Experimental study of nonfuel hydrocarbons and soot in coflowing partiallypremixed ethylene/air flames

Centerline profiles of gas temperature, C1 to C12 nonfuel hydrocarbon conce ntrations, polycyclic aromatic hydrocarbon (PAH) laser-induced fluorescence (LIF), and soot volume fraction are reported for coflowing ethylene nonpre mixed and partially premixed flames with primary equivalence ratios ranging from 24 to 3. Concentrations of acetylene and C4 hydrocarbons were lower i n nearly all of the partially premixed flames than in the nonpremixed flame , whereas concentrations of methane and C3H4 were larger in all of the part ially premixed flames than in the nonpremixed flame. These results indicate thar the primary effect of partial premixing is not to uniformly increase the concentrations of pyrolysis products, but to shift, the pyrolysis mecha nism towards odd-carbon species. The concentration of benzene was larger in several of the richer partially premixed flames than in the nonpremixed fl ame, probably because the shift in pyrolysis mechanism enhances self-reacti on of C3H3 radicals. Increases in soot volume fraction and other aromatics were observed that matched the increases in benzene. Profiles of PAH fluore scence agreed closely with those for specific gas-phase PAH such as naphtha lene, and the maximum PAH signals were a good predictor of the eventual max imum soot volume fractions. Concentrations of oxygenated hydrocarbons such as formaldehyde and ketene were dramatically increased in the partially pre mixed flames; for formaldehyde this trend was confirmed with in situ LIF me asurements. (C) 2000 by The Combustion institute.