Inception and growth of soot particles in dependence on the surrounding gas phase

Abstract

Two sets, each of four sooting laminar ethyne-oxygen-argon flames have been investigated at low pressure. Within each set the pressure, C/O ratio of the flames, and dilution with argon were varied. The flame conditions of the sets were chosen such that the final soot volume fraction was constant within each set and differed by a factor of two from set to set. Mean radii and number densities of soot particles were measured by laser light scattering and extinction. Temperatures are obtained from thermal radiation of soot particles and by applying the sodium-d-line reversal method. The flames were sampled and analysed with on-line mass-spectrometry for concentration of gaseous species. It is found that there is no simple dependence of the soot appearance rates on the ethyne partial pressure, the specific surface area of the soot particles, and the gas-phase temperature. One-dimensional calculations have been performed with a numerical code, that includes a detailed chemical soot model. Calculated profiles of the gaseous species, temperature, number density, and soot volume fraction are in good agreement with the experimental data. The experimentally observed phenomena can be explained in terms of a detailed hydrogen abstraction, carbon addition mechanism. A detailed one-dimensional sensitivity analysis shows that the rate limiting steps of soot formation are gas-phase reactions.