Large Eddy Simulation of the evolution of the soot size distribution in turbulent nonpremixed bluff body flames

Large Eddy Simulation (LES) is used to investigate the evolution of the soot size distribution in a series of turbulent nonpremixed bluff body flames, with different bluff body diameters. The new Bivariate Multi-Moment Sectional Method (BMMSM) is employed to characterize the size distribution. BMMSM combines elements of sectional methods and methods of moments and is capable of reproducing fractal aggregate morphology, thanks to its joint volume-surface formulation, all at relatively low computational cost with fewer transported soot scalars compared to traditional sectional methods. LES results show soot volume fraction profiles agreeing correctly with the experimental measurements, exhibiting significant improvement compared to previous work using the Hybrid Method of Moments (HMOM). The evolution of the particle size distribution function (PSDF) was examined across the flame series and shows that the shape of the size distribution is less sensitive to the bluff body diameter than the overall soot volume fraction, which increases with increasing bluff body diameter. The PSDF across the flame exhibit different features compared to turbulent nonpremixed jet flames. The recirculation zone exhibits a nearly bimodal size distribution, which eventually becomes bimodal in the downstream jet-like region. The rather stark differences in the soot volume fraction predicted by HMOM and BMMSM are due to subtle differences in soot oxidation that are amplified in this configuration due to coupling to the flow field via soot radiation. With HMOM, the inner vortex between the central jet and recirculation zone is weaker, leading to significant soot leakage from the recirculation zone nearer the centerline. With BMMSM, the inner vortex is stronger, leading to a longer recirculation zone but with far less soot leakage and nearer the tip of the recirculation zone away from the centerline. The net result is much larger soot nucleation and condensation rates with BMMSM in both the recirculation zone and jet-like region, comparable to surface growth and oxidation, which dominate with HMOM. This work reveals that accounting for the size distribution can be crucial to both predicting global soot quantities accurately and reproducing fundamental mechanisms at least in some flame configurations.

Novelty and Significance Statement

For the first time, the evolution of the soot size distribution in a series of turbulent nonpremixed bluff body flames is investigated, by leveraging the recently developed Bivariate Multi-Moment Sectional Method (BMMSM) and using Large Eddy Simulation (LES). The analysis includes a comprehensive discussion of the evolution of the soot size distribution in the flame series. Finally, BMMSM predicts even the mean soot volume fraction much more accurately than the Hybrid Method of Moments (HMOM), due to amplifications of subtle differences in the models on soot and its feedback on the flow field through radiation, indicating that consideration of the soot size distribution may be required to accurately predict soot global quantities and unravel fundamental mechanisms in some turbulent sooting flames.