Simulation of soot formation in turbulent premixed flames

A subgrid model for soot dynamics is developed for large-eddy simulation (LES) that uses a method of moments approach with Lagrangian interpolative closure (MOMIC) so that no a priori knowledge of the particles' distribution is required. 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 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 to be in the continuum regime. A relatively detailed multispecies ethylene-air kinetics for gas phase combustion is used here to study the effect of inflow turbulence, the carbon-oxygen (C/O) ratio, and multicomponent species diffusion coefficients on soot production in turbulent premixed flames. The results show that soot formation occurs when the C/O ratio is above the critical value, in good agreement with past observations. Furthermore, we observe that turbulence increases the collision frequency between the soot particles. As a result, the coagulation rate increases and the total average surface area of the soot particles per unit volume decreases. In addition, the rate of surface growth decreases with the increase in the turbulence intensity. Finally, the inclusion of species transport properties is shown to affect the general structure of the flame in the form of wider curvature probability density function tails and higher turbulent flame speed. In addition, the effect on the relative thermal to molecular diffusivity at the subgrid level (Lewis number effect) changes the surface growth rate and the soot production level.