FLOW CHARACTERISTICS OF DOUBLE CONCENTRIC JETS
by
R.S. TARNACHA
Department of Mechanical Engineering
Submitted
In fulfilment of the requirements of the degree of Doctor of Philosophy to the
Indian Institute of Technology, Delhi
June 2009
CERTIFICATE
This is to certify that the thesis entitled "FLOW CHARACTERISTICS OF DOUBLE CONCENTRIC JETS" being submitted by R. S, Tarnacha is a report of bonafide research work carried out by him under our supervision. This thesis has been prepared in conformity with the rules and regulations of Indian Institute of Technology Delhi, New Delhi, India. We further certify that the thesis has attained a standard required for a Ph. D. degree of the Institute. The research reported and results presented in the thesis have not been submitted, in part or full to any other institute or university for the award of any degree or diploma.
Dr. S. N. Singh Professor
Department of Applied Mechanics Indian Institute of Technology Delhi New Delhi — 110 016, INDIA.
Dr. Lajpat Rai Formerly: Assistant Professor Department of Mechanical Engineering Indian Institute of Technology Delhi New Delhi —110 016, INDIA.
Presently: Professor
Dept. of Mech. & Automobile Engineering Institute of Technology & Management, Institutional Area, HUDA, Sec — 23A, Gurgaon - 122017
Date :
Place : New Delhi —110 016
ACKNOWLEDGEMENT
I wish to express my sincere gratitude towards my supervisors, Prof. S. N. Singh and Prof. Lajpat Rai, for their incessant support and encouragement throughout the period of my Ph. D. work. I shall remain deeply indebted to them for their timely advice and support in managing my work keeping in mind my limitations. For me, my association with them during the period of my work at I.I.T. Delhi has been a memorable time and a remarkable process of learning. It has been with their help that the facilities of the C.F.D.
Lab of Department of Applied Mechanics and Turbomachinery Lab of Department of Mechanical Engineering were made available to me to enable me to work for as long as I could.
I am grateful to Prof. V. Seshadri of Department of Applied Mechanics for his advice and support and Prof. M.R. Ravi, Prof. P. M. V. Subba Rao and Dr. Sangeeta Kohli of Department of Mechanical Engineering for their help and advice on problems related to experimental work in the Turbomachinery lab and whenever I felt stuck in my computational work. I also wish to thank Prof. S. M. Yahya and Prof. P.L. Dhar of the Department of Mechanical Engineering for their wonderful coverage of some of the courses I had taken, which laid a strong foundation for me during my work in later years at I.I.T. Delhi. I am thankful to Mr. Y.P. Dogra, former Assistant Registrar, IRD, for his friendly help extended to me during the crucial moments of initial period of my work at I.I.T. Delhi.
During the period of fabrication and installation of the experimental set up I received uninterrupted support from Mr. Rajinder Singh and Mr. Sham Lal Sharma of Turbomachinery Lab of the Department of Mechanical Engineering and Mr. R. P. Bhogal
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of Gas Dynamics Lab and Mr. Diwan Singh of Fluid Mechanics Lab of Department of Applied Mechanics.
My thanks are also due to Cdr R Vijayakumar of Naval Construction Wing at I.I.T. Delhi and my colleagues in the C.F.D. Lab, namely, Dr. Krishnendu Saha, Dr. Parminder Singh, Mr. N.P. Singh, Mr. Rajesh Kumar Singh and Mr. Sunil Chandel, whom I could approach without any hesitation whenever I needed help on problems related to my research work.
I also wish to extend my sincere thanks to Dr. Ashok K. Chauhan, Founder President, R.B.E.F. and Amity Group of Institutions for his support and motivation during my work at I.I.T. My thanks are also due to Prof. B. P. Singh and Prof. D. P.
Tewari of Amity University, Uttar Pradesh, for their continuous encouragement and support whenever I needed, my colleagues at the Amity Institute of Aerospace Engineering Research and Studies and friends who have always kept me motivated.
My absence from home and not being able to associate personally with my wife, Ashokta, on many personal and social engagements must have caused many tense moments to her and she had to very bravely and lovingly handle all the responsibilities of the household herself. I owe a lot to her and to my son and daughter who have been a source of encouragement to me for completing this thesis.
Date: 16 June 2009 R.S. Tarnacha
Place: Delhi —110 016
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ABSTARCT
Double concentric jets are derived from the coaxial jets, which form the basic design feature of any gas turbine combustor liner with the fuel nozzle as the central jet and the primary air supply as annular jet. Mixing between the jet streams takes place in the shear layer between them and in the recirculation zone formed as a result of swirl imparted to the jet streams. The proposed configuration of double concentric jets has a central jet surrounded by two coannular jets and swirl can be imparted to each of the jets. It is known that to ensure proper mixing of air and fuel in a combustor, the air supplied is highly in excess of the stoichiometric requirement. The use of double concentric jets in a combustor, because of the availability of an additional shear layer, is expected to enhance the mixing of fuel and air and thus may lead to a reduction in the excess air requirement.
With swirl in three jets and expanded confinement, the confined space acquires a complex flow structure and is likely to pose difficulties in carrying out the measurements and computational analysis. As the detailed isothermal studies on double concentric jets are not adequately available, such investigations are needed to understand the cold flow characteristics before taking on various aspects of combustion.
The present programme is carried out in three phases, namely, experimental study, validation of code and parametric investigations. The experimental study involves the measurements of mean velocity profiles at the jet exit and at different axial locations of the mixing chamber and wall static pressure along the length of the mixing chamber for non-swirling and swirling flows. The validation of code involves comparison of the results predicted using suitable turbulence models available in commercial CFD code FLUENT with the corresponding experimentally measured data for both non-swirling
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and swirling flows for non-expanded and expanded confinements. The parametric investigations are carried out using the validated CFD code to study the effects of swirl in individual jets, the effect of counterswirl in two inner and all the three jets and the effects of some of the geometrical parameters.
An experimental set up incorporating a configuration of double concentric jets was fabricated. Two centrifugal blowers, one powered by 7.5 hp motor and the other by 1 hp motor, operating at fixed speed of 2850 rpm provide air supply at constant flow rates of 0.532 m3/s and 0.026 m3/s respectively. The bigger blower supplies air through the rectangular diffuser to a settling chamber. An MS pipe of inner diameter 15.8 cm connects the settling chamber with the mixing chamber. The air flow in the settling chamber is split in two parts by providing a collector, which, in turn, is connected to an aluminium pipe of inner diameter 9.5 cm and outer diameter 10.2 cm and located concentrically inside the MS pipe. The air flowing outside the collector, in the settling chamber, passes through the annular space between the MS pipe and the aluminium pipe and forms the outer annular jet. The air entering the collector passes through the annulus between the aluminium pipe and a brass pipe of outer diameter 4.2 cm placed inside the aluminium pipe and forms the inner annular jet. Air supplied by the smaller blower is passed through the brass pipe having inner diameter of 3.9 cm and forms the central jet.
All the three jets exhaust at different velocities into the confinement, referred as the mixing chamber. Two sizes of confinements of length 137 cm have been used and provide expansion ratios of 1 and 1.3 respectively. A 3-hole probe has been used for velocity measurements after calibration against a standard Pitot tube. For generating swirl
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flow in the central jet and annular jets, vane swirlers with vane angle of ±300 and ±45°
have been fabricated and used.
Initial experiments were conducted with non-swirling flow using both the confinements. This has been followed by measurements with flows of +30° swirl and +45° swirl in the central jet for both the expansion ratios. For swirling flows, the tangential velocity has also been recorded. After swirl in the central jet, counterswirling flows in two inner jets and in all the three jets have been taken up with expanded confinement as per certain specified swirl combinations. For some of the swirl combinations, velocity profiles were measured at the jet exit plane only for utilizing them as inlet velocity profiles for additional parametric investigations using CFD code.
Using the measured axial velocity profiles for non-swirling flows at the jet exit plane as velocity-inlet, computations with the Standard k-s turbulence model available in the CFD code were performed for velocity profiles at designated downstream axial locations. Comparison of the computed results with the corresponding experimental measurements has shown a good agreement between them, and as a result, the CFD code with Standard k-s model is taken as validated for non-swirling flows. For swirling flows, the computed results obtained by using RNG k-s model of turbulence in CFD code matched very well with the corresponding experimental measurements. This led to the validation of CFD code FLUENT with RNG k-s turbulence model for swirling flows.
Using the validated CFD code for swirling and non-swirling flows, the parametric investigations have been carried out to study the effects of swirl in the individual jets, and the effects of counterswirl in the two inner jets and counterswirl in all the three jets for non-expanded and expanded confinements. The parameters computed are: (a) the radial
distribution of axial velocity, tangential velocity, total pressure and static pressure at different axial locations of the mixing chamber from jet exit till these profiles attained near uniformity, (b) variation of axial velocity and static pressure along the length of the mixing chamber at different radial distances from the centre line up to the wall, and (c) iso-contours of axial velocity and turbulence intensity in the r-x plane of the mixing chamber. Based on the analysis of above results, an optimum swirl combination has been identified. The effects of geometrical parameters include the effect of expansion ratio for non-swirling flows, the effect of changes in interface thicknesses for non-swirling and swirling flows in non-expanded and expanded confinements and the effect of shape of expansion on mixing and flow development of double concentric jets. For swirling cases optimum swirl combination has been used in the inlet velocity profiles.
Based on the above study the significant conclusions drawn are:
1. A flow with -45° swirl in the central jet, +45° swirl in the annularl jet and -45°
swirl in annular2 jet has been identified as the optimum swirl combination.
2. For the range of inner and outer interface thicknesses investigated, the changes in interfaces thicknesses are seen to have only minor influence on flow development.
Within such a minor influence, for non-expanded confinement, the optimum thicknesses for both interfaces are found as 0.55 cm, and for expanded confinement, the optimum thicknesses for outer and inner interfaces are found as 0.55 cm and 0.35 cm respectively.
3. The 45° straight diffuser and sudden expansion shapes show nearly similar performance. On comparing with them, the curved diffuser shape is found better than them as it forms a CRZ of sufficiently large radial size but smaller axial size.
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CONTENTS
Page No.
CERTIFICATE i
ACKNOWLEDGEMENT ii
ABSTRACT iv
CONTENTS viii
LIST OF FIGURES xii
LIST OF TABLES xxiv
LIST OF PLATES xxv
NOMENCLATURE xxvi
CHAPTER 1 INTRODUCTION 1
1.1 Coaxial Jets 1
1.1.1 Gas Turbine Combustor 2
1.2 Configuration of Jets in Combustors 4
1.2.1 Coaxial Jets 4
1.2.2 Transverse Jets 5
1.2.3 Double Concentric Coaxial Jets 6
1.3 Swirl and Swirl Generators 7
1.4 Mixing Process and Characterization of Mixing 9
1.5 Motivation 11
1.6 Outline of the Thesis 12
CHAPTER 2 LITERATURE REVIEW 16
2.1 Non-Swirling Jets 16
2.1.1 Single Jets 16
2.1.2 Coaxial Jets 17
2.1.2.1 Free Coaxial Jets 17
2.1.2.2 Confined Coaxial Jets 20
2.1.2.3 Salient Parameters of Coaxial Jets 22
2.1.2.4 Flow Structure 25
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2.2 Swirling Jets 31
2.2.1 Single Swirling Jets 32
2.2.2 Coaxial Swirling Jets 38
2.2.3 Multi-Annular Jets 53
2.3 Unexplored Parameters and Scope of the Present Study 65
CHAPTER 3 EXPERIMENTAL FACILITY AND INSTRUMENTATION 73
3.1 Experimental facility 73
3.1.1 Air supply Units 74
3.1.2 Diffuser 74
3.1.3 Settling Chamber 74
3.1.4 Arrangement of Double Concentric Jets 75
3.1.5 Test Section 76
3.2 Traverse Arrangement 77
3.3 Vane Swirlers 78
3.4 Instrumentation 79
CHAPTER 4 EXPERIMENTAL PROGRAM 91
4.1 Initial Conditions 91
4.2 Preliminary Investigations 93
4.3 Final Experimentation and Procedure 93
4.4 Data Processing 94
4.5 Sources of Error 96
4.5.1 Instrument Errors 96
4.5.2 Fabrication Errors 96
4.5.3 Geometrical Errors 96
4.6 Experimental Accuracy 97
CHAPTER 5 EXPERIMENTAL RESULTS AND DISCUSSIONS 102 5.1 Flow Characteristics in Non-expanding Confinement 103
5.1.1 Non-swirling Jets 103
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5.1.2 Swirling Jets 104 5.2 Flow Characteristics in Expanded Confinement 108
5.2.1 Non-swirling Jets 109
5.2.2 Swirl in Central Jet only 110
5.2.3 Counterswirl in Central Jet and Inner Annular Jet 113 5.2.4 Counterswirl in all the Three Jets 118
5.3 Conclusions 121
CHAPTER 6 NUMERICAL PROCEDURE AND VALIDATION OF 139 CODE
6.1 Mathematical Formulation 139
6.1.1 Governing Equations for Flow 140
6.1.2 Turbulence Models Used 141
6.2 Inlet and Boundary Conditions 144
6.2.1 Inlet Conditions 144
6.2.2 Boundary Conditions 145
6.2.3 Grid Consideration 146
6.3 Overview of the Commercial CFD Code 147
6.4 Validation of code 148
6.4.1 Non-Expanded Confinement 149
6.4.2 Expanded Confinement 151
6.5 Conclusion 158
CHAPTER 7 FLOW CHARACTERISTICS OF DOUBLE CONCENTRIC 170 SWIRLING JETS
7.1 Inlet Conditions 171
7.2 Non-expanding Confinement 171
7.2.1 Swirl in individual Jets 171
7.2.2 Counterswirl in Central and Other Jets 178
7.3 Expanding Confinement 184
7.3.1 Swirl in individual Jets 184
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7.3.2 Counterswirl in Central and Other Jets 193
7.4 Conclusion 199
CHAPTER 8 EFFECT OF GEOMETRICAL PARAMETERS ON 274 MIXING OF DOUBLE CONCENTRIC JETS
8.1 Effect of Expansion Ratio for Non-swirling flows 274 8.1.1 Flow characteristics for expansion ratio of 1.6 and 2 275 8.1.2 Comparative study: flow characteristics for different 276
expansion ratios for non-swirling flows
8.2 Effect of Interface Height for Non-swirling and Swirling flows 279
8.2.1 Non-expanded Confinement 281
8.2.2 Expanded Confinement 286
8.3 Effect of Shape of Expansion 291
8.3.1 Straight Diffuser 291
8.3.2 Curved Diffuser and Cusp Diffuser 293
8.4 Conclusion 294
CHAPTER 9 CONCLUSIONS AND SCOPE OF FUTURE WORK 323
9.1 Conclusion 324
9.2 Scope for Future Work 327
References 330
Appendices
Appendix A Experimental set ups used by other investigators 340
Appendix B Reliability of experimental data 342
Papers Published/Communicated based on the Present Work 347
Brief Bio-data of the Author 348
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