Radiative heat transfer in soot-containing combustion systems with aggregation

The effects of soot shape on radiative transfer predictions in soot-containing flames were quantitatively analyzed by using a realistic simulation of the aggregation process. Various spectral properties (phase function, albedo, extinction coefficient, and emissivity) of fractal soot aggregates were computed using the integral equation formulation for scattering (IEFS) for a broad wavelength spectrum from ultraviolet (0.2 mu m) to infrared (6.2 mu m) Total emissions from uniform clouds of aggregates of small spherical particles were then estimated for typical soot volume fractions and flame temperatures. All spectral and total radiative transfer properties computed using IEFS were normalized by the predictions based on the primary (unaggregated) particles. The spectral variation of the extinction coefficient of aggregates relative to that of primary particles has a somewhat unexpected and complex behavior in the ultraviolet and infrared wavelength regions due to the opposite effects of morphology and soot refractive indices. The effects of soot aggregation on spectral and total emissivities are found to be less than 25 and 13%, respectively, for typical particle volume fractions and flame temperatures. The results suggest that the shape effects on emission predictions in soot-laden flames can be neglected, vastly simplifying the engineering treatment of radiation modeling of combustion devices. (C) 1998 Elsevier Science Ltd. All rights reserved.