Radiative properties of flame-generated soot

Approximate methods for estimating the optical properties of flame-generated soot aggregates were evaluated using existing computer simulations and measurements in the visible and near-infrared portions of the spectrum. The following approximate methods were evaluated for both individual aggregates and polydisperse aggregate populations: the Rayleigh scattering approximation, Mie scattering for an equivalent sphere, and Rayleigh-Debye-Gans (R-D-G) scattering for both given and fractal aggregates. Results of computer simulations involved both prescribed aggregate geometry and numerically generated aggregates by cluster-cluster aggregation; multiple scattering was considered exactly, considered using the mean-field approximation, and ignored using the R-D-G approximation. Measurements involved the angular scattering properties of soot in the postflame regions of both premixed and nonpremixed flames. The results show that available computer simulations and measurements of soot aggregate optical properties are not adequate to provide a definitive evaluation of the approximate prediction methods. The simulations involve either exact solutions for small aggregates where effects of multiple scattering are small, or approximate solutions of uncertain accuracy for the large aggregates of interest for practical flames. The measurements are limited to conditions where soot aggregate structure is not known, and for relatively large scattering angles where the various approximations yield similar results. Within these limitations (for aggregates larger than the Rayleigh scattering regime) the approximate theories performed as follows: Rayleigh scattering generally underestimated scattering, Mie scattering for an equivalent sphere yielded unreliable results, while basic and fractal aggregate R-D-G scattering yielded best results for given and fractal aggregates, respectively. However, existing simulations suggest significant effects of multiple scattering for soot aggregates (except near soot inception conditions) that are not included in R-D-G scattering so that improved approximate optical theories for soot aggregates should be sought.