Numerical investigation of soot formation from microgravity droplet combustion using heterogeneous chemistry

The different steps of soot evolution were quantified, and localized in the region between the flame front and the soot shell, where particle velocity is directed inwards because of thermophoresis, and residence times are much higher than what usually found in diffusion flames. As a result, growth, coalescence, and aggregation steps are significantly enhanced, and soot accumulates in the inner shell, with an evident modification of the particle size distribution, if compared to what observed in conventional combustion conditions. The model exhibits a satisfactory agreement with experimental data on flame temperature and position around the droplet, while for larger droplets an increasing sensitivity to the radiation model was observed. It is found that the latter has a significant impact on the production of soot, while scarcely affecting the location of the soot shell. On the other side, the inclusion of incomplete thermal accommodation in the thermophoretic law brought about more accurate predictions of both volume fractions and shell location, and highlighted the primary role of thermophoresis in these conditions, as already found in literature through more simplified approaches.