Soot aerosol dynamics in a stagnation point diffusion flame

Abstract

A soot aerosol undergoing simultaneous growth and coagulation in a stagnation point propane diffusion flame has been simulated numerically. The detailed thermochemical flame structure has been calculated by solving for a conserved scalar, the fuel atom mass fraction, and using empirical data to derive temperature and species concentrations. With the computed flow field and flame structure, soot aerosol behaviour was simulated with free molecular coagulation rates and assumed particle source terms near the flame reaction zone. Forty discrete classes of particle size were used and C3H8, C2H4 and C2H2 were permitted to contribute to surface growth with simplified kinetics. The general features of the computed soot volume fraction, size and number density were in agreement with measurements. Interpretation of the results on the basis of a light scattering experiment showed that use of a monodisperse assumption overestimated mean particle diameties by 50% and underestimated number densities by a factor of 2.6. A self-preserving size distribution (SPSD) was able to yield a reasonable correction to number densities obtained with the monodisperse assumption towards the end of the aerosol history. At earlier times the aerosolcould not be described in either self-preserving or lognormal forms, indicating the need for independent measurements of size distributions in regions of particle production and growth. It was found that particle production rates had a weak effect on overall soot loadings. However, with simultaneous growth and coagulation the effect is not negligible and in some circumstances it could account for observed variations in soot loading.