Large Eddy Simulation of Turbulent Supersonic Combustion and Characteristics of Supersonic Flames Wenlei Luo Submitted in accordance with the requirements for the degree of Doctor of Philosophy Under the supervision of Professor Mohamed Pourkashanian Professor Derek Ingham Dr Lin Ma The University of Leeds School of Chemical and Process Engineering Energy Technology and Innovation Initiative (ETII) September, 2014 I The candidate confirms that the work submitted is his own, except where work which has formed part of jointly authored publications has been included. The contribution of the candidate and the other authors to this work has been explicitly indicated below. The candidate confirms that appropriate credit has been given within the thesis where reference has been made to the work of others. The work contained within this thesis is my own primary research. The other authors were my supervisors who acted in an advisory role and gave suggestions regarding the research direction and analysis methods. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. © 2014 The University of Leeds and Wenlei Luo - II - Achievements and publications International conferences: W. Luo, L. Ma, D.B. Ingham, M. Pourkashanian, Z. Fan and W. Huang, Numerical investigation of reacting flow in a scramjet engine using flamelet and flamelet/progress variable models, 8th Mediterranean Combustion Symposium, Cesme, September 8-13, 2013. (Chapter 5) Journal papers: Zhouqin Fan, Weidong Liu, Mingbo Sun, Zhenguo Wang, Fengchen Zhuang, Wenlei Luo, Theoretical analysis of flamelet model for supersonic turbulent combustion. Science China Technological Sciences, 2012. 55(1): p. 193-205. (Cited as a reference) W. Luo, M. Pourkashanian, D.B. Ingham, L. Ma, Hybrid Reynolds- Averaged Navier-Stokes/Large Eddy Simulation of supersonic combustion with a strut flame holder using flamelet and unsteady flamelet/progress variable models, in preparation. (Chapter 5) W. Luo, M. Pourkashanian, D.B. Ingham, L. Ma, Z. Fan, Characteristics of a supersonic flame and flameholding mechanisms in a cavity-based supersonic combustor, in preparation. (Chapter 6) - III - Acknowledgements I would like to express my sincere gratitude to my primary supervisor, Professor Mohamed Pourkashanian, for offering me an opportunity to pursue a PhD under his guidance and for his steadfast support. Also I thank him for considering my research interests carefully and providing the excellent world-class computing facilities to perform my research work. I wish to express my deep sense of gratitude to my co-supervisors, Professor Derek Ingham and Dr Lin Ma, for the immense patience, support and encouragement that they have given to me. I also would like to express my very great appreciation to them for considering my ideas and interests carefully and keeping me on track. Also, I am particularly grateful for the support and assistance given by Professor Zhenguo Wang, in the National University of Defense Technology in China. Further, I would like to acknowledge the help provided by Dr Kevin Hughes, Miss Lisa Flaherty and Miss Charlotte Kelsall. Also, I am grateful to the following people who have helped me, Mr. Yang Fei, Mr. Guangyuan Xu, Miss Yanling Yang, Mr. Guangping Dong, Miss Fangting Hu, Mr. Jun Yang, Mr. Xin Yang, Mr. Chunlei Ren, Mr. Ning Wang, Dr Mingbo Sun and Dr Zhouqin Fan. I give my gratitude to the financial support from the University of Leeds and China Scholarship Council (CSC) for funding my studies and living costs in Leeds. Finally, I extremely appreciate the support that I received from my family and Miss Yuanyuan Zhou for being very supportive especially at hard times. - IV - Abstract In this thesis we investigate the supersonic combustion in scramjet combustors with strut and cavity flame holders through the Reynolds-Averaged Navier–Stokes (RANS) and Large Eddy Simulation (LES) strategies. Firstly, the Unsteady Flamelet/Progress Variable (UFPV) model for turbulent combustion in low-speed flows is introduced and extended to supersonic flows and a new strategy is developed to create probability density function look-up tables for the UFPV model. Secondly, the RANS modelling is employed to a strut-based scramjet combustor using the flamelet and UFPV models and the latter shows a better performance. Subsequently, the LES modelling is performed with the UFPV model and the UFPV model gives good predictions on comparing the numerical results to the experimental data. Thirdly, the LES modelling is employed to a cavity-based scramjet combustor. The results obtained indicate that the local extinction and autoignition events are very common phenomena in the supersonic flame and the UFPV model is able of predicting these events with reasonable accuracy. Further, an activation-energy- asymptotic-based Damköhler number concept is a valuable metric to identify flame weakening and extinction in supersonic flames. Together with the OH radicals, the distribution of the HO2 radicals can assist in identifying the autoignition events in the supersonic flame. - V - Finally, analysing the flameholding mechanisms of the cavity, it is found that the cavity provides a stable ignition source to the fluid. Further, the combustion in the cavity is dominated by flame propagation. However, on the outer interface of the air and hydrogen streams, the combustion is mainly dominated by autoignition. Both autoignition and flame propagation contribute to the combustion in the mixing layer. Also the combustion in the cavity mixing layer has effects on the induction reactions in the wake of the hydrogen jet and reduces the induction time of the autoignition. - VI - Contents Achievements and publications ............................................................................... II Acknowledgements ................................................................................................. III Abstract .................................................................................................................... IV Contents ................................................................................................................... VI Figures ....................................................................................................................... X Tables .................................................................................................................... XIX Nomenclature......................................................................................................... XX Chapter 1 Introduction ............................................................................................. 1 1.1. Introduction ................................................................................................. 1 1.1.1. Supersonic combustion ramjet engine cycle ................................... 2 1.1.2. History and evolution of scramjet development ............................. 6 1.1.2.1. The first generation scramjet development (Beginning: 1946–1973) ......................................................... 7 1.1.2.2. The second generation scramjet development (Airframe Integration: 1974–1985) ....................................... 11 1.1.2.3. The third generation scramjet development (NASP: 1986–1994) ........................................................................... 12 1.1.2.4. The fourth generation scramjet development (Resurgence: 1995–present) .................................................. 15 1.1.3. General state of scramjet technology ............................................ 21 1.2. Motivation and objectives ......................................................................... 22 1.3. Thesis outline ............................................................................................ 25 Chapter 2 Review of the scramjet combustion numerical simulations .............. 28 2.1. Numerical simulation of compressible turbulent flow .............................. 29 - VII - 2.1.1. RANS approach ............................................................................ 30 2.1.2. LES approach ................................................................................ 32 2.1.3. DNS approach ............................................................................... 36 2.2. Modeling of turbulent combustion ............................................................ 38 2.2.1. Modelling of non-premixed turbulent combustion ....................... 40 2.2.2. Modelling of premixed turbulent combustion............................... 44 2.2.3. Modelling of partially premixed turbulent combustion ................ 46 2.2.4. Modelling of turbulent supersonic combustion............................. 48 2.3. Characteristics and problems of combustion in high speed flows ............ 51 2.3.1. Characteristics of combustion in high speed flows ....................... 51 2.3.2. Effects of compressibility ............................................................. 51 2.3.3. Self-ignition, extinction and reignition ......................................... 52 2.3.4. Effect of shock waves ................................................................... 53 2.4. Closure ...................................................................................................... 53 Chapter 3 Turbulence LES modelling and numerical approach ....................... 55 3.1. Turbulence LES modelling ......................................................................