Subfilter transport modeling for large eddy simulation of turbulent nonpremixed sooting flames

Soot is formed by combustion of rich mixtures and is rapidly oxidized before being transported by turbulence into lean mixtures. Furthermore, different soot evolution mechanisms are dominant over distinct regions of mixture fraction (Z). For these reasons, a new subfilter PDF is proposed to account for this distribution of soot in Z space. In this model, the sooting mode of a bimodal soot subfilter PDF is locally activated only at rich mixture fractions where surface growth is locally faster than the oxidation. At the same time, Direct Numerical Simulation (DNS) studies of turbulent nonpremixed jet flames have revealed that Polycyclic Aromatic Hydrocarbons (PAH) are confined to spatially intermittent regions of low scalar dissipation rates due to their slow formation chemistry. The length scales of these regions are on the order of the Kolmogorov scale or smaller, where molecular diffusion dominates over turbulent mixing irrespective of the large-scale turbulent Reynolds number. A strain-sensitive transport model is developed to identify such species. Using a conventional nonpremixed "flamelet" approach, these species are then modeled with their molecular Lewis numbers, while the remaining species are modeled with an effective unity Lewis number. These two models are implemented within a Large Eddy Simulation (LES) framework, applied to a series of turbulent nonpremixed sooting jet flames, and validated via comparisons with experimental measurements of soot volume fraction. Compared to previous models, the soot volume fraction is significantly increased and in much better agreement with the experimental measurements.