Inversion method and experiment to determine the soot refractive index: application to turbulent diffusion flames

Experimental and numerical studies have been performed to determine the soot refractive index in methane turbulent diffusion flames with two oxidizers: air and oxygen. In the flame zone, soot particles were sampled with a cooled probe. Measurements of optical soot properties have been carried out to obtain extinction and vertical-vertical (90°) scattering coefficients. The size distributions were obtained by electrical mobility analysis. Using these distributions, the optical properties have been computed with the Rayleigh-Debye-Gans theory for fractal aggregates by considering the morphology of soot aggregates and using morphological parameter values based on literature reports for other similar systems. Then, the refractive index has been obtained from a numerical inversion method by matching the measured and computed optical coefficients. This refractive index determination method is new to our knowledge. In turbulent diffusion methane oxygen flames the soot refractive index averaged value found is m = 1.95(±0.13)-0.51i(±0.12), and in the air flame m = 2.10(±0.12)-0.48i(±0.06). In view of the uncertainties, the refractive index is independent of the oxidizer type, the aerodynamic conditions and the flame zone location for the sampling. A sensitivity analysis has been carried out to study the influence of some morphological and experimental parameters on the refractive index value.