Soot Morphology and Optical Properties in Nonpremixed Turbulent Flame Environments

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Abstract
Motivated by the importance of soot to the emission of particulates and other pollutants from combustion processes, current understanding of soot morphology and optical properties is reviewed, emphasizing nonpremixed flame environments. The understanding of soot morphology in flames has grown rapidly in recent years due to the development of methods of thermophoretic sampling and analysis by transmission electron microscopy (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 aggregate size distributions with the largest aggregates containing thousands of primary particles and reaching dimensions of several urn. The optical properties of soot aggregates generally are not suited for the Rayleigh and Mie scattering approximations which has led to the development of approximate Rayleigh-Debye-Gans (RDG) scattering models for polydisperse fractal aggregate populations of soot Evaluation of RDG models for conditions where both soot structure and scattering properties are known indicates encouraging agreement between predictions and measurements at both visible and infrared wavelengths, as well as reasonable accuracy for Rayleigh scattering theories in the infrared. Thus, there is potential for nonintrusive measurements of both soot concentrations and structure in flame environments, which should be helpful for diagnosing problems of particulate and pollution emissions from combustion processes. However, additional work is needed to realize this potential, including: reliable solutions of the inverse problem, to yield soot properties from scattering and extinction measurements, must be developed; existing uncertainties about soot refractive indices, including 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.