  Large Eddy Simulation (LES) was 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 costs 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 HMOM.
The evolution of the particle size distribution function (PSDF) was examined
across the flame series and show that 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
long residence times in the recirculation zone leads to a nearly bimodal size
distribution, which eventually becomes bimodal in the downstream jet-like
region. Further analysis indicates that a size distribution model is needed to
correctly predict the soot evolution. Remarkably, due to improved descriptions
of oxidation with BMMSM compared to HMOM, significant nucleation and
condensation rates in both the recirculation zone and jet-like region were
found using BMMSM, on the same order of magnitude as surface growth and
oxidation, leading to the improved prediction of mean soot volume fraction
compared to HMOM. This work reveals that the need of size distribution is
crucial to both predict global soot quantities accurately and reproduce
fundamental mechanisms.
