SPECTRAL EXTINCTION COEFFICIENTS OF SOOT AGGREGATES FROM TURBULENT-DIFFUSION FLAMES

The spectral extinction coefficients of soot aggregates were studied i n the fuel-lean (overfire) region of buoyant turbulent diffusion flame s. Extinction measurements were carried out in the wavelength region o f 0.2-5.2 mu m for flames fueled with acetylene, propylene, ethylene, and propane, burning in air. The present measurements were combined wi th earlier measurements of soot morphology and light scattering at 0.5 14 mu m in order to evaluate the spectral soot refractive indices repo rted by Dalzell and Sarofim (1969), Lee and Tien (1981), and Chang and Charalampopoulos (1990). The specific extinction coefficients and emi ssivities were predicted based on Rayleigh-Debye-Gans theory for polyd isperse fractal aggregates, which has been recently found to be the be st approximation to treat optical cross sections of soot aggregates. T he results indicated that available refractive indices of soot do not predict the spectral trends of present measurements in the ultraviolet and infrared regions. Soot complex refractive index was inferred to b e m = 1.54 + 0.48i at 0.514 mu m, which is surprisingly in best agreem ent with the values reported by Dalzell and Sarofim (1969). Additional ly, specific extinction coefficients of soot aggregates varied with wa velength as lambda(-0.83) from the visible to the infrared. Finally so ot refractive indices were found to be relatively independent of fuel type for the visible and infrared spectral regions over the H/C ratio range of 0.08-0.22.