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 ethylen e and acetylene burning in coflowing air. The objective was to evaluat e scattering predictions based on the Rayleigh-Debye-Gans (RDG) approx imation 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 p ower-law (large-angle) regimes, were dominant, in order to supplement earlier work for conditions where the power-law regime was dominant. S oot structure was measured using thermophoretic sampling and analysis by transmission electron microscopy (TEM) to yield primary particle di ameters, distributions of the number of primary particles per aggregat e, and the aggregate mass fractal dimensions. Soot optical property me asurements included vv, hh, hv, and vh differential scattering cross s ections, total scattering cross sections, and the albedo at 514.5 nm, as well as several soot structure parameters inferred from these measu rements using the approximate theory. The approximate RDG theory gener ally provided an acceptable basis to treat the optical properties of t he present soot aggregates over a range of conditions spanning the Gui nier 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 s ingle mean squared radius of gyration (from the Guinier regime), Mie s cattering for an equivalent sphere, and Rayleigh scattering-the last u nderestimating differential scattering levels by a factor of roughly 1 00 for the present test conditions.