Soot modeling with temperature- and size-based collision efficiency for nucleation and condensation

Temperature- and size-based collision efficiencies for the collision of dimerizing gas-phase species and for the condensation of dimers onto soot particles were estimated based on theoretical principles. These collision efficiencies were then implemented within a detailed soot model with the Hybrid Method of Moments (HMOM). For the size dependence, taking advantage of the inherent ability of HMOM to demarcate nucleation and aggregation modes of the soot size distribution, a different condensation collision efficiency is used for each mode to account for the substantial differences in size between the two modes and the effect on condensation collision efficiency. Simulations were performed using detailed chemistry for laminar ethylene flames and were validated for premixed atmospheric, premixed pressurized, and coflow atmospheric diffusion flames. Compared to the base model with constant collision efficiency for a given dimerizing species and 100% collision efficiency for condensation, the proposed model results in a change in the budget of nucleation and condensation rates with increased nucleation and decreased condensation. This change was found to increase the surface growth rate due to an increase in the number of primary particles resulting from increased nucleation/reduced condensation and the corresponding increase in soot surface area. This change was found to improve the prediction of soot volume fraction significantly for growth-dominated flames. For the coflow diffusion flame in particular, the maximum soot volume fraction shifted from the centerline to the wings of the flame, consistent with experimental measurements, in stark contrast to the base model with limited soot in the wings of the flame.