Large Eddy Simulation of turbulent nonpremixed sooting flames: Presumed subfilter PDF model for finite-rate oxidation of soot

Abstract

Modeling soot evolution in turbulent reacting flows using Large Eddy Simulation (LES) is challenging due to the complex subfilter soot-turbulence-chemistry interactions. Soot particles form at fuel-rich mixtures and are subsequently oxidized as they are transported toward fuel-lean mixtures. In previous work, this phenomenology was explicitly encoded into a presumed subfilter PDF model for soot by confining soot strictly to mixtures where growth rates exceed oxidation rates. However, this model implicitly assumed that oxidation is infinitely fast. In this work, a new presumed subfilter PDF model for soot is proposed to account for finite-rate soot oxidation. The distribution of soot with respect to the flame structure (mixture fraction) is modeled by comparing the local relative motion of diffusionless soot particles with respect to mixture fraction iso-contours with the local oxidation rate. When the oxidation rate is suppressed or the transport rate is very fast, soot is allowed to penetrate further into fuel-lean mixtures. This model can allow for soot leakage across the flame, a critical phenomenon in smoking flames. The new model is validated a priori against Direct Numerical Simulation (DNS) databases of turbulent nonpremixed jet flames and then a posteriori against experimental measurements in a laboratory-scale turbulent jet flame. A priori results show remarkably good agreement with filtered DNS data and are shown to correctly allow for soot leakage at low Damköhler number. Finally, LES results of the turbulent sooting jet flame show an improvement of soot prediction using the new soot subfilter model compared to previous works. In this flame, as in experiments, evidence of soot leakage is found near the beginning of the sooting region, and this soot leakage cannot be predicted with previous subfilter models that presume infinitely fast oxidation.