Pressure dependence of primary soot particle size determined using thermophoretic sampling in laminar methane-air diffusion flames

Optical techniques, such as the light extinction and scattering as well as the laser-induced incandescence and the spectral soot emission, have been used routinely to measure soot concentrations in atmospheric flames. Laser-induced incandescence and light scattering have been proven to measure the primary soot particle size as well in atmospheric flames; however, using these two techniques for the purpose of primary soot particle sizing in diffusion flames at elevated pressures has been found to be problematic. One of the popular methods of studying soot morphology and primary size in atmospheric flames is thermophoretic sampling followed by transmission electron microscopy analysis. A high pressure thermophoretic sampling system was built and used successfully to measure the size of primary soot particles in laminar diffusion flames of methane at pressures above atmospheric. The multi-probe sampling system was fitted inside the high-pressure combustion chamber that had been used previously for high-pressure soot formation studies. Soot samples taken at various pressures were analyzed subsequently by transmission electron microscopy to estimate the primary soot particle sizes. The soot volume fractions and soot temperatures were measured by spectral soot emission technique at the same height above the burner rim as the thermophoretic sampling. The mean primary soot particle size, measured at a constant height from the burner exit at all pressures, decreased about 35% from 2 to 10 atm whereas the soot volume fraction increased by a factor of more than 50. Experimental results of mean primary soot sizes and the corresponding soot volume fractions imply that the number of soot nuclei in soot inception region of the laminar diffusion flames must have a strong sensitivity to pressure. The higher amounts of soot are mainly determined by the increasing nucleation leading to higher primary soot particle number densities as the pressure is increased.