Soot formation in laminar counterflow flames

Abstract

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 overventilated 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.

Soot Emission Simulations of a Single Sector Model Combustor Using Incompletely Stirred Reactor Network Modeling

“…In this work, computations are also based on a detailed physicochemical sectional model that has been developed and extensively tested by D'Anna and co-workers (see [39] and references therein). This model has been demonstrated to successfully predict soot formation for various simple fuels and flame configurations while supporting the latest experimental evidence of soot evolution in laminar flames [1,3,40]. A simplified version of this approach for ethylene, also called the NAPS soot model, has been recently presented in Refs.…”

Chemical effects of hydrogen addition on soot formation in counterflow diffusion flames: Dependence on fuel type and oxidizer composition

“…An alternative would be to employ a quasi-one-dimensional counterflow diffusion flame (CDF). Due primarily to its simpler flow field and relative ease of modeling, CDF is particularly suitable for studying the fundamental chemistry of soot evolution [33][34][35][36], e.g., the fuel mixing effects on soot formation [37,38] as performed in this work. Additional benefits of CDF include its relevance to the laminar flamelet model [39], its resistance to buoyancydriven instability especially under high-pressure conditions [40], its capability to provide a sooting zone without interference from soot oxidation [41,42].…”

Combustion in the future: The importance of chemistry

“…Recent reviews summarize important aspects of the particle-forming reaction sequences as well as methods to analyze the soot formation process [51 , 84 , 85 , [193] , [194] , [195] . Further work has described the physico-chemical characterization of particle emissions from engines [130 , 196 , 197] and of flame-sampled, aircraft-type soot with potential impact on cloud formation [198] .…”