Soot Volume Fraction Measurements in the Soot-forming Regions of Ethylene-Air Turbulent Partially-premixed Flames

Soot formation characteristics of ethylene-air turbulent partially premixed flames have been experimentally determined by measuring the soot volume fraction, via laser-induced incandescence. Measurements were made in the near-burner soot-forming region up to axial distance of 25 burner diameters. The amount of soot formed in these flames varies non-monotonically as the equivalence ratio is lowered from that of non-premixed flames. Initial premixing with air results in a substantial increase in the integrated soot volume fraction, up to a factor of nearly 2 in comparison to non-premixed flames. Only at overall equivalence ratio near 10, the amount of soot is brought below the level of non-premixed flames in the region. Further premixing with air results in a continuous decrease in the soot volume fraction. The radial profiles of soot volume fraction show more evenly distributed soot particles in partially-premixed flames, indicating that soot-forming regions are significantly broadened in these flames. The injection location of premixing air has a significant effect on the sooting characteristics of partially-premixed flames. In the current co-axial jet flame burner configuration, when the premixing air is added co-axially at the burner exit plane, the partial premixing always resulted in a decrease in the integrated soot volume fraction. Moving the injection point upstream caused the amount of soot to increase rapidly. The pdf's of the soot volume fraction are consistent with the above observations in that there is a shift in the pdf toward larger soot volume fraction when there is an increase in the integrated soot volume fraction. Except, the effect of the air injection location is manifest through small but finite probabilities of large soot volume fractions, while the peak of the pdf remains at small soot volume fractions. These results point out some important trends in the sooting characteristics as a function of partial premixing parameters.