Optical Properties of Soot in Buoyant Laminar Diffusion Flames

The structure and optical properties of soot were studied in the fuel-rich (underfire) region of buoyant laminar diffusion flames of ethylene and acetylene burning in coflowing air. The objective was to evaluate scattering predictions based on the Rayleigh–Debye–Gans (RDG) approximation for polydisperse fractal aggregates of spherical primary soot particles having constant diameters, for conditions where the Guinier (small angle) regime, and the transition between the Guinier and the power-law (large-angle) regimes, were dominant, in order to supplement earlier work for conditions where the power-law regime was dominant. Soot structure was measured using thermophoretic sampling and analysis by transmission electron microscopy (TEM) to yield primary particle diameters, distributions of the number of primary particles per aggregate, and the aggregate mass fractal dimensions. Soot optical property measurements included vv, hh, hv, and vh differential scattering cross sections, total scattering cross sections, and the albedo at 514.5 nm, as well as several soot structure parameters inferred from these measurements using the approximate theory. The approximate RDG theory generally provided an acceptable basis to treat the optical properties of the present soot aggregates over a range of conditions spanning the Guinier and power-law regimes. Other scattering approximations were less satisfactory with performance progressively becoming less satisfactory in the order: RDG polydisperse fractal aggregate scattering using a single mean squared radius of gyration (from the Guinier regime), Mie scattering for an equivalent sphere, and Rayleigh scattering—the last underestimating differential scattering levels by a factor of roughly 100 for the present test conditions.