Simultaneous optical measurement of soot volume fraction and temperature in premixed flames

Abstract

The performance of a three-wavelength optical probe technique for measuring soot volume fraction and temperature was assessed by conducting experiments in the homogeneous environment of a premixed flame. Using a premixed ethylene/air flame, the temperatures and soot volume fractions ( f va , based on absorption measurements at 633 nm and f ve , based on emission measurements at 900 nm and 1000 nm) were compared with previously reported results. Although the temperatures and mean soot volume fractions compared favorably, the discrepancy between f va and f ve prompted new measurements to evaluate the importance of source wavelength on the f va measurements, scattering by soot particles, light absorption by “large” molecules and the use of different indices of refraction reported in the literature. The experiments on the degree of soot scattering and light absorption by “large” molecules indicated that these effects cannot reconcile the observed discrepancy in the soot volume fractions. The measured soot volume fractions were, however, sensitive to the absorption constant and therefore varied significantly when different sets of refractive indices were used. Furthermore, the agreement between f va and f ve was improved when extinction measurements were performed with longer wavelength light sources. Isokinetic soot sampling experiments were also performed to compare with the optically-measured soot volume fractions. This technique does not rely on the refractive indices of soot and therefore provides an independent measure of the soot volume fraction. The soot volume fractions measured using this technique compared favorably with the optically measured values (calculated using various indices of refraction).