Modeling the optical properties of combustion-generated fractal aggregates

Combustion-generated carbonaceous aerosols are generally in the form of fractal aggregates (FA's) with shapes that vary from long chain-like structures to much more compact, almost spherical structures, depending upon the mode or stoichiometry of the combustion process. Typically, as combustion moves from fuel-lean to fuel-rich, aggregate morphologies change from the former to the latter. Accompanying this change in morphology is a change in the chemistry of the aggregates as the percent of carbon in the aggregates also decreases. These combined changes produce radically different scattering and absorption signatures that define their radiative transfer properties. To improve our ability to predict how these optical properties change, experiments were conducted to measure both the physical and optical properties of these aggregates for both flaming and non-flaming modes of combustion. Using the aggregate property data from these experiments, numerical calculations were then performed using both the discrete dipole approximation (DDA) and the Rayleigh-Debye-Gans (RDG) approximation to generate their characteristic scattering and absorption signatures. This paper presents the experimental results, the comparison of the modeling results with the experimental results and discusses those parameters most important to obtain agreement between the modeling and the experiments.