Analysis of light scattering from soot using optical cross sections for aggregates

Abstract

Soot formed in flames usually consists of aggregates (clusters or agglomerates) of a variable number of nearly spherical, monodisperse primary particles (monomers or spherules). In this work, the optical properties of polydisperse aggregates are used to analyze light scattering data from a coannular ethene diffusion flame. In previously reported studies, data have been obtained on the local extinction and volumetric scattering cross sections from laser scattering experiments, on the flame velocity field from laser velocimetry, and on the primary particle sizes determined by electron microscopy. The present analysis yields the average number of primary particles per aggregate, the mean-square radius of gyration, the soot volume fraction and the aggregation rate. It is found that sustained collisional growth of the aggregates occurs while their primary particles growth through heterogeneous reactions low in the flame, and contract through surface oxidation in the upper half of the flame. A recent value of the refractive index gives internally consistent moment ratios of the aggregate size distribution function. This method of analysis provides a more detailed and complete description of the formation, growth and oxidation of soot aggregates in a diffusion flame.