Soot production modeling in a laminar coflow ethylene diffusion flame at different Oxygen Indices using a PAH-based sectional model

A numerical study is carried out to, first of all, investigate the capability of a Polycyclic Aromatic Hydrocarbon (PAH)-based sectional particle dynamics soot model in the prediction of soot production in laminar coflow ethylene diffusion flames. The effects of different oxygen mole fractions in the oxidizer stream, called Oxygen Index (OI), ranging from 17% to oxygen-enriched conditions up to 33% are investigated. Secondly, the relative importance of species responsible for the increase in both soot formation and oxidation rates with increasing the OI is analyzed. The soot production model considers a detailed description of nucleation via collisions among heavy PAHs, particle aggregation, PAH condensation, surface growth and oxidation through the hydrogen abstraction acetylene addition (HACA) mechanism, and fragmentation of soot aggregates. Model predictions are compared with previously-published experimental data and numerical predictions obtained with a semi-empirical acetylene-based soot production model [Comb. Flame 160: 786-795 (2013)]. Results indicate that the flame structure, soot volume fraction and flame cross-section integrated soot volume fraction predicted by the PAH-based sectional soot model are in good agreement with the experimental data over the entire range of OI considered. The temperature and the concentrations of soot precursors increase with the OI, leading to higher soot nucleation, condensation, surface growth, and oxidation rates. Results show that the PAH-based sectional soot model represents a significant improvement over the semi-empirical acetylene based two-equation soot model studied earlier, especially in conditions far from the normal air conditions (21% of O-2). With increasing the OI, the non-dimensional zone of influence does not change for the flame cross-section integrated soot formation rates but increases for the integrated soot oxidation rate. Nucleation occurs just above the burner rim and displays the largest increase in its rate with increasing the OI. Nucleation and condensation are mainly dependent on the concentrations of BGHIF and BAPYR. Condensation is also affected by the increase in the number of aggregates available for collision. Soot formation is mainly dominated by surface growth, through HACA, with a significant influence of soot condensation closer to the burner surface, regardless of the level of OI. The surface growth through HACA is mainly controlled not only by the acetylene concentration and the associated kinetic rate, but also by the specific soot surface area. Oxidation by O-2 is found to dominate at the top of the flame, while oxidation by OH is dominant at the middle height of the flame. The oxidation rate by O-2 increases more rapidly with increasing the OI than that by OH.