Growth of Soot Particles in Premixed Flames by Surface Reactions

Abstract

Mean radii, number densities and specific surface areas of the soot particles generated in laminar premixed flames are measured by means of laser light extinction and scattering. Temperatures are obtained from thermal radiation of soot particles and by applying the sodium-D-line reversal method. Gas samples of the flames are analysed by means of mass-spectrometry. The experimental part of this work covers two series of experiments in sooting ethyne-oxygen-argon flames. These experiments were performed to elucidate some hypotheses for soot formation and surface growth: The dependence of soot appearance rates on the surface area of the soot particles is investigated. For this flames are seeded with alkali metal solutions to prevent the soot particles from coagulation. These flames are compared with the equivalent unseeded flames. The experimental results give the appearance rates of soot as beeing independent of the outer surface of the soot particle aggregates. The influence of chemical composition - in particular the dependence of the soot appearance rate on the ethyne partial pressure - on soot formation has been experimentally investigated. For this, flames with different pressure, flow rates, and composition of the feed have been investigated. The experiments show that the appearance rate of soot is not simply proportional to the ethyne partial pressure as stated in frequently used surface growth models but rather influenced by a number of flame properties. An explanation of the experimental findings is given on the basis of the frequently discussed HACA-mechanism for surface growth of soot particles with the C-H sites accessible for growth being spread about the inner surface of the soot particle aggregates.