Optical properties of soot emitted from buoyant turbulent diffusion flames

Extinction and scattering properties at wavelengths of 250–5200 nm were studied for soot in the overfire region of large buoyant turbulent diffusion flames where soot properties are independent of position in the overfire region and characteristic flame residence time. Flames burning in still air and fueled with both gas (acetylene, ethylene, propane and propylene) and liquid (benzene, toluene, cyclohexane and n-heptane) hydrocarbon fuels were considered. Measured scattering patterns and ratios of total scattering/absorption cross sections were in good agreement with predictions based on the Rayleigh-Debye-Gans (RDG) scattering approximation in the visible. Measured depolarization ratios were correlated by primary particle size parameter, completing RDG methodology needed to make soot scattering predictions. Measurements of dimensionless extinction coefficients were in good agreement with earlier measurements for similar soot populations and were independent of fuel type and wavelength except for reduced values as the near ultraviolet was approached. Present measurements of the refractive index function for absorption, E(m), in the visible were in good agreement with earlier independent measurements of Dalzell and Sarofim (1969) and Stagg and Charalampopoulos (1993) and were independent of fuel type. Present values of the refractive index function for scattering, F(m), were similarly independent of fuel type and agreed with these earlier measurements for wavelengths of 400–550 nm but otherwise increased with increasing wavelength more rapidly than the rest. Present values of E(m) in the infrared were larger than those from earlier ex-situ measurements of Felske et al. (1984) and Dalzell and Sarofim (1969). In addition, present values of F(m) were larger than earlier reflectrometry measurements in the infrared. The comparison between present and earlier measurements of the real and imaginary parts of the complex refractive index was similar to E(m) and F(m). The ratios of the scattering/absorption refractive index functions were independent of fuel type and were in good agreement with earlier measurements. Ratios of total scattering/absorption cross sections were relatively large in the visible and near infrared, with maximum values as large as 0.9, suggesting greater potential for scattering from soot particles to affect flame radiation properties than previously thought.