COMPUTATIONAL EVALUATION OF APPROXIMATE RAYLEIGH-DEBYE-GANS FRACTAL-AGGREGATE THEORY FOR THE ABSORPTION AND SCATTERING PROPERTIES OF SOOT/

A computational evaluation of an approximate theory for the optical pr operties of soot is described, emphasizing the small-angle (Guinier) r egime. The approximate theory (denoted RDG-FA theory) is based on the Rayleigh-Debye-Gans scattering approximation while treating soot as ma ss-fractal aggregates of spherical primary particles that have constan t diameters and refractive indices. The approximate theory was evaluat ed by more exact predictions from the solution of the volume integral equation formulation of the governing equations, ruing the method of m oments, and based on the ICP algorithm of Iskander et al. (1989). Nume rical simulations were used to construct statistically significant pop ulations of soot aggregates having appropriate fractal properties and prescribed numbers of primary particles per aggregate. Optical propert ies considered included absorption, differential scattering, and total scattering cross sections for conditions typical of soot within flame environments at wavelengths in the visible and the infrared Specific ranges of aggregate properties were as follows: primary particle optic al size parameters up to 0.4, numbers of primary particles per aggrega te up to 512, mean fractal dimensions of 1.75, mean fractal prefactors of 8.0, nod refractive indices typical of soot. Over the range of the evaluation, ICP and RDG-FA predictions generally agreed within numeri cal uncertainties (ca. 10 percent) within the Guinier regime, compleme nting similar performance of RDG-FA theory in the power-law regime bas ed on recent experiments. Thus, the use of approximate RBG-FA theory t o estimate the optical properties of soot appears to be acceptable-par ticularly in view of the significant uncertainties about soot optical properties due to current uncertainties about soot refractive indices.