Relation between Particle Size Distribution Function and Morphology of Soot Formed in Atmospheric-Pressure, Premixed Ethylene-Oxygen-Argon Flame

摘要

The transition from apparent unimodal size distribution to bimodal distribution is studied for premixed ethylene-oxygen-argon flat flames at equivalence ratio phi = 2.07 over a range of maximum flame temperatures. The objective of the study is to provide insights into the relation between the evolution of particle size distribution function (PSDF) and particle morphology. Experiments were carried out using an in-situ probe sampling methodology in tandem with a scanning mobility particle sizer (SMPS). The evolution of PSDFs was followed as a function of distance from the burner surface. The morphology of the particles and especially the onset of aggregate appearance and evolution were examined by transmission electron microscopy (TEM). The transition of the unimodal PSDF in a high temperature flame (T_f ＞ ～ 1800 K) to a bimodal PSDF of lower temperature flames (T_f ＜～1800K) shows a continuous shift of the trough between the nucleation and coagulation mode. The location of the trough was previously thought to be fixed. TEM evidence suggests that for these flames the coagulation mode is dominated by spherical, primary particles, and the appearance of this mode does not always correspond to particle aggregation. The experimental finding suggests that in the post flames the mean size of the primary particles increases as the flame temperature is lowered.