Refractive Indices at Visible Wavelengths of Soot Emitted From Buoyant Turbulent Diffusion Flames

Abstract

Measurements of the optical properties of soot, emphasizing refractive indices, are reported for visible wavelengths (350–800 nm). The experiments considered soot in the fuel-lean (overfire) region of buoyant turbulent diffusion flames in the long residence time regime where soot properties are relatively independent of position in the overfire region and residence time. Flames fueled with acetylene, propylene, ethylene, and propane that were burning in still air provided a range of soot physical and structure properties. Measurements included soot composition, density, structure, gravimetric volume fraction, and scattering and absorption properties. These data were analyzed to find soot fractal dimensions, refractive indices, refractive index functions, and dimensionless extinction coefficients assuming Rayleigh-Debye-Gans scattering for polydisperse mass fractal aggregates (RDG-PFA theory). RDG-PFA theory was successfully evaluated using measured scattering properties. Soot fractal dimensions were independent of both fuel type and wavelength, yielding a mean value of 1.77 with a standard deviation of 0.04. Refractive indices were independent of fuel type within experimental uncertainties and were in reasonably good agreement with earlier measurements for soot in the fuel-lean region of diffusion flames due to Dalzell and Sarofim (1969). Dimensionless extinction coefficients were independent of both fuel type and wavelength, yielding a mean value of 5.1 with a standard deviation of 0.5, which is lower than earlier measurements for reasons that still must be explained.