A modeling approach for soot formation in non-premixed flames with elevated stoichiometric mixture fraction

Abstract

Most soot formation models have been developed with particular applications in mind and as such, are valid only for the range of fuel-air combustion conditions representative of the target application. This limits the applicability and versatility of most models, especially for combustion processes wherein the stoichiometric mixture fraction, Zst, could vary widely. A soot-producing flame could become non-sooting (blue) with an increase in Zst even while maintaining constant flame temperature. This paper presents a novel modeling approach to account for the unique flame characteristics at elevated-Zst environments and their effect on soot formation. This modeling approach is designed to capture both the formation and the reversible processes that occur on the fuel-side of a diffusion flame in a way which is robust, simple, and can be utilized in diverse applications. Additionally, a new semi-empirical model is developed for extending two widely-used models–Leung-Lindstedt and Moss-Brookes–both of which render inaccurate predictions under varying Zst conditions. When applied to the counterflow flame system, the soot volume fraction profiles predicted by incorporation of our new and modified models agree well with experimental observations reported in the literature for low Zst. Using this modeling approach also resulted in the prediction of blue (soot-free) limit condition in a non-premixed counterflow flame for the first time. We show that this result cannot be obtained by considering formation processes alone and that the performance of semi-empirical soot formation models can be dramatically improved when applied to higher-Zst flames if the reversible nature of soot formation at high temperature is considered.