Soot primary particle size dependence on combustion pressure in laminar ethylene diffusion flames

A multi-probe thermophoretic soot sampling system, installed inside a high pressure combustion chamber, was used to collect soot samples at elevated pressures from ethylene-air laminar diffusion flames. Ethylene was diluted with nitrogen at a ratio of 1/3 by mass, and a constant mass flow rate of ethylene-nitrogen mixture was maintained at pressures of 5, 10, 15, and 20 bar. Selected mass flow rate of ethylene, 0.72 mg/s, provided stable and non-smoking laminar flames with a height of about 16mm at all pressures considered. Soot samples were collected on transmission electron microscope grids at three heights above the burner rim, 3, 8, and 12 mm. The images of the soot particles were captured by transmission electron microscope and the primary soot particle diameters were determined using an automated edge detection method. The mean primary soot particle diameter increased from the sampling height of 3mm to 8 mm, which corresponds to the mid-height of the flames where the peak soot volume fractions are observed. The soot diameters decreased from the mid-height of the flame to the sampling location of 12 mm, near the tip of the flame. The mean diameter of the primary soot particles increased with increasing pressure up to 15 bar; at 20 bar, the mean soot diameter seemed to reach a plateau or start decreasing. Measured soot volume fractions at sampling locations of 3 and 8mm above the burner rim indicate that, in view of the measured mean soot diameters at the same locations, soot number density should be increasing intensely with pressure.