Relationship between O2 Concentration and Soot Formation in Propane Laminar Diffusion Flame

Oxygen (O2) in a flame is one of the important substances for formation and oxidation of soot. Then it is necessary to investigate the effect of O2 concentration in a flame on soot formation for clarification of soot formation mechanism. In this study, soot formation and soot oxidation mechanism inside propane laminar diffusion flame were focused. O2 concentration of ambient air was adjusted as 21% and 18%. Laser induced fluorescent (LIF) method and laser induced incandescent (LII) method were applied to measure O2 concentration and soot concentration distribution inside the flame. PM size distribution inside the flame was analyzed using scanning mobility particle sizer (SMPS). As the results, according to comparison between soot LII signal and O2 concentration along flame center, the LII signal was detected in a region of more than 2% O2 concentration. It means that soot was not generated under O2 concentration less than 2%. Inside the flame, number concentration of PM at 21% O2 condition was larger than that at 18% O2 condition. The difference of in-flame PM number concentrations between 18% and 21% O2 conditions became small with increasing distance from a nozzle, and at outside of the flame, number concentration of PM at 21% O2 condition became smaller than that at 18% O2 condition.