Optical Properties of Overfire Soot in Buoyant Turbulent Diffusion Flames at Long Residence Times

The optical properties of soot were studied for the fuel-lean (overfire) region of buoyant turbulent diffusion flames in still air. Results were limited to the long residence time regime where soot structure is independent of position in the overfire region and residence time for a particular fuel. Measurements included scattering, absorption, and extinction cross sections at 514.5 nm and extinction cross sections at 632.8 and 1152 nm for flames fueled with acetylene, propylene, ethylene, and propane. The measurements were used to evaluate scattering predictions based on the Rayleigh-Debye-Gans (RDG) approximation for randomly oriented poly disperse fractal aggregates of spherical primary soot particles having constant diameters. The present soot aggregates exhibited significant departures from Rayleigh-scattering behavior at 514.5 nm, with forward scattering roughly 100 times larger than wide-angle scattering and ratios of scattering to absorption cross sections in the range 0.22–0.41, increasing with increasing propensity of the fuel to soot. The approximate RDG theory generally provided an acceptable basis to treat the optical properties of the present overfire soot aggregates, although additional measurements in the Guinier (small angle) regime are needed for a definitive evaluation of model performance.