Evolution of soot size distribution during soot formation and soot oxidation-fragmentation in premixed flames: Experimental and modeling study

The goal of this research was to provide better insights into the evolution of particle size distributions (PSDs) during soot formation, oxidation and fragmentation in premixed flames. Soot formation was studied in premixed ethylene/air, ethylene/benzene/air and benzene/air flat flames under different flame temperatures and C/O ratio conditions. The results demonstrated the major differences in the evolution of the PSDs, both measured and modeled, of soot derived from these flames. The model included reaction pathways leading to the formation of nano-sized particles and their coagulation to larger soot particles. The PSDs for these flames, both experimental and modeled, evolved from a single particle mode to a bimodal PSD. An important distinction between these flames was the behavior of the nucleation mode which persisted at all heights above the burner (HAB) for ethylene/air whereas it decreased for ethylene/benzene/air and was greatly suppressed at greater HAB for the pure benzene/air flames. The explanation for the decreased nucleation mode at higher elevations in the benzene flames was mainly associated with the decrease of soot precursors after the main oxidation zone of the benzene flames.;Soot oxidation via O2 and OH* studies were carried out experimentally in a two-stage burner under fuel lean and slightly rich conditions. Soot was produced in a first-stage ethylene/air flame, while in a second stage, the soot was oxidized. Results for the leanest ethylene/air flame, showed a decrease in particle mean diameter and an increase in number concentration for ultra fine-sized particles (Dp < 10 nm) with increasing HAB, which indicates fragmentation of the fine-sized particles. At higher HAB, the soot oxidation process was dominated by soot burnout, evidenced by the decrease in number and mass concentration. The fuel rich conditions did not show particle fragmentation and the PSDs were governed by soot burnout. Soot oxidation rates were calculated using experimental and modeled concentrations for O2 and OH*. Oxidation via O2 promoted fragmentation and was favored under fuel lean conditions close to the burner surface. Oxidation via OH* produced faster oxidation rates and it appeared to be the major path for soot burnout under near-stoichiometric and fuel-rich conditions.