Swaminathan N., Bray K.N.C. (eds.) Turbulent Premixed Flames
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TURBULENT PREMIXED FLAMES
A work on turbulent premixed combustion is timely because of in-
creased concern about the environmental impact of combustion and the
search for new combustion concepts and technologies. An improved un-
derstanding of fuel-lean turbulent premixed flames must play a central
role in the fundamental science of these new concepts. Lean premixed
flames have the potential to offer ultra-low-emission levels, but they are
notoriously susceptible to combustion oscillations. Thus sophisticated
control measures are inevitably required. The editors’ intent is to set out
the modelling aspects in the field of turbulent premixed combustion.
Good progress has been made recently on this topic. Thus it is timely
to edit a cohesive volume containing contributions from international
experts on various subtopics of the lean premixed flame problem.
Dr. Nedunchezhian Swaminathan is a Lecturer in the Department of
Engineering at the University of Cambridge and Director of Studies at
Robinson College. He has published more than 90 research articles on
turbulent flames and combustion and on the numerical simulation of
turbulence and combustion.
Professor K. N. C. Bray is Professor Emeritus in the Department of
Engineering at the University of Cambridge. He is the author of nu-
merous refereed research publications. Among his many honors, he
was elected a Fellow of the Royal Society.
Turbulent Premixed Flames
Edited by
Nedunchezhian Swaminathan
University of Cambridge
K. N. C. Bray
University of Cambridge
cambridge university press
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Singapore, S
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ao Paulo, Delhi, Tokyo, Mexico City
Cambridge University Press
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Nedunchezhian Swaminathan and K. N. C. Bray 2011
This publication is in copyright. Subject to statutory exception
and to the provisions of relevant collective licensing agreements,
no reproduction of any part may take place without the written
permission of Cambridge University Press.
First published 2011
Printed in the United States of America
A catalog record for this publication is available from the British Library.
Library of Congress Cataloging in Publication data
Turbulent premixed flames / [edited by] Nedunchezhian Swaminathan,
Kenneth Bray.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-521-76961-7 (hardback)
1. Combustion engineering. 2. Flame. 3. Turbulence. I. Swaminathan,
Nedunchezhian. II. Bray, K. N. C. (Kenneth Noel Corbett), 1929– III. Title.
QD516.T84 2011
621.402
3 – dc 222010041887
ISBN 978-0-521-76961-7 Hardback
Cambridge University Press has no responsibility for the persistence or accuracy of
URLs for external or third-party Internet Web sites referred to in this publication
and does not guarantee that any content on such Web sites is, or will remain, accurate
or appropriate.
Contents
Preface page ix
List of Contributors xi
1 Fundamentals and Challenges ............................ 1
1.1 Aims and Coverage 1
1.2 Background 3
1.3 Governing Equations 6
1.3.1 Chemical Reaction Rate 8
1.3.2 Mixture Fraction 9
1.3.3 Spray Combustion 10
1.4 Levels of Simulation 11
1.4.1 DNS 11
1.4.2 RANS 11
1.4.3 LES 12
1.5 Equations of Turbulent Flow 13
1.6 Combustion Regimes 14
1.7 Modelling Strategies 16
1.7.1 Turbulent Transport 17
1.7.2 Reaction-Rate Closures 20
1.7.3 Models for LES 27
1.8 Data for Model Validation 31
references 33
2 Modelling Methods ..................................41
2.1 Laminar Flamelets and the Bray, Moss, and Libby Model 41
2.1.1 The BML Model 42
2.1.2 Application to Stagnating Flows 48
2.1.3 Gradient and Counter-Gradient Scalar Transport 50
2.1.4 Laminar Flamelets 52
2.1.5 A Simple Laminar Flamelet Model 54
2.1.6 Conclusions 60
v
vi Contents
2.2 Flame Surface Density and the G Equation 60
2.2.1 Flame Surface Density 61
2.2.2 The G Equation for Laminar and Corrugated Turbulent
Flames 64
2.2.3 Detailed Chemistry Modelling with FSD 68
2.2.4 FSD as a PDF Ingredient 71
2.2.5 Conclusion 74
2.3 Scalar-Dissipation-Rate Approach 74
2.3.1 Interlinks among SDR, FSD, and Mean Reaction Rate 76
2.3.2 Transport Equation for the SDR 77
2.3.3 A Situation of Reference – Non-Reactive Scalars 78
2.3.4 SDR in Premixed Flames and Its Modelling 81
2.3.5 Algebraic Models 97
2.3.6 Predictions of Measurable Quantities 100
2.3.7 LES Modelling for the SDR Approach 101
2.3.8 Final Remarks 102
2.4 Transported Probability Density Function Methods for
Premixed Turbulent Flames 102
2.4.1 Alternative PDF Transport Equations 105
2.4.2 Closures for the Velocity Field 107
2.4.3 Closures for the Scalar Dissipation Rate 108
2.4.4 Reaction and Diffusion Terms 109
2.4.5 Solution Methods 110
2.4.6 Freely Propagating Premixed Turbulent Flames 111
2.4.7 The Impact of Molecular-Mixing Terms 113
2.4.8 Closure of Pressure Terms 114
2.4.9 Premixed Flames at High Reynolds Numbers 121
2.4.10 Partially Premixed Flames 124
2.4.11 Scalar Transport at High Reynolds Numbers 126
2.4.12 Conclusions 130
Appendix 2.A 132
Appendix 2.B 133
Appendix 2.C 134
Appendix 2.D 135
references 135
3 Combustion Instabilities ..............................151
3.1 Instabilities in Flames 151
3.1.1 Flame Instabilities 152
3.1.2 Turbulent Burning, Extinctions, Relights, and Acoustic
Waves 166
3.1.3 Auto-Ignitive Burning 168
3.2 Control Strategies for Combustion Instabilities 173
3.2.1 Energy and Combustion Oscillations 174
3.2.2 Passive Control 176
3.2.3 Tuned Passive Control 187
3.2.4 Active Control 189
Contents vii
3.3 Simulation of Thermoacoustic Instability 202
3.3.1 Basic Equations and Levels of Description 202
3.3.2 LES of Compressible Reacting Flows 206
3.3.3 3D Helmholtz Solver 215
3.3.4 Upstream–Downstream Acoustic Conditions 219
3.3.5 Application to an Annular Combustor 221
3.3.6 Conclusions 229
references 229
4 Lean Flames in Practice ...............................244
4.1 Application of Lean Flames in Internal Combustion Engines 244
4.1.1 Legislation for Fuel Economy and for Emissions 245
4.1.2 Lean-Burn Combustion Concepts for IC Engines 256
4.1.3 Role of Experiments for Lean-Burn Combustion in
IC Engines 304
4.1.4 Concluding Remarks 307
4.2 Application of Lean Flames in Aero Gas Turbines 309
4.2.1 Background to the Design of Current Aero Gas Turbine
Combustors 312
4.2.2 Scoping the Low-Emissions Combustor Design Problem 313
4.2.3 Emissions Requirements 314
4.2.4 Engine Design Trend and Effect of Engine Cycle
on Emissions 317
4.2.5 History of Emissions Research to C.E. 2000 318
4.2.6 Operability 321
4.2.7 Performance Compromise after Concept Demonstration 323
4.2.8 Lean-Burn Options 324
4.2.9 Conclusions 331
4.3 Application of Lean Flames in Stationary Gas Turbines 335
4.3.1 Common Combustor Configurations 336
4.3.2 Fuels 338
4.3.3 Water Injection 339
4.3.4 Emissions Regulations 340
4.3.5 Available NO
x
Control Technologies 342
4.3.6 Lean Blowoff 345
4.3.7 Combustion Instability 345
4.3.8 Flashback 348
4.3.9 Auto-Ignition 348
4.3.10 External Aerodynamics 349
4.3.11 Combustion Research for Industrial Gas Turbines 349
4.3.12 Future Trends and Research Emphasis 350
references 351
5 Future Directions ...................................365
5.1 Utilization of Hot Burnt Gas for Better Control of
Combustion and Emissions 365
5.1.1 Axially Staged Lean-Mixture Injection 367
viii Contents
5.1.2 Application of the Concept to Gas Turbine Combustors 374
5.1.3 Numerical Simulation towards Design Optimization 375
5.2 Future Directions and Applications of Lean Premixed Combustion 378
5.2.1 LPP Combustors 378
5.2.2 Reliable Models that Can Predict Lift-Off and Blowout
Limits of Flames in Co-Flows or Cross-Flows 383
5.2.3 New Technology in Measurement Techniques 386
5.2.4 Unresolved Fundamental Issues 390
5.2.5 Summary 395
5.3 Future Directions in Modelling 396
5.3.1 Modelling Requirements 396
5.3.2 Assessment of Models 398
5.3.3 Future Directions 400
references 401
Nomenclature 407
Index 415