Numerical Prediction of Air-preheating Effect on Soot Formation in Diffusion Flame During Early Transience Following Ignition

A CFD-based numerical model has been developed for the determination of the volume concentration and number density of soot in a laminar diffusion flame of methane in air, under transient condition following ignition of the flame. The transience is studied from the point of ignition till the final steady state is reached. The burner is an axi-symmetric co-flowing one with the fuel issuing through a central port and air through an annular port. The air is considered to be without and with preheat. Attention is focused on various soot forming and destruction processes, like nucleation, growth and oxidation, during the transient phase to evaluate their relative importance. The contribution of surface growth towards soot formation is more significant than that of nucleation during the early periods following ignition. Once the high temperature reaches the oxygen-enriched zone beyond the flame, the soot oxidation becomes important. Coagulation, on the other hand, limits the soot particle number. Preheating of air increases the soot volume fraction in the steady flame significantly. However, there is not much difference in the qualitative development of the soot-laden zone during the flame transient period.