phenomena, which can be described with mathematical dependences, has determined character. However there are a great deal of physical phenomena, which have non-determined character, and can not be described with mathematical dependences. There is no possibility of predicting the value of such a signal in any moment in the future. These signals are random by nature, therefore the result of each particular observation is only one of the infinitive numbers of possible results, which can happen. That is why these phenomena have to be described with averaged statistical characteristics (4). So far, the description of the phenomena, which accompanies the two-phase flow, is the result of simplification of a complicated character of the process. The next reason for the simplification is limited experimental verification of the results, because analog measurement

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methods enable the comparison of stabilized processes only. We made an assumption that fluidization is a stationary process, because nonstationary processes preclude the extraction of a sufficient number of time functions (series or random functions). And because characteristics of unsteady random processes are time functions, which can be specified only by averaging actual values in random functions ensamble which form the process, we are not able to acquire exact measurement of these characteristics. This circumstance makes the development of practical measurement and analysis methods difficult (5). 18.3.2.1 Random signal analysis To describe the main qualities of random signals we apply, among others, the statistic functions such as probability, density, and autocorrelation. Probability density function describes the properties of the process in value (amplitude) domain. Autocorrelation function gives us the information of the process in the time domain. Probability density function of the random signal determines the probability of an event, which means that the values of the signal at any moment is included in the particular range. The main goal of the probability density of the physical signal measurement is to establish the statistical laws concerning the distribution of its actual values. Nevertheless this function can be applied to distinguishing the harmonic signal from the random signal. Besides distribution and shape of the probability density, fiction allows the experienced specialist to reveal the non-linearity of physical effects. To simplify, the autocorrelation function of the random signal determines the overall dependence of the actual value to the value in other moments. The main application of the autocorrelation function of the random signal is the examination which consists of determining to what degree the value of a process at a particular moment has impact upon the value of that process in a moment in the future. In the case of determined signal, the autocorrelation function 'lasts' for all time displacements. And autocorrelation function of the random signal approaches zero for big values of displacement. In that case autocorrelation function is a good instrument for determined processes detection, which can be 'masked' by random noise (5). 18.3.2.2 Results The experiment was two-way guided. Two methods used and combined at the end. First of all the fluidization process was investigated by visual methods and presented as a change of gray value function [Fig. 18.5(a)]. From the data obtained this way, graphs of probability density distribution [Fig. 18.5(b)] and autocorrelation [Fig. 18.5(c)] were made. The results were elaborated for various two-phase gas-solid flow patterns.

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Fig. 18.5 Change of gray level, probability density, and autocorrelation graphs - examples (5)

The image that is generated by video camera is two-dimensional. Therefore the events, which happen near the back wall of the channel, can not be recorded by objective of the camera. In connection with that, there is proposal of realizing further research in the apparatus with more flat channel to be able to avoid the processes, which happen deep inside the bed. This will allow focusing exactly on the events that happen next to the front wall of the channel. In almost all cases of the autocorrelation function, the graph presents a curve that resembles a sine curve [see Fig. 18.5(c)]. hi all other cases the curves demonstrate repeated cyclic formations. This suggests determinated character of the fluidization process, at least in case of blister and gas trap fluidization. In recapitulate there is the possibility of determination of the flow structures with the use of an image recognition method. In addition, by combining the analysis of gray level and random signal analysis methods, we can achieve satisfactory results.

18.4 Conclusions On the grounds of realization the research work and detailed analysis of theoretical basis of the digital dynamic image process and analysis. And by employing the known fundamentals

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217

of two-phase flow pattern recognition using the random signals analysis, it was found that each of these methods and their combination as well, is an appropriate and applicable research tool in the area we talk over. In the multiphased research, where there is the need to predict the specification of the flow structures, the methods presented meet these needs. These methods are very precise and sensitive, which testifies the recording of even the least perturbations, for example the non-uniform lighting, which took place during the research work. For the matter of further research in the field of two-phase flow, it is possible to determine interfacial surface. During the analysis of acquired results, unequivocally identify property have been noticed. It characterizes with the highest gray level values. Taking this in to consideration we can calculate the number of pixels, where brightness is situated in the former specified range characteristic for the interfacial surface. Then we can count the pixels' surface, that is, we can calculate the interfacial surface. There is another variant possible too. By analysing the given flow pattern stochastically, we can create a probability density graph. From it, we can obtain the fraction of pixels whose brightness is contained in the interfacial surface range. In connection with above the digital image processing and analysis method is the research tool with great possibilities, which is precise and modifiable for the particular applications. This method can be support for the two-phase flow research.

18.5 References (1) (2) (3) (4) (5)

R. Ulbrich, Identification of two-phase gas-liquid flow, qualifying as assistant professor work, Opole Technical University, 1989. G. P. Celata, P. Di Marco, and R. K. Shah, Two-Phase Flow Modelling and Experimentation 1999, Pisa 1999. M. Masiukiewicz and D. Zajqc, The analysis of two-phase flow gas-liquid with use of digital image processing, Thesis, Opole Technical University, 2000. N. Szmolke, Method of appreciation of the fluidised bed structure, PAN Chemical Engineering Institute, Gliwice 1997. S. Anweiler, The analysis of two-phase flow gas-solid mixture with use of image recognition, Diploma paper, Opole Technical University, 2000.

R Ulbrich, M Krotkiewicz, N Szmolke, S Anweiler, M Masiukiewicz, and D Zaj^c Heat Technique and Process Engineering Department, Opole Technical University, POLAND © With Authors 2002

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19 Gas/Liquid Mixing: Simultaneous PIV Measurements of Two Phases Mixing Together; High-pressure Spray Application T Boedec and S Simoens

Abstract The aim of this work is to provide an efficient tool to describe the mixing between liquid and gas phases like, for example, high pressure spray atomization in quiet ambient gas. Out of the basic research interest this is of very high importance for car engine or agricultural spray manufactories. We describe a new planar visualization technique which allows simultaneous measurements of droplet and ambient gas velocity. It combines planar Mie scattering diffusion and planar laser induced fluorescence. Digital image treatment and analysis are used to discriminate the two phase flow information necessary to apply PIV. The basic principle is to seed the ambient gas and to tag the liquid with fluorescent. A synchronized system combining two different CCD cameras (one mounted with a filter) and two Yag lasers, respectively used to record images and to generate light sheets, was designed. We show the steps involved in discriminate gas from liquid information. This technique is efficient independently from the droplet shape. Furthermore this technique furnishes some instantaneous results (velocity field and droplet apparent diameter) which is of high importance for gas/liquid mixing. Instantaneous liquid and ambient gas velocity field examples are given in case of 6 MPa pressure spray.

19.1 Introduction in the past twenty years, some optical techniques were developed for characterizing liquid or gas velocity in two-phase flow studies. Few of them are able to deliver simultaneously gas and

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Optical Methods and Data Processing in Heat and Fluid Flow

liquid characterization (velocity and shape) at the same location. Recently, single point optical measurements were largely used, like laser Doppler velocimetry (LDV) (1, 2) and phase Doppler particle analysis (as PDFA) (3, 4, 5, 6). They demonstrate accuracy for measuring the size and velocity of isolated particles in the case of spray atomization phenomena. Unfortunately, in such case (spray atomization), the presence of non-spherical particles is frequent and these kind of techniques (PDPA) can only characterize spherical droplets (7). Besides, even if seeding particles and droplets are both spherical it might be difficult to extend these methods (which can separate respective seeding by sizing) to discriminate between the different phases because (8) the droplets size distribution generally overlaps with that of the seeding particles, for example. Some partial solutions have been found like tagging liquid with fluoresced dye (9,10). On the other hand, there is a large use of PIV (11) or qualitative visualization techniques which contribute to a better knowledge of velocity fields or spatial structure organization (12) of liquid and gaseous phase. The direct visualization of the droplet images is obtained regardless of their size or their position. For the studies of spray with mixing of liquid phase with the ambient gas and characterization of the velocity field of both phases, we need, unambiguously, to discriminate between the droplets issued from the spray atomization and particles seeding the ambient gas (13). Previous works (14, 15) are concerned with the application of visualization techniques to this kind of flow. But to our knowledge, few are able to describe velocity statistics simultaneously for the ambient gas and the liquid phases in the same measurement location. Consequently, we developed a visualization technique to achieve the discrimination of water droplets and particles seeding the gas (16). Associated with PIV, this allows us to simultaneously determine velocity fields of gas and liquid phases in the same location. Combined techniques are now currently used to simultaneously obtain different scalar quantities or same quantities from different flows mixing together. Recently we have noticed the mass fluxes measurements (17,18,19), the segregation coefficient measurements (20), or the velocity field determination of impinging jets (21). The first one is used for quantitative measurements both in air and water and provides simultaneously at the same location the velocity and the concentration fields of turbulent flows. A turbulent flow is seeded simultaneously with fluorescent dye or solid particles for concentration determination and solid particles for PIV analysis. The second one is devoted to the mixing of two turbulent flows and allows simultaneous concentration fluctuation determination, of two different passive scalars, at the same location. Two turbulent flows are seeded with different fluorescent dyes. In spite of spectrum overlapping, separation of fluorescent intensity was obtained. This provides quantitative segregation coefficient for two species. The third one allows qualitative simultaneous determination of the velocity fields of two impinging jets. The first flow is seeded with droplets tagged with fluorescent dye and the other one with nontagged droplets. This allows droplets seeding both flows to be separated in order to apply PIV. This is a combined approach using LIF and PIV. Unfortunately the principle which uses color PIV (22) doesn't succeed in separating the flows at two succesive instants t and t + 5t and would be difficult to apply to spray atomization study. Our approach doesn't contain such problems. The spray is a complex flow whose understanding is crucial, for example, for car engine improvment. This is a liquid jet which is submitted to an atomization providing liquid droplets which have their own dynamic and entrain the ambient gaseous phase in the core of the flow. It is necessary to determine the correlation of the dynamic of both phases,

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furthermore, simultaneous information leading to phase contact quantification will improve our knowledge of mixing processes for such flow. In the present work velocity fields of both phases are obtained by cross-correlation PFV. It necessitates images from non intrusive particle or droplet which seed separately, both phases. For the liquid phase this was furnished naturally by the images of droplets issued from the atomization. The gaseous phase necessitates another seeding in order to be discriminated. This is obtained with micrometer solid particles. Nevertheless separation would be able only if different wavelength are issued from the respective seeding. Our choice was similar as the one used in (10) or in (17). Fluorescent dye tagging the liquid provides droplet images at a higher wavelength than solid image particles which diffused at the initial wavelength by Mie Scattering Diffusion (MSD). Wavelength filtering is thus necessary to separate information. Digital image processing is used to avoid ambiguities and to provide supplementary information on the droplet shapes. After a description of the set-up apparatus, we detail the image processing analysis used in the technique. The last part presents results for velocity field relative to the application with the high pressure spray. Original results on induced gaseous phase are presented to illustrate the use of this technique.

19.2 Experimental procedure 19.2.1 Experimental facility The technique is applied to study a high pressure (injection pressure = 6-12 MPa) spray discharging in quiet gas inside a non-closed cylinder (Fig. 19.1). The bottom and top of this cylinder are totally opened to avoid any recirculation. On the cylinder, four windows are disposed at 0°, 90°, 180°, and 270°. Two Nd:Yag lasers are used in order to generate a sheet of light whose thickness was about 200 u.m. Two opposite windows are used to homogeneously illuminate both sides of the spray. This allows mainly the total illumination of droplets (without shadows). High energy of enlightment (300 mj) is delivered by each of the two Nd:Yag. It is necessary for obtaining two successive images for PIV analysis. The two other windows provide an optical access (perpendicularly to the sheet of light) for two opposite and synchronized CCD cameras: LHESA (LH 510, CCD camera) and TSI (PIVCAM, CCD camera) PFV systems. One of the cameras is equipped with a pass-band filter and mounted with a macro-ring + a lens of 135 mm focal length. The second camera receives directly MSD intensity and Fluoresced intensity re-emitted by the incense particles and by water tagged droplets. It is mounted with a bellows + a lens of 85 mm focal length. The two CCD cameras are cross-correlation cameras. The spray issues from an injector nozzle (indiameter d = 0.2 mm, length 1 = 0.8 mm) at a velocity around 90 m/s. A digital image containing droplet images laser induced fluorescent (LIF) intensity information and a second digital image containing droplets and incense particles information (MSD intensity information) are recorded simultaneously. Our inter-pixel distance is 18 urn for gas phase images. It is 11.5 urn vertically and 16.7 u,m horizontally for the liquid phase images. This corresponds for both phase to a magnification around 2.

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Optical Methods and Data Processing in Heat and Fluid Flow

Fig. 19.1 Optical configuration for the spray experiment

19.2.2 Physical principles of phase separation To separate the phase informations, the liquid phase is tagged with Rhodamin 6G dye (with a concentration of 200 ppm) and the ambient gas is seeded with incense particles of 0.1-3 urn diameter. Tests were carried out to show that their presence doesn't perturb the flow. This principle was previously used to discriminate droplets from solid particles (9, 10, 23) for single point measurements. LIF intensity (at Xf = 575 nm) and MSD intensity (at A.Q = 532 nm) are re-emitted by droplets or solid particles after illumination with a laser sheet in the area of interest. Intensities are recorded simultaneously on the two synchronized cross-correlation BAY CCD cameras. The laser sheet is obtained with a double pulsed Nd:Yag lasers (Xo)

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coupled with a sphero-cylindrical lens system. Each camera is synchronized with the double pulsed laser. This furnishes a pair of images at instant t and at instant t + 8t. In the area of interest, the droplets reemit light at the initial wavelength (\0) and at the fluoresced wavelength (Xf). The physical principles are described in (17) or (19) when applied to the determination of mass fluxes for turbulent dispersion of passive scalar in water or in air. Using a pass-band filter on the first CCD camera (CCD1), we obtain images containing only the fluoresced intensity (at A.f), i.e. only the droplet images. Images with the other CCD camera (CCD2), without any filter, contain both MSD light (at A,0) and LTF light (at Xf), i.e. droplets and solid particles. MSD intensity has a higher level than LIF intensity for a given droplet diameter. This can provide, on CCD2, images from the finest droplets, which are not present on CCD1. This could be the main problem inherent to the principle chosen here. This was also developed and explained in (21). The sensibility of CCD1 is largely greater than the one of CCD2. We perform manual adjustments on apertures of the objective lenses of CCD1 and CCD2 to avoid such problem. And we performed tests to check that such problems are avoided. After some digital image treatment and analysis [Binary operations and Pattern Recognition (PR)], we are able to separate the droplet images from the solid particle images without ambiguities. Consequently, with such separated information, PIV, and digital image analysis, we simultaneously obtain the velocity of liquid and gaseous phases. A problem, which could exist with the technique, is the creation of MSD information, which is not issued from solid particles but from droplets. This could be due to: a) Inherent problems to the present available set-up. This mainly concerns with: - the two different available CCD cameras used here; - and the choice of two different CCD camera locations (induced by the available laser energy) in place of use of splitter [as in (21)]. b) LIF droplet images, which could be different from MSD droplet images due to reflection on surface droplets. This last problem concerns with large droplets on MSD images, but the digital PR process used here recognized them without ambiguity on LIF images. No part of them can be confused with solid particle images at the end of the process before applying PIV for gas phase. The only problem is that a) induces 'noise error'. In the present application, CCD cameras are disposed opposite each other avoiding the use of beam splitter as in (21). Such use allows recording of total re-emitted intensity. This solution is chosen, in spite of supplementary geometrical uncertainties induced by the opposite locations, to avoid cancelling of less intense droplet images (unavoidable if intensity decreases by a factor 2 with the use of a splitter). With our camera this is a crucial constraint to record the finest droplet images (always difficult when a CCD camera has not enough sensibility). In our operating mode (Yag lasers, emission at 532 nm), we can not excite the Rhodamin 6G on the exact maximum absorption peak of the dye. This gives us a weaker fluoresced intensity than expected at 515 nm excitation wavelength. Nevertheless even with optimal apparatus (semi-transparent mirror, higher sensibility, ...) such problems exist due to laws of emission intensity. Then we use different apertures. 19.2.3 Digital image treatment We now present the digital image processing (binary operations and PR process), which is performed on a SUN workstation before applying PIV analysis.

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We consider at this stage of our process two digital images of the same area of interest, which are recorded at the same instant t separately by the two synchronized CCD cameras. We call a pair of images these two images. One contains LIF images of droplets, I'p. The other, I'M, contains the corresponding MSD images of droplets and the MSD images of solid particles seeding the ambient gas (where i = 1, N; N being the number of statistical realizations). In our case 500 realizations were recorded. The next stage is the use of digital image treatment and analysis (the process is described in Fig. 19.2) in order to dissociate droplets contained in the images I'M and IV [Figs 19.3(a) and 19.3(b)], from solid particle images contained only in I'M [Fig. 19.3(a)]. We choose binary image analysis and pattern recognition (24).

Fig. 19.2 Description of the complete digital process to determine images of solid particles from images containing particle images and droplet images

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Fig. 19.3(a) Original Mie intensity image I'M, (b) Original Fluoresced intensity image I'F, (c) Binarized Mie intensity image I'MI» (d) Binarized Fluoresced intensity image lVt» (e) Final binary image from incense particles I'm

Firstly, we binarize the images, which are coded with 256 grey-level values. The resulting images are I'MH [Fig. 19.3(c)] and iVb [Fig. 19.3(d) after geometrical corrections]. Secondly,

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we have to recognize the objects of the pairs of images, to code them (25, 12), to compare them in order to subtract binary droplets images from I'mb- The binary subtraction, giving IT2 is an absolute one and is not sufficient because of the noise error. Criteria used are the apparent diameter (AD) and centroid criteria. They are geometrical parameters quantifying droplets, sufficient to help with recognizing them. The first one was obtained as following: • Freemann coding (26) of the boundary objects (BO). • Perimeter measured as follows: - between adjacent pixel center, if centers are aligned horizontally or vertically - °5 if not. Counting of pixel inside the BO, including pixels describing BO gives the droplet image area Aj. Then we compare i'2 with I'Mb in order to remove from I'2 all objects having no equivalent object on I'Mb. The equivalence means to have the same AD and centroid with a tolerance of ±5%. We use a hypothesis of spherical shape to deduce the instantaneous and individual (for each droplet) AD. The real diameter measurements are very sensible to magnification and sampling problems, thus we deduce the instantaneous (for individual droplets) AD from area measurements Aj, which is a less sensible parameter than the perimeter estimation (24). For the PR process this is a measurement which needs to give the same result when applied to LIF or MSD images (reproducibility). Centroid is the classical one with pixels detected as part of a droplet. A priori some differences could exist in apparent shape due to fluorescent wavelength and initial reemission wavelength. Effectively, intensity reemission laws are different and can be w/rittpn ac fnllr\w7C'

for fluorescence (10,11), and:

for Mie diffusion, where A and B are constants. Indeed as consequence of (19.1), (19.2), and difference of sensibility of the CCD cameras, some particle images present in I'M could be not present on I1 F. Larger sensibility is needed to recover the complete range of droplet diameter from LIF images. This is our case and adjustment of apertures of the 2 CCD cameras were done to record on both cameras the intensity emitted by the finest droplets. After thresholding, the binary image I'ub, issued from I'M, is subtracted from the binary image I'pb, issued from IV, giving an image I'o which contains binary information due to incense and information due to droplets. This last information is some noise and we need to remove it. This is achieved in three stages. 1. Multiplying I'o by I'pb gives Ii which contains only non desirable information due to binary fluoresced particle images. 2. Subtraction between I'i and I'o gives 1*2 which contains information due to incense and due to non desirable Mie diffusion droplet images.

Gas/Liquid Mixing: Simultaneous PIV Measurements of Two Phases Mixing Together; High-pressure Spray Application

3.

227

1i2is constituted of individual objects. For these objects we determine their centroi'd and their AD, as described previously. A comparison (of AD and centroi'd) is performed with objects contained in Ii mb. If an object contained in 1*2 has not an equivalent object in I'mb we remove it from 1*2, otherwise we preserve it. At the end of procedure an image I'ib contains only binary images due to incense particles. Multiplication of I'n, by I'M gives some apparent incense particles (seeding the gas phase) only on a definitive image l\. This image contains the original grey-level values describing the particles images used for PIV analysis of the gas phase.

19.3 Results All the procedures described were conducted to provide valid information usable for simultaneous and instantaneous PIV analysis both for gas and liquid phases mixing together. Figure 19.4 shows simultaneous instantaneous velocity field for gas and droplet velocity for the same area of interest for spray application. It is clear here that such a map contains frequently undetermination of gas velocity when there is numerous droplets.

Fig. 19.4 Instantaneous gas velocity field (left), corresponding instantaneous droplet velocity field (right)

The laser was synchronized with both systems. Pairs of images were recorded at t and t + St where 8t was 5 u,s. We used 64 x 64 pixels for the elementary meshes. Gaussian method is used for sub-pixel accuracy in PIV analysis. The center of the images corresponds to spray axis at 900d from the nozzle exit.

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The first results concerning the application of the combined technique are shown in Figs 19.5 and 19.6. The common area corresponds to 50 mm2. The mean axial velocities of liquid and gas are measured. It shows that there is a large difference between liquid velocity field and gas velocity field. This fact indicates that it is important to discriminate phases in order to describe the gas penetration in to the liquid phase in atomization studies. The gas velocity is higher than velocity of the finest droplets and close to the large one. This indicates that the ensemble of large droplets entrains mainly the gaseous phase (solid particles) whereas the smaller droplets do not induce an important entrainment. It has to be noticed that there is no reason for smaller droplets to have the same velocity as the (small) solid particles. Smaller droplets have their own momentum due to atomization whereas solid particles 'wait' for entrainment. Smaller particles have not enough energy to entrain in their wake large gas part.

Fig. 19.5 Mean longitudinal velocity for liquid and gas phase at x = 900d

Fig. 19.6 RMS longitudinal velocity fluctuations for liquid and gas phase at x = 900d

19.4 Conclusions We show here the use of a combined technique allowing, simultaneously, measurements of two phase flow velocity. The main errors for velocity field determination of both phases are those which can be deduced classically from PIV technique applied (this was not the aim to

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229

describe them here) on both phases separately. It is clear that the flow studied is complex due to the presence of probable dense liquid part or lack of solid particles seeding the gas phase. The apparatus used here lead to a certain amount of low errors for velocity field measurements which could be obviously removed or decreased with optimal set-up (higher sensitivity of CCD cameras, higher dynamical range, ...). It has to be noted that our application of this visualization technique, due to our set-up, does not discriminate droplet images smaller than 11.5 um. This is a limitation which doesn't provide a problem for applying the technique to any two phase flow study. In contrast the saturated problem of the largest droplet images could be removed also with optimal apparatus and adequate filters. Here for our first application, this prohibits certainly any gas velocity measurement very close to the largest droplets.

19.5 References (1) (2)

(3) (4) (5)

(6)

(7) (8) (9)

(10)

(11) (12)

(13)

Obokata, T., Hashimoto, T., and Takahashi, H. (1990) LDA analysis of diesel spray and entrainment air flow, Int. Symp. COMODIA 90, pp. 231-236. Araneo, L., Brunello, G., Coghe, A., and Cossali, G. E. (1998) Entrained gas field produced by a diesel spray impinging on a flat wall, ILASS-Europe Congress, pp. 141146, Manchester. Faeth, G. M. (1987) Mixing, transport and combustion in sprays, Prog. Energy Combust. Sci., vol. 13, pp. 293-345. Zheng, Q. P., and Jasuja, A. K. (1996) Laser sheet imaging of dense gas turbine sprays, C516/044 MechE. Boedec, T., Champoussin, J.-C., and Marie, J.-L. (1998) Experimental study of droplet dispersion from a plain jet atomizer, ILASS-Europe Congress, pp. 334-339, Manchester. Mohammad!, A., Miwa, K., Ishiyania, T., and Abe, M. (1998) Investigation of droplets and ambient gas interaction in a Diesel spray using a nano-spark photography method, SAE Paper No. 981073. Kim, J. Y., Chu, J. H., and Lee, S. Y. (1999) Improvement of pattern recognition algorithm for drop size measurement, Atomization and Sprays, Vol. 9, pp. 313-329. Bachalo, W. D. (1994) Experimental Methods in Multiphase Flows, Int. J. Multiphase Flow, Vol. 20, pp. 261-295. Georjon, T. (1998) Contribution a 1'etude des interactions gouttelettes-gaz dans un ecoulement diphasique de type 'jet diesel', These de Docteur de 1'Ecole Centrale de Lyon, 24 Juin. Sankar, S. V., Brena De La Rosa, A., Isakovic, A., and Bachalo, W. (1990) A technique for studying mixing in swirl combustors, 5th hit. Symp. on Laser Techniques to Fluid Mechanics, Lisbon. Kozma, J. M. H. (1996) Air entrainment into a transient diesel spray visualized using particle image velocimetry, Thesis of Master science, University of Wisconsin-Madison. Yamada, N., Ikeda, Y., and Nakajima, T. (1998) Multi-intensity-layer PIV for spray measurement, 9* Int. Symposium Applications of Laser Techniques to Fluid Mechanics, Lisbon, pp. 12-6-1—12-6-8. McGregor, S. A. (1991) Air entrainment in spray jets, Int. J. Heat and Fluid Flow, Vol. 12, No. 3, September.

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(14) Benatt, F. G. S., and Eisenklam, P. (1969) Gaseous entrainment into axisymmetric liquid sprays, Journal of the Institute of Fuel, pp. 309-315, August. (15) Briffa, F. E. J. and Dombrowskim, N. (1966) Entrainment of air into a liquid spray, A.I.Ch.E. Journal, Vol. 12, No. 4, pp. 708-717, July. (16) Boedec, T. and Simoens, S. (1999) PIV, PLIF and PMSD combination for description of the velocity fields applied to turbulent mixing of spray and ambient air flows, 3rd Int. Workshop on Particle Image Velocimetry, pp. 139-144, Santa Barbara USA, Sept. 1618. (17) Simoens, S. and Ayrault, M. (1994) Concentration flux measurements of a scalar quantity in turbulent flows , Experiments in fluids, Vol. 16, No. 3-4, pp. 273-281. (18) Simoens, S., Ayrault, M., Primon, R., and Verduzzio, G. (1996) Simultaneous velocity and concentration measurements using Mie scattering and particle image velocimetry in a turbulent air jet, Optical Methods and Data Processing in Heat Fluid Flow, in IMechE Conference Transactions, 1996-3 MEP Publications, London, Part. C, pp. 301-308. (19) Vincont, J.-Y., Simoens, S., Ayrault, M., and Wallace, J. (2000) Passive scalar dispersion of a line source downstream an obstacle, accepted in Journal of Fluid Mechanics. (20) Broquet, L., and Simoens, S. (1995) Mesure du melange de deux especes par double fluorescence induite par laser, 6eme Colloque National de Visualisation et de Traitement d'Images en Mecanique des Fluides, Saint-Etienne. (21) Towers, D. P., Towers, C. E., Buckberry, G. H., and Reeves, M. (1999) A colour PFV system employing fluorescent particles for two phase flow measurements., Meas. Sc. Tec., Vol. 10, pp. 824-830. (22) Stefanini, J., Cognet, G., Vila, J. C., Merite, B., and Brenier, Y. (1993) A colored method for PIV technique, pp. 247-258, Fluid Mechanics and its applications, Vol. 14, Kluwer Acad. Publish. (23) Georjon, T., Chale, H. G., Champoussin, J-C., Marie, J-L., and Lance, M. (1997) On the potentialities of a droplet tagging method to perform LDA discrimination in dense sprays, FEDSM97-3084, ASME Summer Meeting, Vancouver. (24) Serrat, J. (1982) Image analysis and mathematical morphology, Academic Press. (25) Simoens, S. (1992) Applications de Panalyse d'images a des phenomenes de melange et de dispersion turbulents, These de Doctoral de PEcole Centrale de Lyon. (26) Gonzales, C. and Wintz, P. (1977) Digital Image Processing, Addison-Wesley Publishing company.

T Boedec and S Simoens Laboratoire de Mecanique des Fluides et d'Acoustique, Ecole Centrale de Lyon, Ecully, France © With Authors 2002

20 Flame Visualization Enhancement by Image Processing W B Ng, K Y Cheung, and Y Zhang

Abstract This Chapter presents flame image visualization enhancement using image capturing and processing techniques. Test cases of the enhancement of flame visualization have been demonstrated. It has also been shown that the images could be pre-processed by the manipulation of the shutter speeds (or gating times) of a camera, in order to obtain the desired aspects of flame dynamics such as the time and spatial scales.

20.1 Introduction In the last few decades, laser technology has developed progressively which enables laserbased optical diagnostic technique to play a dominant role in combustion research. Direct photography and its potential, on the other hand, has been overlooked even though it has been applied to flame studies ever since the first camera was invented. This is due to the fact that the flame light emission is integrated along the line of sight, which leads to ambiguity in spatial resolution. As a result, quantitative analysis is difficult. In addition, the sensitivity of a film is often too low to capture the flame dynamics and the images are either blurred or too dim to be of any use. The situation has changed recently since we are in the middle of a revolution of imaging techniques, sparked by the rapid progress not only in advanced image sensing technology but also in digital image processing power. Conventional film cameras are being replaced by high-end digital cameras. The captured images can be easily uploaded in to a computer directly for enhancement and processing. Much useful physical insight and information can be gained by digital image processing. By using proper combinations of the shutter speed, sensitivity, framing rate, and resolution of a camera, the time scales of combustion may be resolved and the corresponding flame structures may be extracted temporally and spatially. For example, using a lower shutter speed

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and light sensitivity of a CCD (charge-coupled device) sensor, an averaged image of the flame is obtained as long as the image is not over exposed or saturated. Within the exposure time (or gating time) the flame may be locally created or destroyed many times. What is recorded is the averaged flame pattern. If a higher shutter speed is applied only smaller time-scale flame movements are recorded and the flame image is more 'instantaneous'. The flame structures at different time-scales are needed to gain a better understanding of the processes involved. Therefore, with the aid of advanced imaging equipment, imaging capturing can also be regarded as imaging processing. In the following sections, the experimental set-up is presented first. Then the various examples of digital enhancement of flame visualization are given.

20.2 Experimental setup All the flame images are obtained from an impinging burner. The experimental rig consists of a burner, a steel plate, a mixing chamber, a fuel, and air supply system. The steel plate was positioned above the burner nozzle [Fig. 20.1(a)]. The reactant jet came out of the burner nozzle and impinged on the plate. The burner was attached to an adjustable platform so that the height between the burner nozzle and the plate could be adjusted. The air and fuel (propane) flows were controlled separately by rotameters and pressure gauges. The two flows were then mixed in the swirling mixing chamber, which was connected to the burner by a long flexible pipe. The pipe was bent deliberately to create secondary flow so that the fuel and air could be further mixed. Three turbulent generators [Fig. 20.1(b)] were used to modify the flame turbulence intensity. They were perforated discs, 50 mm in diameter. Each disc was cut from an aluminium sheet, 1 mm thick. Some experiments were conducted without a turbulent generator. V-shaped flames could also be created by placing a steel rod above the nozzle exit. 20.2.1 V-shaped premixed impinging jets Five steel rods with diameters of 5.5, 6.5, 8.0, 10.0, and 12.0 mm [Fig. 20.1(c)] were used to create V-shape flames of different characteristics. The first step of this experiment was to create a conic flame (1) with uniform turbulent flow of premixed air and propane at the nozzle exit. The steel rod was then placed on top of the nozzle at the center position, either lined up parallel to the camera lens or perpendicular to the lens. As a result, an impinging V-shape flame was formed. The plate and nozzle distances were adjusted and set for each set of experiments. The details of the different cases are shown in Table 20.1.

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Fig. 20.1 Experiment apparatus

Table 20.1 Experimental conditions Fuel/Air equivalent ratio

Nozzle exit velocity (m/s)

Ratio of distance between nozzle/plate to nozzle diameter (HID)

Rod Diameter

Set 1 (5 cases)

1.25

4.18

2.425

Set 2 (4 cases)

1.25

4.18

1.75

5.5, 6.5, 8, 10, 12 5.5

Set 3 (3 cases)

1.25

4.18

1.75,2.425,3.00

8

TGO, TG1 , TG2, TG3 TG3

Set 4

1.20

1.657

3.00

Nil

TG2

Data Set

Turbulence Generator

Flame Type

TG3

V-shaped premixed V-shaped premixed V-shaped premixed 'Star' shaped diffusion

(mm)

A CCD-TRV224 Sony camcorder was used to record the different flame patterns and flame transitions. The camera has four imaging modes but only two modes were used. These are the normal mode and high shutter speed mode. The normal mode is operated at a shutter speed of l/50th second, and the high shutter speed mode was at l/4000th second. All modes have a framing rate of 25 frames per second. Images were recorded on video 8 mm cassettes. A Kodak EktaPro HS4540 Motion Analyser was also used in this experiment. It was operated at 4500 frames per second. It can only capture images in B/W format.

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20.2.2 Diffusion impinging jets The experimental rig cited above was also modified for diffusion impinging jet studies by adding a small diameter fuel tube to the central axis of the nozzle. Pure propane fuel comes out the small fuel tube and coaxial air flows through the burner nozzle itself. As a result, an impinging diffusion flame is formed. The distance between the steel plate and the burner nozzle was fixed at 120 mm. An Olympus E-100RS digital camera was used to capture the flame images. The camera speed can be set as high as 1/10 000th second. All captured images were initially stored in SmartMedia (memory) card in digital format before transferred to PC through a cable. The camera was positioned at an inclined angle to the steel plate. Images were taken at different shutter speeds but at a fixed framing rate of 15 frames per second. The flame structures of the obtained images were useful in resolving time-scale information. Eight shutter speed modes of l/500th second, 1/lOOOth second, l/2000th second, l/3000th second, l/4000th second, l/5000th second, l/8000th second, and 1/10 000th second were applied respectively. 20.2.3 Laser sheet tomography of premixed impinging jets Details of the laser sheet tomography experimental setup have been reported elsewhere (2). The plate and burner exit separation was 25 mm and the mean burner exit velocity was fixed at 3.1 m/s. The turbulent generator was located 20 mm below the exit of the burner nozzle. They are 2 mm thick and have hole diameters of either 2 mm or 3 mm. The perforated discs have the same blockage ratio of 0.5. The flow was seeded with fine silicon oil droplets generated by a glass atomizer. After chemical reaction in the very thin flame front, the oil droplets are destroyed. Therefore only the reactant part of the flow field could be visualized when a two-dimensional thin laser sheet (0.6 mm) was shone through the flow field. The thin laser sheet was created by a long focal length spherical lens to focus the laser beam and a cylindrical lens shortly after to expand the beam in one plane. A Kodak EktaPro HS4540 Motion Analyser was used for imaging. The camera was operated at 9000 frames per second. For each test the digital camera could store 2025 images, which is equivalent to 225 ms in real time. The captured images are stored in the RAM bank of the camera, which can then be transferred to VHS videotape. The images stored on a VHS tape have to be digitized. A video player is connected to a computer image grabber card. The captured video clips are saved in AVI format.

20.3 Data evaluation by image processing In this section, the post processing of combustion database (raw images) to deliver information on flame structures, flame dynamics, flame behaviours, and time-scales of combustion process has been demonstrated. It starts with the images of LST, followed by the V-shaped premixed flame and ends with the diffusion flame. The pictures in Fig. 20.2 give an impression of the images to be evaluated.

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Fig. 20.2 (a) V-shape turbulent impinging jet taken by a Kodak HS4540 (4500 fps) (b) V-shape turbulent impinging jet taken by a Sony camcorder (l/4000th second) (c) V-shape turbulent impinging jet taken by a Kodak PC600 (l/50th second) (d) LST premixed impinging jet taken by a Kodak HS4540 (9000 fps) (e) 'Star' shaped turbulent diffusion flame taken by an E-100RS Olympus digital camera (1/4000th second)

20.3.1 LST Figure 20.2(d) shows a typical flame image captured by LST. The grey areas represent the reactant and the thin white line marks the wall position. There are 600 images in sequence cropped to a specified rectangle shape. Most of the images of the LST experiment have intensity faded non-uniformly, the appropriate threshold value may have to vary throughout the image. Therefore, instead of global thresholding, adaptive thresholding is used to create the binary image: 1 for the burned region and 0 for the unburned region, resulting in an instantaneous binary image. The binary images are then superimposed using the arithmetical operation PLUS [Fig. 20.3(c)] and the resultant image is proportional to the degree of combustion progress. Figure 20.3(d) shows the layers of reaction progress variables of 0.8 (the most outer layer), 0.6, 0.4, and 0.2 (the most inner layer).

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Fig. 20.3(a) A typical cropped image with a 256 grey scale

Fig. 20.3(b) A binary image

Fig. 20.3(c) Resultant image of operation PLUS

Fig. 20.3(d) Reaction progress variable contours

Fig. 20.3(e) A single flame front boundary

Fig. 20.3(f) 250 flame front boundaries superimposed

By using a gradient operator [sobel operation (3) in this case], the contours of the images are extracted. The width of each contour is equivalent to 1 pixel-size. Once the flame front boundaries are detected, the flame surface density can be calculated by the method of either Veynante et al. (4) or Deschamps et al. (5).

20.3.2 V-shaped flames The original images of the V-shaped flame captured by the high-speed camera are not suitable for direct information extraction because the flame patterns are almost invisible (Fig. 20.4 left). Histogram-equalization enhancement is applied to improve the visibility of the flame patterns. However camera-streak noise has also been highlighted as well and appeared in the processed images (Fig. 20.4 right). Therefore further image enhancement such as noise removal is required.

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Fig. 20.4 (Left) An original V-shaped flame image, (right) a histogram-equalization version of V-shaped flame image

Basically a digital image is made up of fundamental spatial frequency components that, when combined, make up the form of the image. A frequency-transforming process provides a pictorial view of these spatial frequency components. It converts an image from the spatial domain of brightness to the frequency domain of frequency components and a distinct white point may appear in the frequency-transform image if a repetitive noise pattern exists in the original image. The horizontal streaks shown in existing raw images (Fig. 20.4 right) are a type of repetitive noise pattern. Therefore, a two-dimensional Fourier transform algorithm is used to convert the original image to frequency domains (Fig. 20.5 left). We multiply the frequency images by 0 in the area of the spots. This creates a band-reject filter that attenuates only the frequency of interest. An inverse Fourier transform algorithm is then applied to transform the frequency images back to the spatial domain (3). As shown in Fig. 20.5 (right), the image no longer contains the repetitive noise streaks.

Fig. 20.5 (Left) Fourier-transform image showing horizontal noise patterns as bright spots in the image, (right) inverse Fourier-transform image with periodic noise removed

The quality of all raw images has been improved by means of image processing algorithms. This process allows one to further manipulate and evaluate the post-processed images in order to extract wanted information by intelligently reducing the amount of image data as each digital image contains enormous information and often much of this information is superfluous to solve a specific problem. In the following section, data extraction techniques are elucidated.

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20.3.2.1 Image subtraction It is often the case that the flames have changed little within a very short period of time. As a result, it is difficult to spot the variations between two consecutive images. Subtraction of the two adjacent images would highlight the minor images. This method is often used to detect motion. Consider the case where nothing has changed in a scene; the image resulting from subtraction of two sequential images is filled with zeros and replaced by a homogeneous colour. In contrast, if something has moved in the scene, subtraction produces a non-zero result at the location of movement.

Fig. 20.6 Subtracted images

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The top left sub figure shows the difference between two consecutive frames, i.e. frame 2 is subtracted from frame 1 with a time difference of 0.222 ms. It is very hard to distinguish the differences between the two images. As the gap of the frames increases, the difference and the structure of the flame movement are getting more obvious. In between Frame 5-1 and Frame 6-1 (0.889-1.111 ms), the structure of the flame movement is in its most detailed form. As the separation continues to increase, the images start to lose their information on the structure of the flame movement. When the frames gap is 7 or greater, a false impression of the flame structure movement seems to be shown. Although it still highlights the main area of the flame movement, the details of the structures are not shown. 20.3.2.2 Analysis of rod diameters on V-shaped flame behaviour There are five different sizes of rods used in the experiments. Using the image processing techniques, it is possible to evaluate the flame separation distances and angles. The first measurement is taken from Data Set 1, which has the same variables except for the varying rod sizes, as shown in Table 20.1. For each set of data, the first fifty frames are grabbed from each data clip. All outer boundaries are tracked automatically and superimposed on to one frame, where the average of the flame position can be obtained, shown in Fig. 20.7.

Fig. 20.7 (Left) 50 tracked outer boundaries of each case of Data set 1 are superimposed on to a single image, (right) A zoom in version of the processed image

The inner boundaries tracing can provide the positions as well as the movements of the flame at a specified time interval. Next, 10 frames are grabbed from each test case instead of 50, and there is a 10 frames gap between each frame. For example 1st frame = frame no.l, 2nd frame = frame no.l 1, 3rd frame = frame no.21, etc. The contrast level is increased for easy boundary tracking. Again, all ten tracked boundaries are superimposed on to a single frame (Fig. 20.8). The results give the movement of the flame in a time period of 22.22 ms. In general, these images show that the bottom half of the flame stays fairly steadily. The flame starts to fluctuate more vigorously further away from the rod. Near the tip of the flame, the behaviour is very unpredictable. There is no obvious pattern. The scale of turbulence becomes greater.

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Fig. 20.8(a)-(d) Image contours superimposed in a single plot

The angle of the flame separation is measured from the centre of the rod, where it is marked before the image layer is removed. Two tangential lines are drawn in the inner flame boundary from the centre point of the rod. The results of the angles with different rod diameters are tabulated in Table 20.2.

Table 20.2 Data Set 1

Rod size/mm

Angle/degree

Case 1

5.5

24.9

Case 2

6.5

28.0

CaseS

8

30.6

Case 4 Case 5

10 12

38.1 41.9

Flame Visualization Enhancement by Image Processing

241

Fig. 20.9 Angle measurement from contour-superimposed plot

Figure 20.10 shows the angle increases when the rod diameter increases. From all the measurements taken above, it can be concluded that the larger the rod diameter, the greater the separations between the inner flame boundaries and between the rod and the flame root.

Fig. 20.10 Plot of flame separation angle versus rod diameter

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20.3.2.3 Analysis of the distance between the plate and the nozzle on flame structure This is a variable which defines the vertical distance between the nozzle of the burner and the flat metal plate hanged above. Three cases have been investigated. All the other variables are kept the same, except for the nozzle to plate distance. Using a time gap of every 5 frames, 20 frames are captured in each case. Each of these frames is processed with increased contrast. A comparison of the difference in contrast is shown in Fig. 20.11.

Fig. 20.11 (Left) An original V-shaped flame image, (right) contrast enhanced image

It is easier to highlight the region of the vortex in the inner flame boundary (circle) in the above figure. In general, when a flame is impinging on a wall, it has the following behaviour.

Fig. 20.12 Impinging V-shaped flame

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243

Region 1 The flame has a greater momentum at the exit of the nozzle. The flame is separated by a metal rod with little fluctuation and the creation of small-scale wrinkles. Region 2 This is the impingement region, where the flame hits the metal plate and flows along the wall. The velocity in this region is gradually decreasing due to the friction resistance of the plate and the divergence of the flow. The vortex generated in Region 1 is developing with an increase in its scale. Region 3 This is the forward vortex region, as the flame is separated at the root at an angle. The flow of the flame is rotating and rolling along the plate, and thus vortices are formed. The mixing and the diffusion are weak in this region, although the momentum is supplied from Region 1. Region 4 is similar to Region 3 due to the symmetry of the problem. 20.3.2.4 Effect of turbulence generators on V-shaped flame behaviour There are three different turbulence generators used in the experiments for comparison. TGO = No turbulence generator. TGI = Big and relatively few holes TG2 = Smaller holes with a higher number density of holes. TG3 = Smallest holes with the highest number density of holes. The subtraction technique is used to highlight the structures of the flame. As seen from Fig. 20.13, the main structure of the flame is generally situated at the center region of the flame, where the rough structure of the flame can be seen for each case. It can be seen that as for TGO, the flame fluctuates very little. When a turbulent generator is used, the flame structure is more apparent, as the generator will create more turbulence. Therefore, changes in flame movement will be more vigorous.

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Fig. 20.13 Subtracted image of TGO, TGI, TG2, and TG3 at top left, top right, bottom left, and bottom right respectively

20.3.3 Time scales analysis of the diffusion impinging flame Image capturing itself is an image processing process due to the limited exposure time (or gating time). It is not possible for a capturing process to resolve flame time scales which are smaller than the camera exposure time. Therefore image capturing is a time averaging process. The content of an image is an ensemble of various flame stages, within the chosen camera exposure time. Longer exposure times, i.e. low shutter speed, result in an averaged flame pattern and shorter exposure times give a more instantaneous look of a flame. Figure 20.14 shows images of a 'ring' flame formed by a pure fuel central jet and a co-flow air jet impinging on a plate. The global fuel-to-air equivalence ratio is 2 and the ratio of the plate to nozzle separation distance (H) over the air nozzle diameter (D) is H/D = 3. A range of shutter speeds (1/3 Oth second to 1/1600th second) was used. It can be seen that the averaged flame patterns, i.e. at low shutter speed, are very different from the more 'instantaneous' flame patterns captured at high shutter speed. It is obvious that an image at low shutter speed has 'gathered' more flames than at the high shutter speed.

Flame Visualization Enhancement by Image Processing

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Optical Methods and Data Processing in Heat and Fluid Flow

Fig. 20.14 'Ring' flame images captured by high-end digital camera of different shutter speeds

20.4 Conclusions Flame visualization enhancement using digital image processing has been demonstrated. Traditional digital image processing techniques such as contrast enhancement and noise reduction are very valuable in improving the visualization quality. It was found that image subtraction between consecutive images are effective in identifying otherwise hidden flame structures. However the time separation between the two images has to be considered carefully. Contour tracking of the flame boundaries is useful in visualizing the dynamics and extent of flame movement. Further quantitative information extraction of the processed flame boundaries would give an indication on the global characteristic of each flame. The variable shutter speeds of modern digital camera are a valuable asset for the studies of flame time scales.

References (1) (2) (3) (4) (5)

Foat, T., Yap, K. P., and Zhang, Y., Combustion and Flame, Vol. 125, pp. 839-951, 2001. Zhang, Y., Rogg, B., and Bray, K. N. C., Combust. Sci. and Tech, Vol. 113-114, pp. 255-271, 1996. Jahne, B., Digital Image Processing - Concepts, Algorithms, and Scientific Applications, 4th Edition. Springer, 1997. Veynante, D., Duclos, J. M., and Piana, J., Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, pp. 1249-256, 1994. Deschamps, B. M., Smallwood, G. J., Prieur, J., Snelling, D. R., and Gulder, 6. L., Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, pp. 427-435, 1996.

Bibliography Pope, S. B. Int. J., Engng. Sci. Vol. 26, pp. 445^69, 1988. Zhang, Y. Experiments in Fluids, S282-S290, pp. 282-290, 2000. W B Ng, K Y Cheung, and Y Zhang Mechanical, Aerospace and Manufacturing Engineering Department, UMIST, Manchester, UK

21 Optical Diagnostics - Automatic Data Processing and Application in Fundamental Studies and Control Systems V S Abrukov, I V Andreev, and P V Deltsov

Abstract Practical realization of the broad possibilities of optical methods requires the automation of measurements. The present work generalizes some previous results of the authors concerning the automation of image decoding and image analysis, presents a new automatic procedure, as well as the results of its use in experimental investigations of combustion and detonation. Representation of the image, as a collection of black and white pixels, is used in two developed computer programs. Originally, all computer programs were intended for interferometry that has broad and unique opportunities in combustion research. At present, they can be used for other optical methods. New results of the use of neural networks for combustion interferogram analysis are presented also.

21.1 Introduction Optical methods have good possibilities at experimental investigations of combustion. But there are also some problems with their use. One of the problems is connected with the stage of image decoding and its evaluation. Image decoding has an essential influence upon the accuracy of measurements. The results of decoding often have a subjective nature and depend greatly on the experience of the operator in the image decoding. It limits the level of usage of optical methods in the quantitative analysis of combustion. The automation of image decoding and analysis of received data is necessary for the full realization of optical method possibilities and this must ensure the accuracy and reliability of the results.

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Interferometry is a perspective optical method that has broad and unique opportunities in combustion research. It allows unique measurements to be carried out (1, 2), that can not be fulfilled by other methods. It is a sole method, which can be used to determine the integral characteristics of a flame, for example, mass of a flame (mass of gas in a flame area), Archimedes lifting force acting on a flame, quantity of heat in a flame, etc. But, interferometric images of combustion processes have a very complex structure and an automation of decoding for interferometry has an even greater value, than for other optical methods. The problem of automation of interferogram decoding has been studied for a long time (3-11). Nevertheless, the problem is not decided completely till now. The commercial packages currently available that provide a large range of image processing facilities can not be used for solving the very specific tasks of scientific research because the commercial packages do not allow the decoding and analysis of specific and complex images of combustion. Here the authors present new results concerning the automation of combustion interferogram decoding and analysis. In the authors opinion the developed programs may be used for image decoding and analysis of other optical methods, and for usage in monitoring and controlling of combustion systems. The results of the investigation of polymer vapour ignition by laser radiation and of geometric measurement of exhaust of pulsed detonation engine (12) are presented below.

21.2 Experimental apparatus The standard schemes of holographic interferometer and polarizing shift interferometer were used for obtaining interferogram presented in this Chapter. The standard video camera was used for registration of exhaust of pulsed detonation engine, there are detailed descriptions of interferometers in (13, 14), and a technique of realization of experiments with pulse detonation engine in (12). The authors would like to mark here, that the main purpose of this Chapter is the representation of new solutions to problems with the automation of decoding and analysis of interferometric and other optical images. The description of setting of experiments in combustion interferometry is stated in detail in the thesis of one of authors of this work (13,14), in English, see also (16-21).

21.3 Interferometric techniques in combustion research In this section some main possibilities of quantitative interferometry are described. For the complete description of the interferometry possibilities see references (1, 2, 22, 23).

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21.4 An integral approach: the determination of integral characteristics of burning gaseous phase Interferometry is an integral technique by nature. In accordance with the basic interferometric equation the phase difference distribution, S(x, y), in the interferogram plane is an integral of the refractive index distribution, n(x, y, z), within the flame:

where S(x,y) is measuring in unit 2n, n 0 is the refractive index of the undisturbed medium surrounding the flame and x,y,z are the Cartesian co-ordinates, with the Oz axis directed along the light beam passing through the flame. By equation (21.1) and Gladstone-Dale equation:

where p is density, k* is an average value of Gladstone-Dale constant, the following formula would be derived with the help of double integration of equation (21.1):

where m is mass of gaseous phase of object, I= X ff S(x,y)dxdy, V is object volume. Equation (21.3) provides an insight in to the physical meaning of I:

Equation (21.4) shows that the double integral divided by k represents the difference of the mass of the substance falling within the viewing area of the interferometer before and after the object comes in to view. At this point, it should be noted that equation (21.4) indicates that the interferometry makes possible determination of an «unusual» characteristic, e.g. the «Archimedes lifting force» acting on the heated gas:

This may be useful in studying the effect of convection flow on flame stability and hysteresis phenomena. Mass is the fundamental characteristic of a thermodynamic system. With the volume, pressure, and equation of state known, it allows the following thermodynamic characteristics to be determined: • a temperature corresponding to the equation of gaseous state, T; • an enthalpy (isobaric thermal effect) H=mcp(T-To), where Cp=a+bT is the compositionaverage specific heat of the gaseous mixture; • an average specific enthalpy, Hm=H/m, and enthalpy density, HV=H/V.

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21.4.1 Determination of non-stationary burning rate Burning rate is the fundamental burning characteristic, which allows a variety of other characteristics to be determined. During ignition, the condensed material is converted in to gas, with loss in the sample mass being equal to gain in the gas mass. If the initial period of the laser ignition of propellant, is to be investigated by interferometric technique (on interferometric cine film frames it how's up as a gas «cap» expanding in a gradual manner over the viewing area), the time dependence of the mass of the entire gas being formed, m(t), can be obtained. By differentiating m(t) with respect to time, the time evolution of the burning rate, m'(t) (kg/s), can be estimated. It is important to note that m'(t) can be determined more simple in a different way (24): m'(t) = (I/ k*)dl/dt. This way may be used, if the determination of other integral characteristics is not necessary. 21.4.2 Determination of non-stationary heat release power and other characteristics Heat release power (in unit of Watt) is a characteristic that is non-conventional in combustion theory. The procedure for its determination is similar to that used in the determination of mass burning rate. The time dependence of the quantity of heat released in to the gaseous phase during ignition, H(t), is assessed from the interferometric cine film frames, whereas heat release power H'(t) is determined by differentiating H(t). After determining the H'(t) value, one can also obtain the heat release rate for inflammation of gasification products during propellant ignition (this is also true for gas mixture inflammation by electrical spark): (D= max [H'(t) / V(t)] (in unit of Watt/cm3) 21.4.3 The determination of intraballistic and energy characteristics of propellants Interferometry provides a means of determining, during the initial propellant ignition stages, the time dependence of the total work expended on gas expansion Ft = p0 V. With regard to the ignition of propellant, explosive progress is characteristic and certain points in time being consistent with the maximum realization of the energy resources of the propellant. At the moment at which the gaseous products inflames, the process is almost in line with an adiabatic explosion, conditions under which the reactions occur being similar for different compositions and approaching actual ones. This makes it possible, in accordance with the definition of the concepts of intraballistic characteristics of propellants, to calculate the force of powder F = max[Ft (t)/m(t)] and specific gasification rate Vsp = max[V(t)/m(t)]. This also enables calculation of the energy characteristics of propellant: • a quantity of heat released when a unit mass of the material is burnt EM = max[H(t)M/m(t)]. • a quantity of heat released when a unit volume of the material is burnt Evc = max[H(t)/Vc(t)]. This procedure is more straightforward and less expensive than that regularly employed for determination of intraballistic and energy characteristics of propellants. It is best suited to comparison studies of different new propellant compositions.

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21.4.4 The potentials of interferometry in studies of stationary burning waves with known burning rate - determination of convective heat flux profile and heat release rate profile In addition to the determinability of time dependencies of integral characteristics and, based on them, calculability of integral burning characteristics, there exist possibilities for using, to this end, profiles of integral characteristics aligned with the gaseous phase flow, such as specific enthalpy Hm(y), enthalpy density Hv(y), flow-section-average linear gas velocity v(y) (here y is a flow-aligned co-ordinate). During stationary burning, with its mass burning rate m' known, the profiles make it possible, in the context of a pseudo-one-dimensional approximation, to determine the following: • a profile of the convective heat flux: qc (y) = ni'Hm (y); • profiles of theflow-section-averageheat release rate
where p0 (the ambient pressure) and R (the universal gas constant) can be taken to be constant, and k (the Gladstone-Dale constant), u, (the molar mass), and M = ku. (the molar refraction) vary in the flame volume as a function of the gas composition which varies as the reaction progresses as well as due to the ambient environment diffusion in to the flame. For a single-component gas, the values of k and M are not functions of p and T over a wide range of their variations. For a gas mixture, the molar refraction is determined in terms of the value of molar refraction for each component M; and in terms of its volume concentration xi: M = SMjXj. If equation (21.7) is to be approached following the assumption that variations in T are similar to those in concentration (i.e., M):

the following formula can be derived for a set of the flames zones from the gas «cold» zone with temperature T0 to the reaction zone, where the theoretical highest temperature Tmax can be reached:

Here, M0 is the molar refraction of the gas (or vapor of liquid, or gasification products of condensed system), To is the temperature corresponding to M0 (the initial temperature of the gas, or the surface temperature of the burning liquid, or the surface temperature of the burning

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condensed system), Mf is the molar refraction of the theoretical final burning products mixture, which can be calculated by brutto-equation of chemical reaction. The value of Tmax can be determined also by the value of Mf from equation (21.7). Notice that equation (21.8) is insensitive to small errors in chosen value of T0. It is of importance in determining temperature fields in flames of solid propellants and other condensed systems where determination of surface temperatures presents considerable difficulties. Formula of the type in equation (21.8) for flame regions that are external to the reaction zone can be written as:

where Ma and Ta are molecular refraction and temperature, respectively, of the undisturbed medium that surrounds the flame. In (13), the greatest possible variation of M (in terms of [(M0-Mf )/Mf]) has been calculated for a variety of reaction systems. Calculations have been done using molecular refraction data obtained for different gases (25), a generalized chemical equation of complete reaction processes (brutto-equation of chemical reactions), data from thermodynamic calculations made for propellants burning products composition, and data on the composition of gasification and burning products reported in the literature. Part of the results is given in (26). The following is worthy of note: • for all premixed gas and condensed systems the greatest possible variation of M is within 2-30 per cent; • the greatest possible variation of M runs as high as 100-200 per cent is in the fuel gas pyrolysis zone of diffusion flames. 21.4.6 The other potentials of interferometric technique - short review The above potentialities of interferometry allow investigation in to the simple heat exchange processes to be made without the determination of the temperature field (27). The authors have also suggested that a «template process technique* can be used for measuring the characteristics of a non-stationary complex heat exchange (27). This technique enabling measurements without having to determine a double integral (based on comparing interferometric cine film frames of a process under study and that of a process whose characteristics are determined by special experiment with controlling condition and parameters). The development of integral approach to interferometry enables also the determination of the changing of a mechanical impulse of a non-stationary gaseous stream using interferometric cine films (28), allows the reconstruction of velocity and pressure fields of an arise potential gas flow based on the reconstructive tomographic concept (29), as well as the energetic characteristics of acoustic waves (30), which can be generated by various periodic exited flames, for example «singing flame». Apparently, the interferometry can be applied to measurement of the many various characteristics of combustion processes, gas dynamic, and heat transfer. But practical realization of the broad possibilities of interferometry requires the automation of measurements, first of all, the automation of interferogram image decoding and analysis.

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21.5 Image decoding and analysis The main data that determine the interfere gram decoding are usually the following (1,2): • the phases difference distribution on the interferogram, S(x,y); • the integral S(x,y)dxdy; • the integrals of S(x)dx for various y; • co-ordinates of geometric borders of the object and it parts. The stage of interferogram decoding is the most difficult in interferometric measurements. It demands the very much manual work. For this reason, the authors believe, that all the possibilities of interferometry were not used earlier. The description of image decoding and analysis software that was developed by the author is presented below. The authors hope, that they can essentially help at implementation of wide opportunities of interferometry and to expand the areas of its application. 21.5.1 Algorithms, implementation, and experimental results In this section, the description of algorithms of computer programs and its interface and screens, as well as the results of its application at research of processes of ignition, burning, and detonation are presented. The results of measurements of the several integral characteristics represented: enthalpy, heat release power and hear release rate, Archimedes lifting force as well as the results of determination of temperature field in a flame. Earlier, without automation of decoding of interferometric images, determination of such collection of values demanded a lot of time. At application of our programs, it demands only some clicks of the computer mouse (excluding the program Nl - see below). At present the complex of computer programs, developed by the authors consists of the following. 21.5.1.1 Program Nl The program of determination of the discrete kit of values of the function S(x,y). It works in the dialogue mode and is provided with detailed menu. It uses two marking lines, moveable by the operator by means of the keys of the keyboard on the computer screen. The program vastly accelerates and facilitates the interferogram decoding in contrast with earlier facilities. The operator marks a kit of spots on the interferogram image by marking lines and assigns the values of S(x,y) of each marked spot. The number of the spots usually equals 20-50 depending on complexity of object structure. The further data processing connected with different arithmetical operations of subtraction, summations, and averaging is executed automatically. The program has also the following possibilities: visualization of S(x,y) during the interferogram decoding; the correction of the earlier entered data; the returning to earlier saved decrypted interferogram and its addition. The interferogram images in the bmp-format with the palette '256 gradations of grey' are used, see Fig. 21.1. The preliminary interferogram image editing is not required at usage of this program. Interferogram image is used 'as is'. Program can be used for interferograms obtained at customization of an interferometer on bands of infinite and finite width.

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Fig. 21.1 The screen of interferogram decoding (program Nl) 21.5.1.2 Program N2 The program of the determination of the integral S(x,y)dxdy by means of the 'painting of the interference bands'. This method processes an interferogram image as a collection of black and white pixels. The integral is represented as a sum of the square (in pixels) of black and white interference bands multiplied by the average value of S(x,y) ('weight of band') which correspond to each band. The decoding is made as follows. Operator with one 'mouse' click marks any spot of the concrete interference band; the computer paints the area of that band with the green colour and automatically computes the number of painted pixels, see Fig. 21.2. This number is proportional to the integral of the band. The obtained number is multiplies by 'weight of band', which is indicating by the operator. All interference bands on the interferogram are processed and the results obtained are summarized. The number of 'mouse' clicks usually equals 10-20. The interferogram image of the bmp-format (monochrome variant) is used. A calculation needs a little preliminary image editing. Until recently the Program could only be used for calculation of interferograms obtained at customization of an interferometer on a band of infinite width.

Fig. 21.2 The painting of the first interference band (program N2)

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The program also allows (after full painting of the interferogram) determination if the square of the object image as a whole, the object volume and the square if its side surface (if an object has a cylindrical symmetry), as well as the object image perimeter, see Fig. 21.3 by one mouse click. These geometrical characteristics are determined by simple geometrical formulae linking the border co-ordinates of the image with the volume of the object, the square of its side surface, and the perimeter (envelope) of the image.

Fig. 21.3 The all interferogram area painting (program N2)

The interferometric cine film frames of polystyrene vapour ignition by laser radiation and the integral characteristics of flame and ignition process (see section 21.3): the enthalpy, heat release power, and heat release rate time dependencies are presented in Fig. 21.6. There is the possibility of using this program for other optical images of the combustion flow. The image sequences of exhausts from a pulse detonation engine (12) and curves of allocation of some parameters of the image are presented in Fig. 21.4.

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Fig. 21.4 The images sequence of exhaust of the pulse detonation engine for a threshold equal to 243. Threshold conversion is a process that sets a midpoint between black and white as the threshold and converts all shades above the threshold to white and all shades below to black. Under frames their numbers are given. A total number of frames are 11. The diagrams of change of the area and perimeter of the images for various threshold

21.5.1.3 Program N3 The program of the automatic determination of the 'full' function S(x,y). Unlike the program Nl, it allows it to process automatically all possible cross-sections on the interfere gram image. Usually, the number of cross-sections is near 300-400 and number of separate values of the S(x,y) near 300CMOOO. This program, as program N2, uses the principle of painting the

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interference bands (see the screen of the program N2 on Fig. 21.2). Unlike program N2, N3 determines no square of band, but co-ordinates the borders and middle of all the interference bands on all its length. The work of an operator consists of indicating the band number (usually about 10-20 numerals). If the screens of programs Nl (Fig. 21.1) and program N3 (it is the same as program N2 - Fig. 21.2), are compared it is possible to see that the work of the operator decreases at application of program N3. The interferometric cine film frames and temperature fields during the powder vapour ignition by laser radiation are presented on the Fig. 21.5.

Fig. 21.5 The interferometric cine film frames and temperature fields during the powder vapor ignition by laser radiation. The scale of temperature is given as line with different shades of grey. Under images the values of time (in seconds) are shown

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At present the program N3 can be used only for the decoding of monochrome images (bmpformat) and demands preliminary interferogram image editing. During editing, it is necessary to remove the unnecessary areas of pixels or separate pixels (noise). The program can be use only for interferogram obtained at customization of an interferometer on a band of infinite width. The detailed mathematical description of algorithm of interferogram image decoding is contained in (31). The computer codes of all above-mentioned programs are written in C++ language. At preparation of computer codes of program N2 and N3, the library of standard utilities for operation with bmp-images (the software package Jordan Hargraphix Software 91) was used. The Jordan Hargraphix Software ship is a 'donationware' product that includes BGI drivers for all kinds of SVGA graphics adapters, as well as 'tweaking' VGA in to unusual graphics modes. The product also includes a special mouse driver, because the ordinary mouse driver can not cope with all of the modes. The computer codes and texts of programs as well as the methodical help on operation with programs can be represented on request to the authors. The authors believe that these programs may be used for decoding and analysis of LIF-images (programs Nl-3). There is an interesting possibility of using the N3 program for the automatic determination of histograms of the particles distribution at the analysis of shadow and other images of the aerosol ensemble. There is a possibility of using the N2 program for decoding and analysis of x-ray images. 21.5.1.4 Program N4 Neural network program for interferogram image decoding and analysis. In using program N2, the operator should make approximately 10-20 clicks of the computer mouse. These operations are connected to determine a double integral. The involvement of the operator is obligatory. The operator should have good knowledge of the principles of interferometric image decoding. Therefore the determination of double integral can not be completely automized. It does not allow the use of the N2 program in the control systems of combustion processes, where the instant response to a change of the characteristics of the system is required. Therefore, one of the problems in the work which we have now, is learning all of the possibile uses of neural network's for determining the integrated characteristics of a flame on the basis of incomplete parameters of flame interferometric images. The required integral characteristics of a flame were mass of a flame, the Archimedes lifting force acting on a flame, and quantity of heat in a flame. As incomplete parameters of the interferometric images, the following geometrical parameters of the interferometric image of a flame were used: maximal height (h) and width (w) of the image, image square (s), and perimeter (e). Their determination is considerably more simple, than determination of double integral and can be completely automized. The Neural Network Wizard (NNW) 1.7 of BaseGroup Corporation (www.basegroup.ru) was used in our work. This program works on the basis of back propagation algorithm (32, 33). The NNW was trained with the help of various combinations of the above-stated geometrical parameters of the interferometric image. They moved on an input of an NNW. Three integral characteristics of a flame (mass, Archimedes lifting force, and quantity of a heat) were

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installed on an output of the program. It was necessary to find what combinations of input values give more precise results and to receive (to train) the NNW version, which would allow it to determine the integral characteristics of a flame on basis of incomplete parameters of the interferometric image.

Fig. 21.6 The interferometric cine film frames of the polystyrene vapor ignition by laser radiation and the enthalpy (H), heat release power (FT), and heat release rate (H'v) time dependencies

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The six sets of values of integral characteristics (output parameters) of the six flames (Fig. 21.6) and geometrical parameters of its interferometric images (input parameters) represented in the Table 21.1. The five sets were used at training the NNW. The set N4 was used for testing the program. Twelve various combinations of each set of values of the geometrical parameters were used as input data at training and testing of the NNW. One of the results of testing of the training NNW is shown in Figs 21.7 and 21.8. The horizontal line on Fig. 21.7 specifies the value obtained with the program N2, that is exact value. The vertical columns correspond to the values obtained with the help of the NNW. The each column corresponds to various input data combinations. For example, the signature (hse) shows that the height (h), the square (s), as well as the perimeter of the image (e) were used as the input data.

Fig. 21.7 Results of calculation of quantity of heat at various combinations of input data

Fig. 21.8 Relative errors of calculation of quantity of heat at various combinations of input data

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Table 21.1 The sets of values of integral characteristics (output parameters) and geometrical characteristics of the interference images (input parameters)

NO

1 2 3 4 5

6

w, cm,cm10-2 10 65 88 108 130 142 151

h, cm,10-2

S, cm2

e, cm

m, g

Fa, dynes

H, J

65

0.325 0.624 1.264 2.252

1.991

0.00008 0.00022 0.00052 0.00125 0.00190 0.00267

0.080 0.226 0.566

0.024

1.065 1.422 1.904

0.324 0.430 0.575

85 147 222 274 341

2.989 3.914

2.774

4.088 5.619 6.688 8.114

0.070 0.175

The results show that the NNW can determine the integral characteristics of a flame successfully enough. But the analysis of results also shows that the result of the NNW operation considerably depends from the combination of input data that is used at training. For example, if the combination of values of height and perimeter (he) of the image is used the error is small. The error is much higher if the combination of values of width and height of the image (wh) are used at training. The more detailed analysis shows that the combinations of values that include width and square of the image give higher error. On the other hand, smaller error turns out at usage of combinations of values that include height and perimeter of the image. That is, the values of height and perimeter of the image are more essential parameter to determine the integral characteristics of a flame, than width and square of the image. As a whole the results of operation show, that for calculation of the integral characteristics of a flame it is better to use the programs, trained on one of the following combinations of geometrical parameters of images: e, h, he, whe, hs. We have trained the Neural Network Wizard 1.7 on the basis of five interferograms (the sixth interferogram and its data were used for check of the trained program). If to increase the number of interferograms, it is possible to receive enough good program for calculation of integral characteristics of the flame formed at ignition laser radiation. The advantage of this program will consist of considerably more simple operation of the operator and the much greater speed of operation of the program. It is of great importance for combustion control systems. Determination of square, perimeter, height and widths of a flame are operations, which can be completely automized. It presumes to use the program in applied researches and at development of automatic control systems of combustion processes. Direct usage of the interferometric images in industrial systems is impossible. The interferometric methods can be used, in the main, at research of laboratory models of combustion processes. Therefore in control systems it is necessary to use neural networks trained on the usual video images of a flame, but with the help of exact interferometric methods. That is, input data at training the program will be geometrical and some others the automatically measured parameters of the video images. And the output data will be the integral characteristics of flame determined by interferometric methods. The solution of this problem will essentially allow expanding the opportunities of optical monitoring and control systems of industrial performances.

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21.6 Concluding remarks Visualization has an important value in the research of combustion processes. It is important from the point of view of the interpretation of measurement results, and qualitative confirmation of results of mathematical simulation. Optical methods that simultaneously allow a visualization of combustion flow and carry out quantitative measurements have even more value. The significance of similar methods grows, if these methods allow carrying out a control and process control of burning. The latter requires the automation of all stages of measurement, first the 'slowest' stage - the stage of decoding of optical image. The authors of this work have created several automatic procedures of image decoding and analysis that allow realizing the broad possibilities of optical methods in fundamental research. The using of these procedures and the high-speed video registration can greatly increase the applications of optical methods in scientific research and technical developments. At present, these procedures (programs N2 and N3) can be used only for monochrome images. In the future, we plan to develop procedures for analysis of the images with 256 gradation of grey, as well as colour images. Similar programs may be used in industrial systems where geometrical parameters of the image may be the factor of system monitoring or a system mode control. For example, geometrical parameters of the image of the engine, exhaust plume from the engine, the image of a fuselage can register in 'a black box' and can be used at the analysis of reasons of air catastrophes. For using a program N2 in monitoring of detonation engine it is necessary to provide registration of the exhaust plume image in bmp format (or to create the program similar to program N2 for those image formats in which video registration is realized); to provide the registration of momentary images in real-time and to automate the operation of painting of the necessary area of the image. Authors are planning a work, which will last in this direction. In our work we trained the Neural Network Wizard 1.7 on the basis of five interferograms (sixth interferogram and its data were used) for checkout of operation trained program. If to increase a learning gang of interferograms, it is possible to receive a good enough program for calculation of the integral characteristics of a flame generated at an ignition of powders by laser radiation. The advantage of the program will consist of a considerably more simple operation from the operator and speed of neural program. It has major value in scientific research of combustion processes. The determination of the area, perimeter, height, and the width of a flame are operations, which can be completely automized. It can allow using the neural program in applied research as well as at development of self-acting monitoring and controlling systems of combustion processes. Immediate usage of the interferometric images in industrial systems is impeded. The interferometry can be used as basic science research of laboratory models of combustion processes. Therefore, in hardware systems of control it is necessary to use neural networks, trained on the usual video images of a flame, but with the help of precise interferometric methods. That is, input data at training are geometrical and some other automatically defined

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characteristics of the video image. Output data are the integral characteristics of flame determined by interferometric methods. Global problems of our work are the research of neural network opportunities in the field of the analysis and recognition of complex optical images, and also the usage of neural networks in monitoring and control systems of combustion processes and modern detonation engines. The solution of it will allow essentially expanding opportunities of optical monitoring and control systems of industrial performances.

Acknowledgments This material is based upon work supported by the Russian Foundation for Basic Research under grant rVolga 98-07-03375, and partially by Office of Naval Research (USA) under Contract N 68171-99-M-6466 as well as Ministry of Education of Russian Federation under Contract b-13 of the Chuvash St. Univ., 2001.

References (1)

Abrukov, S. A., Abrukov, V. S., and Maltsev, V. M. (1996) Interferometry for Combustion Processes, Izvestija Natsionalnoy Akademii Nauk i Iskusstv Chuvashskoy Respubliki, 6, pp. 105-124,. (in Russian). (2) Abrukov, V. S., Ilyin, S. V., Maltsev, V. M., and Andreev, I. V. (1998) Interferometric Technique in Combustion, Gas Dynamic and Heat Transfer Research. New Results and Technologies, CD-ROM Proc. Of VSJ-SPIE98 Int. Conference on Optical Technologies and Image Processing in Fluid, Thermal, and Combustion Flow, AB076, VSJ, Yokohama,. (http ://www.vsj .or.jp/vsj spie/). (3) South, R. and Hay ward, B. M. (1976) Temperature Measurements in Conical Flames by Laser Interferometry, Combustion Sciences and Technology, 12, pp. 183-195,. (4) Grischin, M. P. (1976) Automatic input and processing of the photographic image on a computer, Moscow, Izd Energia,- 132 pp. (in Russian). (5) Reid, G.T. (1986) Automatic Fringe Pattern-Analysis - A Review, Opt. Laser Eng, 7, pp. 37-68. (6) Servin, Rodriguezvera, M. R., and Carpio, M. et al. (1990) Automatic Fringe Detection Algorithm Used For Moire Deflectometry, Applied Optics, 29, pp. 32663270,. (7) Gurov, I. P., Goyko, N. A., and Duhopel, 1.1. (1994) The automized optical-electronic system for the high-precision analysis of interferograms, Optichesky jurnal, 10, p. 1518, (in Russian). (8) Judge, T. R. and Bryanston-Cross, P. J. (1994) A Review Of Phase Unwrapping Techniques In Fringe Analysis, Optics and Lasers in Engineering, 21, N 4, pp. 199239. (9) Huntley, J. M. (1998) Automated fringe pattern analysis in experimental mechanics: a review, Journal Strain Anal. Eng., 33, pp. 105-125. (10) Batkovich, V. V., Budenkova, O. N., and Konstantinov, V. B. et al. (1999) About obtaining allocation of temperatures in a liquid and a solid with the help holographic interferometry, Zhurnal tehnicheskoy fiziki, 69, pp. 106-111, (in Russian).

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(11) Mende, N. P., Podlaskin, A. B., and Saharov, V. A. (2000) The automising system of image processing and reconstruction of density fields of the aerodynamic object, Zhurnal tehnicheskoy fiziki, 70, pp. 110-113, (in Russian). (12) Hanson Research Group Home Page/ A laboratory - scale pulsed detonation engine (PDE)/http://navier.stanford.edu/hanson/ (13) Abrukov, V. S. (1984), Interferometric Techniques in Condensed Systems Burning Research, Candidate Sci. Dissertation (PhD Dissertation), The Chernogolovka Institute of Chemical Physics, Chernogolovka, Moscow Region, Russia, 200 pp. (in Russian). (14) Abrukov, V. S. (1994) Interferometric Techniques in Combustion Research, Dr. Sci. dissertation, The Semenov Institute of Chemical Physics, Moscow, Russia, 173 pp. (in Russian). (15) Abrukov, V. S. (1986) Interferometric Techniques in Combustion Research. Methodological Guidelines for Laboratory Work //Prepared by Abrukov, V. S. Chuvashsky Universitet (Chuvash State University), Cheboksary,. 37 pp. (in Russian). (16) Vest, C. M. (1979) Holographic Interferometry, Wiley, New York,. 480 pp. (17) Holographic Interferometry: Applications. Rajpal Sirohi, Wolfgang Osten, Cho Tay, Hua Shang, Fook Chau, Editors. SPIE Vol. MS 170, 2001. 768 pp. http://www.spie.org/web/abstracts/oepress/MS 170.html (18) Bryanston-Cross, P. J. and Towers, D. P. (1993) Quantitative holographic interferometry applied to combustion and compressible flow research. - In: Holographies International '92, Yuri N. Denisyuk; Frank Wyrowski; Eds. Proc. SPIE Vol. 1732, p. 533-546. http://spie.org/scripts/abstract.pl?bibcode=1993SPIE%2el73 2%2e%2e533B&db key=INST&qs=spie&sJype=adv_paper (19) Ginzburg, V. M. (1997) Measurement of high-speed processes by pulsed holography and a shift interferometer Ultrahigh- and High-speed Photography and Image-based Motion Measurement, D. R. Snyder; A. Davidhazy; T. Etoh; C. B. Johnson; and J. S. Walton; Eds. Proc. SPIE Vol. 3173, p. 153-160. http://spie.org/scripts/abstract.pl7bib code=1997SPIE%2e3173%2e%2el53G&db_key=INST&qs=spie&s_type=adv_paper (20) Shakher, C. and Thakur, M. (2001) Lau phase interferometer for the measurement of the temperature and temperature profile of a gaseous flame. - In: Optical Engineering for Sensing and Nanotechnology (ICOSN 2001), Koichi Iwata; Ed. Proc. SPIE Vol. 4416,, p. 246-251. http://spie.org/scripts/abstract.pl?bibcode=2001SPIE%2e4416%2e% 2e246S&db_key=INST&qs=spie&s_type=advjaper (21) Ito, A., Majidi, V., and Saito, K. (1994) Temperature measurement by holographic interferometry in liquids for transient flame spread. Laser Applications in Combustion and Combustion Diagnostics II, Randy J. Locke; Ed. Proc. SPIE Vol. 2122, p. 176-185. http://spie.org/scripts/abstract.pl?bibcode=1994SPIE%2e2122%2e%2el76I&db_key=I NST&qs=spie&s_type=adv_paper (22) Abrukov, V. S., Ilyin, S. V., and Maltsev, V. M. (1995) Interferometric Techniques and Other Optical Methods in Combustion Research. A New Approach. Optical Techniques in Fluid, Thermal, and Combustion Flow, San-Diego, USA,. S. S .Cha and J. D. Trolinger, Eds, Proc. SPIE's Int. Symp., Vol. 2546, pp. 420-26. (23) Abrukov, V. S., Khristoforov, A. V., Nikonorov, V. E., and Andreev, I. V. (1999) Interferometric and shadow technique in analysis of combustion wave shaping and development during ignition of a solid. Optical Diagnostics for Fluids/Heat/ Combustion and Photomechanics for Solids, S. S. Cha; P. J. Bryanston-Cross; and C. R. Mercer; Eds., Proc. SPIE Vol. 3783, p. 347-351. http://spie.org/scripts /abstract.pl?bibcode=1999SPIE%2e3783%2e%2e347A&db_key=INST&qs=spie&s_ty pe= adv_paper full text: www.chuvsu. ru/~victor

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(24) Abrukov, V. S. and Ilyin, S. V. (1997), Interferometric Techniques in Combustion, Gas Dynamic and Heat Transfer Research. Chuvash State Univ., Cheboksary, 32 pp. (in Russian) www.chuvsu.ru/~victor (25) Gardiner, W. C. Hidaka, Y., and Tanzava, T. (1981) Refractivity of Combustion Gases, Combustion and Flame, Vol. 40, N 2, pp. 213-219. (26) Abrukov, V. S. and Maltsev, V. M., (1984) Interferometry of Combustion Processes. Review of the Technique. Issledovanie Protsessov Neustoychivogo Goreniya, Chuvash State University, Cheboksary, pp. 87-104. (in Russian). (27) Abrukov, V. S. and Maltsev, V. M. (1996) Interferometry of Complex Heat Exchange, hvestija Natsionalnoy Akademii Nauk i Iskusstv Chuvashskoy Respubliki, 6, pp. 125129 (in Russian). (28) Abrukov, V. S. and Ilyin, S. V. (1991) Analysis of the Dynamic Characteristics of Non-Stationary Gas Stream Using Interferometry Techniques, Proceedings of SPIE's Int. Symp., Ed. Fu-Pen Chiang, SPIE, San-Diego, Vol. 1554 B, pp. 540-543. (29) Ilyin, S. V., Abrukov, V. S., and Abrukov, S. A., (1994) Tomographic Reconstruction of Velocity and Pressure Fields of Ignition and Explosion Gas Flows Based on Interferometric Measurements. In: Book 'Non-Intrusive Combustion Diagnostics' Ed. by Kenneth K. Kuo, Begell House Inc., N.Y., pp. 294-298. (30) Ilyin, S. V. and Abrukov, V. S. (1994) Potentialities of Interferometry in the Study of the Energy Structure of Combustion - Generated Acoustic Fields. Abstract of Symposium Paper and Abstract of Poster Session Presentation/Twenty Fifth Int. Symp. on Combustion (July 31-August 5, (1994) The Combustion Inst., Pittsburgh. (31) Davydov, A. E., Abrukov, V. S., and Alimov, K. K. et al. (2001) Optical Methods for Combustion Research. Chuvash St. Univ., Cheboksary, Russia,. 160 pp. (in Russian). (32) Jeenbekov, A. A. and Sarybaeva, A. A. Conditions of convergence of backpropagation learning algorithm. Optoelectronic and Hybrid Optical/Digital Systems for Image and Signal Processing, Simon B. Gurevich; Zinovii T. Nazarchuk; Leonid I. Muravsky; Eds. Proc. SPIE Vol. 4148, 2000, p. 12-18. http://spie.org/scripts /abstract.pl?bibcode=2000SPIE%2e4148%2e%2e%2el2J&db_key=INST&qs=spie&s _type=paper (33) Ge Yi, Li Zhenhua, Zhang Li, He Anzhi, and Lu Ai-Ming. (1996) Improved backpropagation algorithm applied to target recognition. In the International Conference on Holography and Optical Information Processing (ICHOIP '96), G. Mu, G. Jin, and G. T. Sincerbox; Eds. Proc. SPIE Vol. 2866, p. 112-115. http://spie.org/scripts/abstract .pl?bibcode=1996SPIE%2e2866%2e%2ell2G&db_kev=FNST&qs=spie&s_type=paper V S Abrukov, I V Andreev, and P V Deltsov Chuvash State University, Cheboksary, Russia

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22 Application of Physical Modelling to Study Combustion Processes and Flow Patterns in Large-scale Boilers and Furnaces J Baranski, W Blasiak, and J Sta.siek

Abstract Physical modelling is an easy way to simulate combustion and flow patterns by visualization in cold, isothermal models (1). This is done in steps beginning with a burner model to investigate the flame and continuing with a boiler model to visualize the flow pattern and combustion processes. These models must be used with certain similarity criteria to visualize real industrial processes properly (2). Together with mathematical modelling high accuracy in simulation will be achieved. This work presents application of modelling tools applied to different fired boilers.

22.1 Introduction Despite great advances, which have been made in combustion modelling over the last decades, its application to real combustion chambers is far from common practice (3). The real industrial combustion process is one of the most difficult to model. Much research has been devoted to developing simplified methods for the optimization of combustion processes, which occur in furnaces or boilers and very often represent a compromise between the accuracy of a model and the production of useful results. There is often a need for modifying and improving the quality of combustion to achieve a reduction of emissions and to get a more rapid and efficient mixing of reactants. This could be done by studying the flow pattern in the furnace and in the combustion zone in particular.

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To model large-scale industrial units, like furnaces and boilers or burners, the physical modelling technique is used. It is an efficient, cheap, and rapid method to optimize or design modern boilers, burners, and furnaces for implementation of low emission of NOx, SOx, CO, soot, and volatiles in real processes.

22.2 Physical modelling Numerous studies in the field of heat and combustion technology have shown that physical modelling is a valuable experimental method involving small-scale water models for the study of aerodynamics of non-isothermal flow and combustion processes occurring in large-scale combustors. The technique makes it possible to visualize the movement and mixing of combustion air, fuel, and exhaust gases. Assuming that combustion process is controlled by turbulent mixing, the shape of the turbulent diffusion flame can be studied and visualized by using acid-alkali technique. The basis of the physical modelling is similarity theory. To physically simulate flow and mixing behavior in two- and three-dimensional, an isothermal water must meet a number of similarity criteria. These are necessary for qualitative and quantitative measurements in the model. By using dimensional analysis and differential equations, some dimensionless numbers of the physical model have the same values as the prototype. To obtain similar flow patterns in the model and in the real combustion chamber, kinematic and geometric similarities are necessary. This is usually done by keeping the Reynolds number the same in the model and the prototype, or, if the combustion process is controlled by the turbulent mixing, by ensuring that the model is kept turbulent. To properly model the interaction between the hot furnace gases and the cooler air jets in a cold isothermal model, the thermal expansion of the jets is taken into account using so called equivalent dimensions principle (thermal similarity). To model mixing between fuel and combustion air it is necessary to create the same momentum ratios between each flow in the model as in the real combustion chamber. There is a strong relationship between studying the flow pattern in order to obtain a better mixing of air, fuel, and flue gases in the furnace room and having a decreased amount of different emissions of combustion products and an increased fuel saving. Therefore, it is interesting to investigate the flow pattern and by cheap means study what effects different changes will do to the boiler. Many times, physical modelling allows processes to be tested, which could never be examined in a real plant because of the cost and the risk for damage to the plant. The model offers numerous possibilities such as changing the position of the burners or air inlet ports and the combustor shape. In practice it is not possible to maintain all similarity criteria at the same time - one is forced to choose. A problem then is to decide which criteria can be neglected. The choice must be based upon experience, comparative experiments, simple calculation, and so one. This is one of the reasons why modelling is sometimes referred to as an art rather than a science. If possible a model investigation should be conducted in three steps: • experiments in a model of an existing plant under known conditions and comparison with experience from the plant - this will verify how well the model simulates real conditions; • the main experiments are carried out in a model of the planned plant, usually a newly proposed one;

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• when the new plant has been taken into operation works trials are conducted in order to verify the results of step 2 - in this way experience is continuously built up so that uncertainties of the model results become smaller and smaller. The final design of the model is therefore often a compromise. Skills are required here in identifying the significant similarity criteria that need to be maintained. Once the model is built, a variety of experimental techniques are available to visualize the flows and obtain detailed measurements of required parameters. 22.2.1 Two-dimensional visualization The most commonly used physical model is the two-dimensional (2D) water table modelling technique. It is used primarily to provide qualitative information about combustion chamber performance. In two-dimensional modelling, water is used to represent the combustion gases or products and their mixing. The two-dimensional model is designed to visualize the flow pattern of the gas phase, and simulates and visualizes the non-isothermal combustion processes despite isothermal conditions by using solids particles (e.g. aluminum powder) and colored dyes. The simplicity of this technique is its main advantage. The two-dimensional model is a simple replica of the boiler's or furnace's vertical or horizontal cross section. The main dimensions of the model are scaled down while maintaining geometrical similarity, but dimensions of the inlets are increased according to and equivalent diameter concept. The concept is based on the Thring-Newby similarity criterion, which takes into consideration thermal expansion of the jet's cross-section when it enters the hot environment of combustion chamber. An original construction of a twodimensional model allows easy changes to be made to the configuration and size in the working section of a real boiler or furnace, which is made from aluminum plates in the water table. The two-dimensional model is arranged in a horizontal plane and placed in a water table, which allows observation and visualization of the flow, Fig. 22.1. Advanced digital video techniques are used so that the general flow patterns in a combustor can easily be seen and characterized depending on the combustor geometry and boundary conditions.

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Fig. 22.1 Example of the two-dimensional (2D) visualization of the flow pattern and mixing performed during physical modelling in the water table (4)

Simple two-dimensional modelling can be the answer to many questions regarding the malfunctioning of the real combustor at an early stage before any three-dimensional modelling is performed. Information concerning the flow pattern helps to indicate the direction of the combustor and combustion process optimization. Thus, two-dimensional modelling provides valuable information required to design a three-dimensional model, which is suitable for studies of, e.g. different low-NOx combustion techniques or modifications in the design of a combustor. 22.2.2 Three-dimensional visualization The three-dimensional models are built of transparent materials, e.g. Plexiglas, on a scale of 1/30-1/50 and have usually a complex design, Fig. 22.2. The geometrical similarity is maintained but as in the two-dimensional case, the concept of equivalent diameter is used to calculate all inlet dimensions. The model need not be a full three-dimensional replica of the plant and in some examples only small sections of the equipment are simulated, often called partial modelling (1). An important feature in the design of the model is the facility of view its interior.

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Fig. 22.2 Examples of the three-dimensional (3D) visualization of the combustion processes performed during physical modelling in different types of boilers: (a) tangentially coal and gas fired (5); (b) wall coal and peat fired (6); (c) oil fired (7)

This technique is used, when it is necessary to simulate flame length and shape in a smallscale isothermal physical model in order to investigate the effects on a plants thermal performance of a change to the system geometry, fuel and air mixing arrangement, or other operating parameters such as air excess factor. In this way the three-dimensional model allows processes to be tested, which could never be examined, in a real plant because of the cost and risk of damage to the plant. The three-dimensional technique is used to visualize flow patterns in the gas phase and to simulate and visualize non-isothermal combustion processes in the isothermal conditions of the physical model. To simulate and visualize combustion in the water model 'neutralization technique', sometimes called 'acid/alkali', is used. Due to the effectiveness and practical usefulness of this technique, it has been developed in to an effective method for the study and design of complex, environmentally clean combustion processes. When used alongside computer simulations, it is a useful tool for the design of new and retrofitting of industrial combustion chambers. Of course, many assumptions and simplifications are made to allow the physical modelling of such complex chemical and non-isothermal processes. The technique simulates the mixing in the flame by using dilute solutions of sulfuric acid and sodium hydroxide to represent combustion air and fuel respectively. The alkali contains colored indicator, which becomes clear on neutralization after mixing with diluted acid. The result is a visual representation of the diffusion flame. This method gives quantitative information that is directly related to the flame characteristics, but it is not appropriate for investigating the complex combustion and aerodynamic processes immediately in front of the burner.

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The technique is developed around two basics concepts: • in a free, expanding turbulent jet, the concentration of entrained fluid as a function of axial distance only depends on the initial force (thrust) of the jet, it depends of Reynolds number provided that fully turbulent flow exists; • the combustion rate in the turbulent diffusion flame is limited by fuel-air mixing and not by the rate of chemical reaction - the 'mixed-is-burned' concept, hence better mixing produces more rapid combustion and thus shorter flames. A similar process exists for a turbulent alkali jet entraining and reaction with the acid. In this case it is the acid entrainment rate alone which determines the progress of neutralization. The relative strengths of the acid and alkali should be arranged so that the ratio of the acid molarity to alkali molarity is equal to the stoichiometric air requirement on a mass basis. The reaction could be strong acid plus strong alkali resulting in neutral salt. As a result, complete neutralization takes place when the two solutions are mixed in the same proportions, as that required for stoichiometric combustion; the product of acid flow rate and acid molarity is equal to the product of alkali flow rate and alkali molarity. There are some indicators, which are colorless in acid solutions, e.g. thymolphthalein and phenolphthalein. A calibrated and reliable pH meter can be used to find the end-point and burnout profiles precisely. Variation of pH values will then correspond to the degree of neutralization and hence to the burnout.

22.3 Application of physical modelling to optimizations and redesign of industrial boilers 22.3.1 Combine combustion system for wall-fired boiler (6) At the Firyskraft power station in Uppsala small solid particles (powders) of coal and peat are fired and the aim for the study of this plant was to develop and to apply by modelling the complicated combustion operations. The combustion system of a 400 MWei pulverized-fired boiler, Fig. 22.3, has been design for two types of fuels and was based on results of experimental, physical and numerical modelling, studies of the boiler furnace aerodynamics, effectiveness of mixing, and combustion processes.

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Fig. 22.3 Modelling of wall coal-peat fired boiler by using: (a) three-dimensional (3D) physical, (b) numerical modelling techniques (comparison of mixing process)

At the beginning of the boiler's retrofit, the burner model should to be able to model representative flame shapes for two types of fuel. In-furnace measures to reduce NOx emission as air staging and fuel staging and flue gas recirculation were studied taking into consideration the boiler geometry and operating conditions. Then, systematic in the model to control the installed OFA (over fire air) system gave that it was in need of service to be able to get an effective NOx reduction. Complete combustion process during under-stoichiometric conditions at the burner level was obtained when 20-30 per cent of the combustion air and 20 per cent recirculated flue gases were supplied via the OFA nozzles. Finally, the distribution of fuel between burner level was found to create even deeper under-stoichiometric conditions in the combustion chamber to reduce NOx. 22.3.2 Reburning system for tangentially-fired boiler The complete combustion system of the Limhamn district heating plant 125 MWa tangentially-fired boiler, Fig. 22.4, with pulverized coal was redesigned based on results from mathematical and physical modelling experiments. The boiler was equipped with Low-NOx burners with an over fire air system (OFA). The aim of the boiler modification was to reduce NOx emissions (about 250 ppm at 3 per cent 02 in a waste gas) by installing a reburning system. As a reburn fuel natural gas was used.

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Fig. 22.4 Modelling of tangentially coal-natural gas fired boiler by using: (a) three-dimensional (3D) physical, (b) numerical modelling techniques; mixing - a, velocity vectors and temperatures - b

Primary in-furnace measures to reduce NOx as air staging and fuel staging were studied taking into account the boiler geometry and operating conditions. On the other hand, the design of a new, low-NOx combustion process was based on experimental (physical modelling) and theoretical (mathematical modelling) studies of the boiler furnace aerodynamics, effectiveness of mixing and combustion processes. The experiments were conducted in a 1:40 scale model, equipped with nozzles that could be tilted in vertical and horizontal directions in order to optimize the size and position of the natural gas nozzles and additional air ports. Before and after the retrofit extensive experiments were carried out. It was found that the combustion and reduction of NOx could be efficiently influenced by the direction of the additional air nozzles, without recirculated waste gas. The direction of the natural gas ports had a little influence on NOx reduction. These results of the modelling were in good agreement with results of in-boiler experiments.

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After installation and adjustement of the reburning equipment plant tests showed that low emission of the NOx, to about 120 ppm, have been achieved. This shows that reburning is an efficient way to reduce nitrous oxides.

22.4 Activities of the Heat Technology Department at TUG The Heat Technology Department in the Faculty of Mechanical Engineering at the Technical University of Gdansk (TUG) undertakes scientific work in various areas of energy transfer, high temperature energy conversion, and fluid dynamics. The group has considerable experience in experimental investigation of high temperature coal gasification and combustion, numerical modelling and turbulent flow in complex geometries, research, and expertise of thermal-power engineering systems. In coal gasification and combustion the Department provides advanced temperature, velocity, and fuel combustion, measuring devices, such as high-speed video camera comprising a digital image processing system. In numerical predictions the moving boundary and coupled wall/fluid interaction problems are also a current interest. The conventional models of turbulence, e.g. k-s, algebraic-stress, Reynolds-stress models have been used in simulations of flows in compact heat exchangers. The group expertise comprises advanced measurements method with opto-electronics, fundamental termography together with comprehensive abilities in CFD (CFX and FLUENT codes). The Heat Technology Department laboratory also provides basic experimental and modelling tools to facilitate in site measurements and physical and numerical modelling of combustion in laboratory scale. Polish Scientific Committee (KBN) and various international agencies (e.g. TEMPUS PhareJEP3594, EPSRC UK) support these works. The Heat Technology Division possesses well equipped heat transfer and combustion laboratories, which are listed below: • • • •

two-dimensional and three-dimensional physical modelling laboratory for simulation of combustion processes in boilers, high temperature reactors, and burners; temperature measurement system using reversed sodium line method; an advanced true-color image processing system based on Frame Grabber and Acquisition Boards of Data Translation Inc. USA and JVC RBG camera; numerical simulations laboratory equipped with workstations: SUN Spare-station LX and Ultra 5, AEA Technology (CFX 4.3) and FLUENT Software, possibilities of using a supercomputer for parallel computing at the Technical University of Gdansk Computer Centre.

The new laboratory for modelling of combustion processes at the Heat Technology Department makes it possible to analyse and study the optimization of very different types of boilers. Together with numerical modelling and in situ measurements this can give answer about ways to get the best mixing in real industrial combustor.

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22.5 Summary The physical modelling technique is an established method for the optimization and redesign of industrial combustion chambers like boilers, furnaces, or burners.

22.6 Acknowledgement Thanks to cooperation between Royal Institute of Technology, Stockholm, Sweden and Technical University of Gdansk, Poland and help of W. Blasiak, J. Stasiek, and J. Baranski from Department of Heat Technology are gratefully acknowledged for participation in researches conducted during visit in Heat and Furnace Technology Group, Department of Metallurgy at Royal Institute of Technology, Stockholm, Sweden. The financial assistance of the State Committee of Research Science (KBN), Poland within Grant No 8T10B04615 is kindly acknowledged.

References (1) (2) (3) (4)

(5)

(6)

(7)

Rhine, J. M. and Tucker, R. J. Modelling of gas-fired furnaces and boilers, British Gas, 1991. Spalding, D. B. The art of partial modelling, Ninth Symp. (intl.) on Combustion., pp. 833-843, Academic Press, New York, 1963. Chomiak, J. Combustion: A study in theory, fact and application, Abacus Press, 1990. Vaclavinek, J., Baranski, J., Helen, C., and Blasiak, W. Analysis of flow pattern in coal grate-fired boiler OSR-32, Division of Heat and Furnace Technology, Royal Institute of Technology, Stockholm, 1998. Bis, Z., Blasiak, W., Magnusson, L., and Collin, R. Reburning - injection methods, analytical and experimental study. Publications from Nordic Gas Technology Centre. ISBN nr.: 87-89309-17-0. Olsson, E., Blasiak, W., Vaclavinek, J., Landtblom, M., Magnussen, B., and Grimsmo, B. Reduction of NOx emissions from an industrial oil fired boiler. 4th European Conference on Industrial Furnaces and Boilers, Porto, Portugal, April 1-4, 1997. Vaclavinek, J., Lidegran, P., Helen, C., Bang, J., and Blasiak, W. Physical modelling of Fyriskraft boiler, KTH, Div. of Heat and Furnace Technology, Stockholm, May 1997.

Bibliography Blasiak, W. and Collin, R., Large scale mixing in a recovery furnace, VI Congreso Latinoamericano De Celuloso y Papel, Torremolinos, Spain, 23-25 June 1992. Blasiak, W. and Vaclavinek, J., Modelling of grate fired incinerators, Eurotherm Seminar No.35, Compact Fired Heating Systems, Leuven, Belgium, May 26-27, 1994. Someya, T., Advanced Combustion Science, Springer-Verlag, Tokyo 1993.

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Vaclavinek, J., Physical modelling of a fluidized bed incinerator - NOx reduction, internal report, Dept. of Heat and Furnace Technology, Royal Institute of Technology, Stockholm, 1993. Wei, D., Helen, C., Bang, J., Blasiak, W., and Vaclavinek, J., Analysis of combustion processes in grate fired boiler at Braviken Paper Mill, Division of Heat and Furnace Technology, Royal Institute of Technology, Stockholm, 1997.

J Baranski and J Stasiek Technical University of Gdansk, Poland W Blasiak Royal Institute of Technology (KTH), Sweden

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23 Pulsed Laser Particle Image Velocimetry using a Fibre-optic Delivery System S S Coltman and C A Created

Abstract The work outlined in this Chapter is concerned with the study of a particle laden turbulent airflow in a pipe of square cross-section. The velocity of the flow is obtained using a particle image velocimetry (PIV) setup where a novel fibre-optic delivery technique is employed (1, 2). Three sizes of glass beads were used as seeding particles in these experiments, with average diameters of 59 ±51, 173 ±53, and 471 ±120 mm, and all of density 2499 ±53 Kg m-3. These particles are referred to as small, medium, and large respectively throughout the rest of this Chapter. These experiments were repeated with a more conventional beam delivery system for comparison. Measurements have shown that the velocity values obtained with the fibre setup are, in some cases, underestimated with respect to the direct beam results. This is because the fibre only illuminates the larger beads in the flow sufficiently for the camera to detect them. Both sets of results also show a skewness of the mean velocity profiles. This is possibly due to light being scattered by particles as it travels across the duct, again resulting in biased illumination of the particles.

23.1 Introduction The work outlined here looks at the behavior of large particles in a fully developed turbulent airflow in a vertical square duct. The particle motion was measured with two different laser beam delivery systems, a conventional direct beam and a novel fibre optic delivery system. As well as velocity profiles and turbulence statistics, the distribution of particles within the flow is also to be considered. Three sets of experiments were carried out with the fibre bundle, one for each size of seeding particle. These experiments were then repeated without the fibre for

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comparison. The expectation was that the large particles would not follow the higher frequency components of the turbulent flow accurately, due to their inertia being greater than a corresponding volume of fluid.

23.2 Experimental setup The experimental setup required to carry out this work involves many different components, some of which are interlinked. For this reason the two main sections of the rig are described separately. The flow setup is shown in Fig. 23.1. Air is drawn into the setup and passes through a flow valve, which is used to control the speed of the airflow. A thermometer next to the air inlet is used to compare the external air temperature with the temperature inside the duct just after the square section. The air temperature is monitored so that the viscosity of the air can be accurately determined, the temperature on the internal thermometer gives a slightly higher reading due to frictional heating within the fluid flow. After the flow valve any seeding particles that are required are fed into the duct.

Fig. 23.1 The airflow setup

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Corn oil particles are produced by an atomizer run by a pressurized cylinder of nitrogen, and have a diameter of approximately 2 mm. The glass beads with average diameters of 59, 173, and 471 mm are fed into the flow by a mechanical feeder consisting of a vibrating plate and a storage chute. As particles are fed into the flow by the vibrating plate the chute slowly empties its contents out, keeping the plate fully stocked. In both cases the particles are fed into the flow at the wall of the pipe and the turbulent flow distributes them across the pipe. After the seeding particles are fed into the flow the straight test section of the rig begins. The first two metres of this section is circular corrugated piping. This is followed by a three metre stretch of square glass duct. This entire straight section gives the flow enough time to become fully developed (1). The air is finally fed through a cyclone separator (which removes any glass beads with diameters greater than 7 mm for flow speeds of the order of 15 ms-1) before reaching the blower which drives the flow. This arrangement ensures that the blower is not damaged by any glass beads and that the beads are not introduced into the free atmosphere.

23.3 Image acquisition setup

Fig. 23.2 The image acquisition setup

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The image acquisition setup involves many interlinked components (see Fig. 23.2). The initial trigger for the whole process comes from the computer software package VidPFV (optical flow systems), which sends a trigger pulse to the timing box. There are two outputs from the timing box; one to the laser timing box; and another to a Kodak Megaplus CCD Camera. When the trigger pulse reaches the camera an exposure event is started. An exposure event in this case involves the camera taking two images, with a time interval of 40 ms between exposures. The first exposure is set to 1.53 ms, whereas the second exposure is fixed (by the camera) to 33 ms. Since the particles are only illuminated sufficiently for detection by the 6-7 ns pulse of light from the laser, the condition of frozen particle images is fulfilled. The only effect of the disparate timings is a brighter background in the latter images, which is compensated for by normalizing both images before analyzing them. The trigger pulse that is sent to the laser goes through another timing box because the laser requires three separate trigger pulses to produce two pulses of light. The first of these triggers the flash lamp which optically pumps the excitation medium in the laser. The other two trigger the laser pulses themselves.

23.4 Fibre optic delivery

Fig. 23.3 A conventional lens setup for producing a light sheet

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This conventional method of beam delivery involves a system of three lenses, two spherical lenses to control the beam width and a cylindrical lens to expand the beam into a light sheet (see Fig. 23.3). However the beam is initially aligned by moving the laser head, since the lens setup cannot control the beam's direction. The laser head is quite large (about 1 m long) so it can be difficult to use in places where space is limited. Also alignment can take a long time since the laser head is quite heavy and difficult to move in a controlled manner. Fibre optics are often used to convey continuous wave laser beams but, due to the high peak powers involved, fibres are usually irreparably damaged when used in conjunction with pulsed lasers. However the technique outlined here allows these high energy pulses to be delivered without damaging the fibres (2).

Fig. 23.4 The fibre optic delivery system

Figure 23.4 shows the basic setup for this beam delivery technique. The diffractive optic element (DOE) is the most important part of the optical setup. The DOE has an uneven surface which causes light that passes through it to be diffracted. Different parts of the incoming laser beam are diffracted by different amounts so that when the beam leaves the DOE its energy is distributed over a small range of angles (the half angle of this distribution is 35 mrad). After the DOE the beam is allowed to expand before being focussed into the fibre bundle. Each fibre in the bundle is 200 mm in diameter, and there are 19 fibres in total. Fibres of this size are multi-nodal, i.e. there is more than one path that a light ray can take down the fibre. If a laser beam is directly focussed into a fibre very few modes are excited and the beam refocuses just inside the fibre, the high energy density at this point causes further refocusing,

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the net result being irreparable damage to the fibre. However, the dispersion caused by the DOE means that in the new setup the beam energy is more evenly distributed across the different modes of the fibre. This means that refocusing does not occur and the fibre remains undamaged. Using an array of fibres means that even higher pulse energies can be transmitted since each fibre carries only a fraction of the total beam energy. At the output end of the bundle the light spreads out (producing a speckle pattern in the far field) and a positive cylindrical lens is used to focus the light into a thin sheet. In our setup a second lens is also used to narrow the angle of divergence of the beam before the cylindrical lens. The advantage of the fibre bundle setup is that the repositioning of the light sheet is made substantially easier and less time consuming. The end result is a more robust experiment setup.

23.5 Results 23.5.1 Mean Flow Results

Fig. 23.5 Comparison of fibre and direct beam results for streamwise velocity

The images were analyzed using a standard cross-correlation technique [3]. A 32 x 32 interrogation window was used as well as a single re-interrogation to reduce random errors. A three point Gaussian peak estimator was used to identify the correlation peak to an accuracy of l/20thofapixel(3).

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Figure 23.5 shows the mean streamwise flow results taken with both setups. The error in these results is due to the peak fitting error, and the error in the calibration value calculated to convert the displacements into real velocities. The large particle streamwise results have larger error due to peak locking problems, making the peak fitting error 1/2 a pixel. Peak locking is normally due to small particle images and so it is odd that it occurs in the large particle results. One possible explanation is the smaller number of particles that are present when the large particles are used. Other possible errors in PIV include out of plane motion and velocity gradient biasing. Due to the symmetry of the flow (see Fig. 23.6) we can take the maximum out of plane velocity as 0.2 ms-1, the largest spanwise velocity for small particles, giving a displacement of 0.008 mm between images. The light sheet is of the order of 1 m and so the out of plane motion is negligible. The velocity gradient is at the most 7 per cent of the mean velocity within an interrogation area, taken from the small particle results in the wall region, and so the influence of velocity gradients can also be neglected (3). The streamwise velocity graphs would be expected to have a maximum at y/D = 0.5, and be symmetric about this point. This is clearly not the case in the small and medium particle results, but the errors in the large particle case make such distinctions impossible. The reason for this skewness will be outlined later. The spanwise results are expected to be antisymmetric about y/D = 0.5 where there should also be a zero point. This profile is due to the fact that in fully developed square pipe flow there are circulation cells as shown in Fig. 23.6.

Fig. 23.6 A view down the length of the duct showing the form of the circulation cells present in fully developed turbulent square pipe flow

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Comparing the streamwise results from both setups an interesting trend is observed. While the two sets of results taken for the small particles compare reasonably well, the results for the fibre have a noticeable velocity lag for the other two particles. The is due to the differences in illumination level for the two setups. While the direct beam has an energy of around 55 mJ the fibre output is only 12 mJ, a loss of 78 per cent. The amount of light scattered by an individual particle is proportional to the particle size, and so larger particles are more likely to be detected by the CCD camera. This biased illumination means that for lower illumination levels some of the smaller particles will not be visible, and so they will not contribute to the velocity estimations. Since these smaller particles move faster than their larger heavier counterparts, this results in an underestimation of the velocity in the fibre results when compared with the direct beam results. It is clear that the full range of sizes have been illuminated in the small particle results, since both graphs compare well, but for the other two cases there is a significant velocity lag in the fibre results. It should be noted that there is no guarantee that the direct beam has illuminated the full range of particles sizes in these cases either; only that it has illuminated a wider range than the fibre. However in more recent experiments it has been found that the laser power had to be slightly reduced to compensate for excessive glare in medium particle experiments. This would seem to suggest that the medium particle results presented here also represent data where all particle sizes have been illuminated.

Fig. 23.7 Comparison of fibre and direct beam results for mean spanwise velocity (top left: small particles, top right: medium particles, bottom: large particles)

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Fig. 23.7 Cont. Comparison of fibre and direct beam results for mean spanwise velocity (top left: small particles, top right: medium particles, bottom: large particles)

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The skewness in the graphs mentioned earlier is most probably due to a similar effect. As the light passes through the beads it is scattered and so the light sheet becomes less intense as it moves across the duct. This would again lead to a preferential illumination of the particles, resulting in the velocities being more greatly underestimated as y/D gets closer to zero (the light is introduced at y/D = 1). This effect could be further compounded by PIV's preference for underestimating velocities, since smaller displacements generally have a larger signal to noise ratio (4). Figure 23.7 shows graphs comparing the spanwise velocity profiles for the three different particle sizes. It should be noted that the large variations near the walls in the fibre results for the small particles, are due to large bands of glare caused by reflections. The graphs seem to suggest that the secondary flow cells are not symmetrical, something that was found to be the case in earlier work with this rig (1). This is most likely due to a slight asymmetry in the setup, at the point where the straight section of circular piping is coupled to the square duct. The results seem to compare reasonably well between setups, although the medium particle velocities in the fibre case are significantly larger than the direct beam values. We would expect these results to compare quite well since the secondary flows are normally around 1-3 per cent of the streamwise velocity, which is not significantly different despite the preferential illumination of particles. 23.5.2 Turbulence statistics

Fig. 23.8 Comparison of turbulent kinetic energy results

Figure 23.8 shows a comparison of the turbulent kinetic energy results for both setups. The turbulent kinetic energy of the airflow should peak near the walls and fall off to a plateau at the centre of the duct. This is the case for the small particles but the other particles show a

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significant departure from this form. Comparing the direct beam and fibre results for each particle size we see that the curves have a similar shape, however the fibre results are always slightly smaller than the corresponding direct beam results. The differing shape of the profiles can possibly be explained by considering the distribution of turbulent eddy sizes within the duct, hi turbulent pipe flow the eddies found near the wall are generally smaller than the eddies found at the centre of the pipe, and we would expect the larger particles to be less sensitive to the motion of the smallest eddies.

Fig. 23.9 Comparison of normal stress results (left: spanwise component, right: streamwise component)

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Figure 23.9 shows the graphs for the streamwise and spanwise normal stresses, again the results for the two setups have the same shape but the fibre results are always smaller. One interesting aspect is that for small particles the spanwise component is noticeably smaller for the fibre result, the first time there has been a large difference for small particle results. Again the small particle results have the expected profile while the other particle results deviate markedly from this.

Fig. 23.10 Comparison of Reynolds stress results

Figure 23.10 shows the Reynolds stress graphs. The two sets follow the trend of the fibre results slightly lagging the direct beam curves. It can also be seen that as the particle size increases the Reynolds stress becomes smaller, possibly an indication of the reduced sensitivity of large particle motions to small scale turbulent eddies.

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23.5.3 Particle concentration and size profiles

Fig. 23.11 Medium particle results Figures 23.11 and 23.12 show plots of the average number and size of particles against position in the duct. These plots were obtained using in-house software which thresholds (any

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pixel above a given intensity is assumed to be a particle) the images before identifying and sizing any particles. Each curve is labelled with the grey level threshold value used, there were 256 grey levels in total.

Fig. 23.12 Large particle results

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For both particle sizes the concentration profiles exhibit the same trends, the curves are slightly curved with a minimum around y/D = 0.5, and as the threshold value is increased the curves move down the y-axis. There is also a large spike at the left-hand-side of each graph, again due to glare from the walls. The second of these effects is due to the fact that as the threshold is increased the dimmest particles are no longer identified by the thresholding. The shape of the curves is caused by the streamwise velocity profile. Imagine that particles are released at each x-position at regular time intervals, and are then carried off by the flow. We can see that they will be spaced by Vt, where V is the flow velocity and t is the time interval, in the flow direction. So the seeding will be the least dense where the flow is fastest, i.e. at y/D = 0.5, and more dense in the slower moving wall areas. The particle size plots are less clear, but it can be seen that as the threshold is increased the gradient of the straight line fits increases. One possible reason for this is that as we move towards y/D = 0 the drop off in light we attributed the skewness to would also cause the particle images to be smaller as well as dimmer. One problem with this is that we would expect the particles images to be smaller on the left-hand-side of the graph than on the right, and while this is true for the medium-sized particles it is not true for the large particles. One possible explanation for this is the way in which the light sheet spreads out, making it wider on the far side of the duct. This could mean that the particles in the top and bottom right corners of the viewing area are not well illuminated since the light sheet is darker towards its edges. This would mean that the particle images in these areas would not be as large as those in the central section of the light sheet, and so would drag down the average particle size on the left of the graph. This supports the view that the skewness could be due to preferential illumination of particles. Finally, Fig. 23.13 shows the average number of vectors per image at each x-location. If all the vectors were valid the result would be a horizontal line at y = 62. The results are similar for each setup, with a drop off as we move to the less densely seeded large particle images.

Fig. 23.13 Profiles of average number of vectors per image

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23.6 Conclusions In conclusion it can be seen that when using a fibre optic delivery system in conjunction with large particles, care must be taken to ensure that all particles are illuminated. If there is a preferential illumination of particles then the mean velocity profiles and turbulent statistics will all tend to be underestimated. As for the possibility of the scattering of light causing a skewness of the mean velocity profiles, there is no marked difference between the two sets of results.

References (1) (2)

(3) (4)

Entwistle, J. D. 'Particle Image Velocimetry Applied to Flow Through a Duct', PhD Thesis, The University of Edinburgh, 1999. Hand, D. P., Maier, R. J., Khun, A., Created, C. A., and Jones, J. D. C. 'Fibre Optic Beam Delivery System for High Peak Power Laser PIV Illumination', Measurement Science and Technology, 10, p 239-245, 1999. Westerweel, J. 'Fundamentals of Digital Particle Image Velocimetry', Measurement Science and Technology, 8, p 1379-1392, 1997. Raffel, M., Willert, C., and Kompenhans, J. 'Particle Image Velocimetry: A Practical Guide', Springer, 1998.

S S Coltman and C A Created Department of Physics and Astronomy, The University of Edinburgh, UK © With Authors 2002

24 Automated Fringe Analysis for Profilometric Mass-transfer Experiments J J Nebrensky

Abstract A new approach has been proposed for the automated analysis of fringe patterns obtained in mass transfer experiments, where the rate at which live fringes cross a given point (and hence the local mass flux) is identified by finding the first-order peak in a time-domain Fourier transform of that point's intensity history during the experiment. A software implementation of this approach has been tested on synthetic data to evaluate the precision of the data produced; several peak frequency interpolation schemes are also compared.

24.1 Introduction Profilometric mass-transfer techniques, such as naphthalene sublimation, have an important place in the investigation of transfer processes. Their use, however, is hampered by the difficulty of the data reduction process: e.g. co-ordinate measuring machines measure at only a selection of points over the test surface and can suffer from errors due to natural convection, whereas optical interferometric methods can quickly and easily record the surface deformation but the conventional manual analysis of the resulting fringe patterns is time consuming. Two major difficulties are that mass transfer coefficient distributions may have multiple maxima or minima and that it is necessary to find absolute, rather than just relative, fringe order values, meaning that the zero-order fringe must be identified. Button et al. (1) and Paler et al. (2) describe techniques that allow the positions of the fringes in an image to be found, but do not discuss the subsequent assignment of fringe order numbers. This phase unwrapping can be quite difficult, as by the nature of the mass transfer process there may be multiple

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maxima or minima in the field of view, so fringe order numbers will not simply vary monotonically across the image. Wang et al. (3) showed that the fringes could be ordered using phase-stepping techniques; but this only determines relative, not absolute, fringe orders and either separate fiducial measurements must be made, or the experiment must be limited so that the zero-order fringe is known to remain within the field of view. These approaches are examples of what may be termed 'off-line' measurement techniques, in which only the total change occurring during an experimental run is considered; frozen-fringe interferometry is an obvious example. Holographic interferometry and ESPI produce fringes that are wrapped into a 2n-wide range of phase, and so a single frozen-fringe pattern does not intrinsically contain any information about the direction and absolute magnitude of any displacement. The problems mentioned above are avoided by 'on-line' methods, epitomized by real-time live-fringe holographic interferometry in which local mass transfer maxima, and minima, are apparent as regions in which fringes appear, or merge together, respectively, and the absolute fringe order may be determined simply by counting the number of fringes passing a given point. The main difficulty in live-fringe work is the amount of raw data produced, and for practical use, an automated analysis system is required. Skeletonizing and ordering the fringes within each frame and then identifying their movement over several frames is a complex process. A new approach (4) has been proposed for the automated analysis of fringe patterns obtained in mass transfer experiments, where the rate at which live fringes cross a given point (and hence the local mass flux) is identified by finding the first-order peak in a time-domain Fourier transform of that point's intensity history during the experiment. Such a scheme has several advantages: it will find the absolute rate of transfer across the object; it can easily be adapted to curved surfaces; it should allow the filtering out of spurious fringe movement; and it naturally ignores any static mechanical deformation of the object that might be caused by the flow.

24.2 Fringe interpretation in mass transfer experiments Profilometric mass transfer measurement methods (of which the naphthalene sublimation technique is probably the best known) involve the loss of material from a coating applied to the test object into the fluid flow. This results in a thinning of the coating which is more pronounced in regions where the mass flux is higher. In order to measure the distribution of the local mass transfer coefficient it is necessary to measure the local rate of surface recession, for example with optical techniques such as holographic interferometry. There has been only limited success in applying interferometric techniques to naphthalene sublimation; instead they have mostly been used with the swollen polymer method (5). This involves coating the test surface with an involatile elastic polymer, which is then swollen by soaking to equilibrium with a suitable solvent. During the course of an experiment the swelling agent is transferred to the surrounding fluid, and the polymer layer shrinks proportionally to the local rate of mass transfer. The local deformation of the elastomer coating can then be measured using techniques such as holographic interferometry or ESPI: examples using double-exposure holography are given by Masliyah and Nguyen (6) and by Kapur and Macleod (7). A variety of optical set-ups can be used - some are reviewed by Law and Masliyah (8).

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Common problems encountered in live-fringe work include: fading - a loss of fringe contrast from the image; woozing - concerted halting or unsteady motion of all fringes across the subject; wavering - localized spurious fringe motions (similar to heat shimmer); and uneven illumination due to the Gaussian beam profile or shadows from, say, the jet apparatus. It is expected that the approach outlined here will apply, not only to holography, but to any method producing continuously moving fringes, possibly including structured-light/moire methods. The interpretation of fringes in mass transfer experiments has been described by several authors, such as Nebrensky and Macleod (9). The equation

relates the local mass transfer coefficient p to the number of fringes N that have passed across that point during an experiment of duration t, with a gas as the working fluid. The other terms represent the properties of the transferred substance (its molecular mass Ms, density ps, and vapour pressure ps ) and the optical system (the illumination wavelength A, and a geometrical factor Q, and Tand R are the ambient temperature and the universal gas constant respectively. While off-line measurement methods try to count the number of fringes A' passing during a long time interval!, with on-line techniques it is possible to directly measure the quantity N/t it is the frequency with which the live fringes are seen to pass over a given point on the test surface. Nebrensky (4) proposed that this could conveniently be done by taking the Fourier transform in the time domain of the intensity at that point (Fig. 24.1). The fringing frequency N/t would then correspond to the first order peak. This approach not only finds the absolute rate of surface recession at each point directly, but it also allows the filtering out of both static fringes, such as those due to deformation of the test object by the flow or to natural convection prior to the experimental run, and of any spurious fringe movements, such as woozing and wavering. By restricting the acquisition of fringes for analysis, it is also possible to exclude those formed during the start-up and shutdown of the fluid flow.

Fig. 24.1 The raw data consist of the intensity I (x, y, t)at each pixel (x, y) at a series of times t (left). Fourier transformation in the time domain converts this to the frequency component magnitudes F (x, y,f) for each pixel at a series of frequencies/. For any pixel, the frequency/of the first-order peak in F corresponds to the fringing rate, N/t

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Fig. 24.2 (above) Simulated intensity variation for a pixel traversed by a fringe once every ten seconds, and (below) the magnitude of the Fourier transform, showing clear peaks corresponding to 0.1 Hz (4)

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A software implementation of this approach has been completed and tested with synthetic data to evaluate the precision of the results produced; several peak frequency interpolation schemes have also been compared. The computational requirements have also been assessed.

24.3 Automated analysis 24.3.1 General approach This fringe analysis scheme is currently realized in the form of a stand-alone program that reads in a series of image files, corresponding to a series of fringe patterns captured at a fixed time interval, and for each pixel performs a Fourier transform on the intensity values and identifies the first-order peak in the frequency spectrum (Fig. 24.2). As a first approximation, the frequency at which this peak occurs is taken to be the fringing rate Nit at that pixel. In practice, the data must consist of a finite number of individuals that can be identified. Each of these frequency components is associated with an integer k, such that

where n is the total number of images and T the time between successive samples. Requiring the peak frequency/to be one of this fixed set obviously limits the accuracy of the analysis; therefore a simplistic interpolation scheme was also implemented, in which the frequency component amplitudes either side of the peak were included to generate a non-integer, interpolated component kInt as

and hence an interpolated fringing frequency

The code is written in ANSI C and has been successfully compiled and run both using DJGPP/GCC 2.7.2.1 under MS-Dos 6 and Microsoft Visual C++ 5 under Windows NT. As the fringing rates involved in real mass-transfer experiments are much less than one per second, for testing purposes it is more convenient to refer to the time periods involved. 24.3.2 Accuracy Preliminary testing has been carried out using synthetic data. The main test set consisted of 250 frames (each with 120 pixels) representing 1 s-interval samples of 120 randomly chosen frequencies with periods in the range 2 to 125 s. The fringing rates measured by the software could thus be compared directly with the values used to generate the data. The results are shown in Fig. 24.3, which clearly shows how the non-interpolated results can take only certain values. Two interpolation schemes have been tested: that described above, in which the Fourier amplitudes are used to linearly interpolate between the first order peak and the neighbouring frequency values (in Hz); and a similar approach but now interpolating between the peak and the neighbouring period values(s).

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The mean relative errors of these approaches are shown in the first row of Table 24.1. Over the whole range, the average error of the measured values was six per cent from the original periods, without any interpolation. The period-weighted interpolation scheme reduces this average error to just over four per cent, and the original (and computationally simpler) frequency-weighting reduces it even further. As the data simulate only a pure sine wave without any noise, the effect of changing the number of grey-scales in the images is not significant.

Fig. 24.3 The measured periods found using several interpolation schemes, compared with the original values (256 level images) Table 24.1 Mean relative error (%) for different sample bit-depths Greys

Wo interpolation

Frq.- weight

Per. weight

256 64 16

6.0 6.0 6.0

3.8 3.8 3.8

4.3 4.3 4.3

It is clear from Fig. 24.3 that the largest errors are associated with the slowest frequencies; hence if one restricts the range of periods over which data are accepted, one can dramatically improve the effective precision of this technique (Table 24.2). By way of comparison, manual analysis of real-time fringes (9) was generally limited to 1/4 of a fringe in at least ten fringes, i.e. a worst case of over two per cent.

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Table 24.2 Mean relative error (%) for different sampling strategies

all periods below 60s below 30s

No interpolation

Frq.-weight

Per. weight

6.0 3.3 1.5

3.8 2.1 0.9

4.3 2.1 1.0

As a guide to performance, 255 frames of 192 by 144 pixels could be processed in about five minutes on a 233 MHz PC. Analysis times will scale proportionally with the number of pixels, but will increase much more rapidly with the number of frames sampled. It is clearly feasible to implement such a fringe analysis system on a standard desktop PC.

24.4 Example A couple of fringe patterns from a synthetic 96 by 72 pixel sequence are shown in Fig. 24.4. The sequences were generated by using empirical correlations to find the mass transfer coefficient at the point corresponding to each pixel, and then using equation (24.1) to calculate the fringe order at that pixel at that instant. The pixel intensity / can then be found from

where /o is the maximum intensity to be displayed in the image and int (N) represents the integer part of N (this assumes that the zero-order fringe is bright, as in thermoplastic holography, rather than dark).

Fig. 24.4 Extracts from a sequence of synthetic fringe patterns representing a swollenpolymer experiment with an impinging circular jet (Re = 1000, z/D = 2.5, image width 6 jet diameters)

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The pattern on the left is equivalent to a frozen fringe (double exposure) interfere gram obtained after 560 s. It is circularly symmetric and consistent with the expected monotonic distribution. That on the right (from 350 s), however, shows a discontinuity in the fringes that would have been missed with only a double exposure, 'off-line' experiment. This feature (at r/D = 2) can be clearly seen in the analysed data, Fig. 24.5.

Fig. 24.5 Mass transfer (Sherwood number SA) distribution beneath the simulated impinging circular jet illustrated in Fig. 24.4 (frequency-weighted interpolation) In this particular example the discontinuity is not a feature of the flow but instead the boundary between two of the experimental mass-transfer correlations (10) used to generate the fringe patterns. However, similar issues arise with this flow geometry at higher Reynolds numbers, with the appearance of secondary maxima and minima of the local mass transfer coefficient that can not be identified from only a single interferogram but become readily apparent from a sequence of images (11).

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Conclusions Software to perform a one-dimensional discrete Fourier transform, in the time domain, of the intensity of each pixel in a series of two-dimensional images has been written and tested. The accuracy of this method is as good as manual analysis of real-time live fringes, and may be improved further by making use of better peak interpolation algorithms. This approach is, therefore, a viable data extraction technique for mass-transfer experiments, either as a standalone solution for solely on-line measurement or to provide fiducial information for other analysis methods. The synthetic data used here have represented 'ideal' fringe patterns, whereas in practice one would expect various spurious movements of the fringes and changes in their contrast. An important feature of this approach is that it allows the filtering out of both static fringes, such as those due to deformation of the test object by the flow or to natural convection prior to the experimental run, and of any spurious fringe movements, such as woozing and wavering. The performance of the software on archived experimental data is currently being assessed.

References (1)

(2)

(3)

(4)

(5)

(6) (7)

(8)

(9)

Button, B. L., Cults, J., Dobbins, B. N., Moxon, C. J., and Wykes, C. (1985) The Identification of Fringe Positions in Speckle Patterns. Optics and Laser Technology;\1'(4) pp. 189-192. Paler, K., Crennell, K. M., Kittler, J., Dobbins, B. N., Button, B. L., and Wykes, C. (1987) Identification of Fringe Minima in Electronic Speckle Pattern Images Pattern Recognition Letters 6 (1) pp. 33-44. Wang, L. S., Kapasi, S., Jambunathan, K., and Dobbins B. N., (1994) Application of Phase Stepping Speckle Interferometry and Swollen Polymer Technique to Mass Transfer Measurement Proceedings of the International Seminar on Optical Methods and Data Processing in Heat and Fluid Flow, City University, London; pp. 207-211. Institution of Mechanical Engineers. Nebrensky, J. J. (1998) A New Scheme for Fringe Analysis in Mass Transfer Experiments in Optical Methods and Data Processing in Heat and Fluid Flow pp. 171177, Professional Engineering Publishing. ISBN 1 86058 142 . Kapur, D. N. and Macleod, N. (1974) Determination of Local Mass Transfer Coefficients by Holographic Interferometry - I. International Journal of Heat and Mass Transfer 17pp. 1151-1162. Masliyah, J. H. and Nguyen, T. T. (1974) Qualitative Study in Mass Transfer by Laser Holography Canadian Journal of Chemical Engineering 52 pp. 664—665. Kapur, D. N. and Macleod, N. (1975) The Use of Holographic Interferometry for the Measurement of Local Mass Transfer Coefficients. Journal of Photographic Science 23 (2) pp 81-84. Law, H-S. and Masliyah, J. H. (1984) Coherent Optical Measurement Techniques in Profilometric Determination of Local Mass Transfer Coefficients'Optics and Lasers in Engineering 5 (4) pp. 211-229. Nebrensky, J. J. and Macleod, N. (1996) Natural Convection Mass Transfer Measurement in Air Using the Swollen Polymer Technique Proceedings of the International Seminar on Optical Methods and Data Processing in Heat and Fluid Flow, City University, London, April pp. 27-37.

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(10) Gholizadeh, N. (1992) Interferometric Measurement of Local Mass Transfer Rates to an Impinging Jet by Non-Photographic Holography, PhD Thesis. University of Edinburgh, Edinburgh, Scotland. (11) Nebrensky, J. J. and Macleod, N. (1994) Mass Transfer Measurement by Holographic Interferometry for a Turbulent Jet Impinging on a Flat Plate at Moderate Reynolds Numbers Paper 32.2 at LADOAN 7: The 7th International Symposium on the Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal. 1 lth-14th July.

J J Nebrensky Department of Electronic and Computer Engineering, Brunei University, Uxbridge, UK

Authors' Index A Abrukov, V S Allen, J Andreev, I V Anweiler, S Amott, AD

H 247-266 1-12, 37^14 247-266 207-218 93-102

1-12, 185-196 1-12 103-124 79-92

K

B Baranski,J Blasiak, W Boedec, T Briicker, C Buick, JM

Hargrave, GK Heath, J Heyes, A L Hobson, P R

267-278 267-278 219-230 151-160 93-102, 133-142

Kanar,J Khachaturyan, V M Kocik, M Kovrizhina, V N Krotkiewicz, M

161-184 143-150 161-184 143-150 207-218

L

C Campbell, D M Charbonnier, J M Cheung, KY Choy, K L Ciofalo, M Coltman, S S Cosgrove, J A Craig, G

Leblanc,R 133-142 13-24 231-246 103-124 125-132 279-294 133-142 79-92

Masiukiewicz, M Matcher, S J Meglinsky, IV Meledin, V G Mizeraczyk, J Murata, S

161-184 247-266 13-24 197-206 13-24

Nareid, H Naumov, IV Nebrensky, J J Ng, W B Nicholls, J R

185-196 103-124 59-68 79-92 59-68

Pavlov, V A Pieroni, G G Pitcher, G Podlinski, J

D Dekowski, J Deltsov, P V Dizene, R Doolan, C J Dorignac, E

M 207-218 45-58 45-58 25-36 161-184 69-78

N

F

Fairweather, M Feist, JP Fomin,N Foresti, GL Fuentes, C

13-24

79-92 25-36 79-92, 295-304 231-246 103-124

P

G Gentili, S 79-92 Created, C A .. 93-102, 133-142, 279-294 Green, R B 197-206

25-36 79-92 1-12, 37^14 161-184

S Saulnier, J-B 59-68 Schlicke, T 93-102 Simoens, S 219-230 Stasiek, J 125-132, 161-184, 267-278 Szmolke,N 207-218

T Thomas, NH Tuhault, J-L

93-102 59-68

207-218

Yasuda,N

69-78

Z

w Wasilewski, J Watson,J Wierzbowski, M Wigley, G

161-184 37-4 185-196

Y

u Ulbrich, R

Wilczyhski, L Williams, P Williams, T C

161-184 79-92 125-132 1-12, 3 7 - 4

Zajac, D Zhang, Y Zharkova, G M

207-218 231-246 143-150