Comprehensive soot particle size distribution modelling of a model Rich-Quench-Lean burner

Soot particle size distribution (PSD) evolution in an ethylene lab-scale swirl Rich-Quench-Lean (RQL) combustor is investigated using a detailed physicochemical sectional soot model coupled with the Conditional Moment Closure turbulent combustion model and Large Eddy Simulation. The aim is to develop predictive capability for the local soot PSD and to explore differences in soot PSDs with different conditions of a burner configuration widely used in practice for emissions control. Two such conditions are studied by varying the airflow provided in the burner primary and dilution regions, which has a drastic effect on soot emission as shown by previous experiments. The results show a reasonably good agreement with experiments for the mean reaction zone and soot locations and their variations with dilution. The predicted PSDs at the burner exit are fairly well captured for the high-dilution condition but show too few and too small particles for the dilution-free condition, which may be due to an over-prediction of the oxidation rates or the unity Lewis number assumption used here that enhances penetration of soot towards the oxidiser stream. The results are analysed to reveal the hierarchy of reaction pathways during soot evolution and indicate how dilution air modifies the soot PSD within the primary zone. In particular, the presence of dilution leads to a broadly sustained uni-modal soot PSD shape and a decrease in particle size. The results demonstrate a framework for capturing soot PSD in realistic combustion devices, which may help meet future regulations based on particle number and size distribution.