Experimental study on thermophoretic deposition of soot particles in laminar diffusion flames along a solid wall in microgravity

Soot deposition process in diffusion flames along a solid wall has been investigated experimentally under a microgravity environment. An ethylene (C2H4) diffusion flame was formed around a cylindrical rod-burner with the surrounding air velocities of Va = 2.5, 5, and 10 cm/s, the oxygen concentration of 35%, and the burner wall temperature of 300 K. A laser extinction method was adopted to measure the distribution of soot volume fraction. The experiments determined the trace of maximum soot concentration together with the relative distance of the trace of flame. Results showed that the distance was about 2-5 mm. As the surrounding air velocity increased, the region of the soot particle distribution moved closer to the burner wall. The soot particles near the flame zone tended to move away from the flame zone because of the thermophoretic force and to concentrate at a certain narrow region inside the flame. Because of the simultaneous effects of convection and the thermophoresis, soot particles finally adhered to the burner wall. It has been found that there existed an optimal air velocity for the early deposition of soot on the furnace wall.