A study of the effect of oxygen concentration on the soot deposition process in a diffusion flame along a solid wall by in-situ observations in microgravity

The effect of oxygen concentration on the soot deposition process from a diffusion flame to a solid wall was investigated in a microgravity environment to attain in-situ observations of the process. An ethylene (C₂H₄) diffusion flame was formed around a cylindrical rod burner in oxygen concentrations of O₂ = 21, 35, and 50% with a surrounding air and wall temperatures of 300 K. Laser extinction was adopted to determine the soot volume fraction distribution between the flame and burner wall. The experimental results show that the soot particle distribution region moves closer to the surface of the wall and that more deposition occurs with increasing surrounding oxygen concentrations. The experiments determined the trace of the maximum soot concentration position, defined as the "soot line", and it was comparable to that established with numerical calculations. A numerical simulation was also performed to understand the motion of soot particles in the flame and the characteristics of the soot deposition to the wall. The results successfully predicted the differences in the motion of soot particles by different oxygen concentrations near the burner surface and are in good agreement with observed soot behavior, ie the "soot line", in microgravity. A comparison of the calculations and experimental results led to the conclusion that a consideration of the thermophoretic effect is essential to understand the soot deposition on walls.