Subgrid Modeling of Soot Formation in Non-Premixed Turbulent Flames

A subgrid model for soot dynamics is developed for large-eddy simulation (LES) using the Method of Moment with Interpolative Closure (MOMIC). The soot model is implemented within a subgrid mixing and combustion model so that reaction-diffusion-MOMIC coupling is possible without requiring ad hoc filtering. The combined model includes the entire process, from the initial phase, when the soot nucleus diameter is much smaller than the mean free path, to the final phase, after coagulation and aggregation, where it can be considered in the continuum regime. The soot diffusion and the effect of the thermophoretic forces are included in the model. A relatively detailed but reduced kinetics for ethylene-air is used to simulate an experimentally studied non-premixed ethylene/air jet diffusion flame. In this formulation, acetylene is used as a soot precursor species. The soot volume fraction order of magnitude, the location of its maxima, and the soot particle size distribution are all captured reasonably. Along the centerline, an initial region dominated by nucleation and surface growth is established followed by an oxidation region. A possible initial log-normal distribution followed by a more gaussian distribution downstream the centerline is observed. Limitations of the current approach and possible solution strategies are also discussed.