Experimental investigation of soot morphology and primary particle size along axial and radial direction of an ethylene diffusion flame via electron microscopy

The effect of oxygen concentrations on the formation and evolution of soot particles was investigated by analyzing soot morphology using SiC fiber deposition technique and thermophoretic sampling method in a co-flow diffusion ethylene flame. Soot particles were examined via transmission electron microscopy at different heights along the flame centerline. Results show that the flame temperature exhibits a bimodal distribution. As the flame height increases, the flame distribution gradually changes from bimodal to unimodal, and the increase in oxygen concentration not only causes the flame height to decrease, but also causes the bimodal distribution of the flame more pronounced. The morphological evolution of soot deposits are strongly dependent on the oxygen concentration, radial and axial position, and flame temperature. Soot deposits along the flame centerline begin to be oxidized into dense flocculent and fibrous mesh structures with the flame temperature increases. In front of the flame, the oxidation is enhanced with temperature rise at the same height, resulting in more dense morphology of the soot deposits and the decrease in primary particle size. The results of thermophoretic sampling show that soot growth undergoes various stages of nucleation, growth, coagulation, agglomeration and oxidation, and the average particle size distributions of soot increase first and then decrease. The increase in oxygen concentration leads to advances in all stages of soot formation, including surface growth, agglomeration and oxidation. Additionally, the flame temperatures increase sharply as the increase of flame heights, leading to the soot aggregates to be oxidized to loose chain-like agglomerates.