Numerical investigation of soot formation mechanisms in partially-premixed ethylene–air co-flow flames

Recently, an improved chemical mechanism of PAH growth was developed and tested in soot computations for a laminar co-flow non-premixed ethylene–air diffusion flame [Dworkin et al., Combust. Flame 158(9) (2011) 1682–1695]. With the intention of testing the robustness of the solution methodology on partially-premixed systems, this work used the same algorithm as that in the study of Dworkin et al. for computations of two sets of sooting partially-premixed co-flow laminar ethylene–air flames. The results show very good qualitative and good quantitative agreement with the experimental results for soot volume fractions and soot precursors, without any changes to the parameters of the model. The soot yield was found to initially increase with decreasing primary equivalence ratios, and then to decrease for Φ < 24, reaching levels lower than the non-premixed case for Φ < 10. On the flame centerline, both PAH and acetylene-related processes were found to be important for soot growth. The initial increase in the soot yield was linked to higher inception rates. On the wings of the flame the dominant soot growth process was found to be HACA growth. The initial increase in the soot yield was mostly due to higher acetylene yield leading to faster surface growth. The primary air was also found to influence the soot oxidation process by increasing OH radicals in both the centerline and the wings region.