Relative influence of soot oxidation kinetics and subfilter soot-turbulence interactions on soot evolution in turbulent nonpremixed flames

Oxidation of soot is a critical process in determining the evolution of soot in turbulent reacting flows. On the one hand, the kinetic rates of soot oxidation remain one of the most significant uncertainties in soot models. On the other hand, turbulent transport plays a significant role in the distribution of soot with respect to the flame structure so the location of soot in regions where oxidation can actually occur. The objective of this work is to assess the relative importance of small-scale turbulent transport and oxidation kinetics on the prediction of soot evolution in turbulent nonpremixed flames. A series of Large Eddy Simulations has been performed in a turbulent nonpremixed piloted jet flame of ethylene and air using a series of soot subfilter PDF models (with different assumptions regarding the small-scale turbulent transport of soot) and a series of kinetic rates for soot oxidation (with differences in kinetic rates for both OH and O₂ oxidation). Similar to previous work, the results show that accounting for the disappearance of soot in lean mixtures due to soot oxidation is extremely important to avoid excessive “spurious oxidation” of soot otherwise artificially present in lean mixtures. In addition, the sensitivity to soot oxidation kinetics is found to be significantly less important than this effect. However, once this effect is taken into account in the soot subfilter PDF, the sensitivity to soot oxidation kinetics becomes rather significant compared to further refinements to the soot subfilter PDF models. More specifically, the assumption in the soot subfilter PDF models whether soot oxidation is infinitely fast or finite rate compared to small-scale turbulent transport is a less significant model sensitivity compared to the sensitivity to the kinetic rates of soot oxidation. Furthermore, oxidation by OH and O₂ are revealed to play fundamentally different roles in soot evolution.