Many practical soot-emitting combustion systems such as diesel and jet engines rely on diffusion flames for efficient and reliable operation. Efforts to mitigate soot emissions from these systems are dependent on fundamental understanding of the physicochemical pathways leading from fuel to soot in laminar diffusion flames. Existing diffusion flame−based soot studies focused primarily on over-ventilated coflow flame where the fuel gas (or vapor) issues from a cylindrical tube into a co-flowing oxidizer, and counterflow flame, where a reacting zone is established between two opposing streams of fuel and oxidizer. As a canonical diffusion flame configuration, laminar counterflow diffusion flames have been widely used as a highly controllable environment for soot research, enabling significant progress in the understanding of soot formation for several decades. In view of the possibility of fuel/oxidizer premixing in practical systems, counterflow partially premixed flames have also been studied. In the present work we intend to provide a comprehensive review of the researches on various aspects of soot formation utilizing counterflow flames. Major processes of soot formation (formation of gas phase soot precursors, soot inception and surface reactions, as well as particle-particle interactions) are examined first, with focus on the most recent (post-2010) research progress. Experimental techniques and associated challenges for the measurement of soot-related properties (some knowledge of which is helpful for full appreciation of the experimental data to be reviewed) are then introduced. This is followed by a detailed description of soot evolution in counterflow flames, which is complemented by a discussion on the similarity and differences of the sooting structures between counterflow and coflow diffusion flames. Parametric studies of the effects of fuel molecular structure, fuel additive, partial-premixing, pressure, temperature, stoichiometric mixture fraction, and residence time on soot formation in counterflow flames will then be addressed in detail. This review concludes with a summary of the knowledge and challenges gathered and demonstrated through decades of research, and an outlook on opportunities for future counterflow flame−based soot research towards a more complete understanding of soot formation and the development of novel techniques for soot mitigation in practical combustion devices.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The preparation of this manuscript was supported by the National Natural Science Foundation of China (51606136) and the Clean Combustion Research Center of KAUST. The authors are grateful to Drs. S. Mani Sarathy, Hong G. Im and William L. Roberts of KAUST, Dr. Abhijeet Raj of Khalifa University and Dr. Sungwoo Park of the Korea Aerospace University for collaboration on relevant projects. YW also acknowledges stimulating and informative discussions with Dr. Alessandro Gomez of Yale University, Dr. Dongping Chen of Beijing Institute of Technology, Dr. Dong Liu of Nanjing University of Science and Technology, Dr. Wei Ren of The Chinese University of Hong Kong, Dr. Xunchen Liu of Shanghai Jiaotong University, Dr. Yuan Xiong of ETH-Zurich, and Dr. Lei Zhou of Harbin Institute of Technology (Shenzhen). Technical assistances from Lei Xu, Wei Dai, Mengxiang Zhou and Wei Wang of Wuhan University of Technology in organizing some of the cited references are also acknowledged.