Optical Studies of Soot‐Formation Processes in Premixed Flames

Abstract

Abstract Soot formation in flat, premixed flames of ethylene, benzene, and pyridine with air is investigated using laser light scattering and fluorescence and extinction measurements. — In the earliest stages of the soot formation process in all of the flames examined, a fluorescent emission is excited by the laser (extending from 450 nm to beyond 700 nm when excited at 488 nm). In pyridine and ethylene flames their intensity increases downstream or remains constant. In benzene flames, it passes through a sharp maximum just as the first particles are detected by light scattering. Measurements of optical extinction by soot particles are complicated by broad‐spectrum gas‐phase absorption which may dominate the soot absorption early in the flame. This absorption is stronger at shorter wavelengths and appears to be related to the molecules responsible for the fluorescence observed in these flames. The continuum fluorescence is also responsible for the apparently high degree of depolarization of the elastically scattered light early in the flame. The actual depolarization of the light scattered from the soot particles is of the order of 1% for ethylene and pyridine, 0.2% for benzene flames. — Combination of the corrected particle extinction and scatter measurements yields values for the particle size and number density. Soot particles grow by surface growth which depends on the nature of the parent fuel, the mixture C/O ratio, and the temperature and by coagulation. The measured coagulation rated are largely independent of these parameters and in agreement with the rates predicted from kinetic theory.