  A numerical study of the effects of air dilution on soot formation and
particle dynamics in a lab-scale rich-quench-lean (RQL) combustor is presented
using large-eddy simulations (LES) with tabulated chemistry. The modelling
approach comprises a flamelet generated manifold (FGM) turbulent combustion
model and an efficient discrete sectional method with clustering of sections
(CDSM) to model soot formation. Three operating conditions are studied
including a reference case without secondary air dilution and two cases with
varying air dilution levels. For the latter, the split between primary and
dilution air is varied from 80:20 to 60:40 expressed in percentage values. The
study aims to investigate the effect of air dilution on the formation and
oxidation of soot for various air splits using a direct comparison with the
available experimental data. The results show a good correlation between
predicted flame topology and spatial distribution of the soot volume fraction
with the experimental observations. The introduction of air dilution is found
to limit the production of soot with a more drastic reduction for the 40%
dilution case compared to the 20% condition. Predicted particle size
distributions (PSD) from the case without secondary air dilution correlate well
with scanning mobility particle sizer (SMPS) measurements although fewer and
smaller particles are predicted with air dilution. Leaner mixtures and enhanced
oxidation, resulting from the interaction with air dilution jets, favor the
decrease in soot formation.
