SOOT MORPHOLOGY AND OPTICAL-PROPERTIES IN NONPREMIXED TURBULENT FLAMEENVIRONMENTS

Motivated by the importance of soot to the emission of particulates an d other pollutants from combustion processes, current understanding of soot morphology and optical properties is reviewed, emphasizing nonpr emixed flame environments. The understanding of soot morphology in fla mes has grown rapidly in recent years due to the development of method s of thermophoretic sampling and analysis by transmission electron mic roscopy (TEM). The results show that soot consists of nearly spherical primary particles, having diameters generally less than 60 nm, which collect into open structured aggregates that are mass fractal objects. Aggregates grow by cluster/cluster aggregation to yield broad aggrega te size distributions with the largest aggregates containing thousands of primary particles and reaching dimensions of several mu m. The opt ical properties of soot aggregates generally are not suited for the Ra yleigh and Mie scattering approximations which has led to the developm ent of approximate Rayleigh-Debye-Gans (RDG) scattering models for pol ydisperse fractal aggregate populations of soot. Evaluation of RDG mod els for conditions where both soot structure and scattering properties are known indicates encouraging agreement between predictions and mea surements at both visible and infrared wavelengths, as well as reasona ble accuracy for Rayleigh scattering theories in the infrared. Thus, t here is potential for nonintrusive measurements of both soot concentra tions and structure in flame environments, which should be helpful for diagnosing problems of particulate and pollution emissions from combu stion processes. However, additional work is needed to realize this po tential, including: reliable solutions of the inverse problem, to yiel d soot properties from scattering and extinction measurements, must be developed; existing uncertainties about soot refractive indices, incl uding effects of fuel type and flame conditions, must be resolved; and more definitive assessment of the limitations of existing approximate theories for soot optical properties must be obtained.