Temperature dependence of the fuel mixing effect on soot precursor formation in ethylene-based diffusion flames

Soot formation is a complex process and can be affected by a variety of factors including base fuel type, fuel mixing, temperature and pressure. Under real-world conditions, these factors can interact with each other to influence soot formation jointly. The present work aims to provide an understanding on the temperature dependence of the fuel mixing effect on the formation of molecular soot precursors. In particular, the effects of propane and methane addition on aromatic formation in ethylene counterflow diffusion flames were studied experimentally by gas chromatography measurements and computationally utilizing a detailed chemical reaction mechanism. Two series of flames under high (with peak flame temperature similar to 2020 K) and low temperature conditions (similar to 1660 K) were covered to investigate the fuel mixing effects and its interaction with flame temperature. The experimental results clearly demonstrated the temperature-dependence of propane's mixing effects on benzene formation: Compared to the ethylene baseline flame a notable enhancement of benzene formation was observed with propane addition in the high temperature flames; while a reduction of benzene concentration was seen under low temperature conditions. For the methane mixing cases, the addition of methane decreased benzene formation in both the high and low temperature flames, with the effect being more pronounced under low temperature conditions. Numerical simulations of the tested flames were also conducted with a previously developed reaction mechanism (Wang et al, Combust. Flame, 160 (2013) 1667-1676). Through a detailed reaction pathway analysis based on the numerical data, it was found the addition of C3H8 and CH4 would inhibit the C-4 + C-2 but enhance the C-3 + C-3 benzene formation pathway. However, the relative importance of the two pathways to total benzene production rate was strongly dependent on flame temperature. This fact was identified to be the primary mechanism leading to the temperature dependence of the C3H8/CH4 mixing effect. The qualitatively different effect of C3H8 and CH4 addition on benzene formation in the high temperature flame was believed to originate from their different influences on CH3 and C3H3 radical concentrations. The present work highlighted the fact that the effects of fuel mixing on benzene formation can be highly dependent on flame thermal conditions. As a result, great care is needed when generalizing literature data of fuel doping effect on aromatic and sooting tendencies.