The effects of premixing on soot production in nonpremixed flames fueled with unsaturated hydrocarbons

An experimental investigation has been conducted of the effects of premixing with air on soot production in initially non-premixed flames. The measurements were made with laser-induced incandescence, and a procedure was implemented that allowed us to determine the maximum centerline soot volume fraction as a function of primary equivalence ratio with high precision and high resolution in equivalence ratio. Results from propene and ethylene flames with varying fuel flow rates and varying nitrogen dilutions showed that the effects of premixing-positive or negative-combine synergistically with other perturbations that reduce the overall soot-forming tendency. Seven different fuels were studied; in two cases (ethylene and acetylene), slight premixing produced an increase in soot production, while in fife cases (ethane, propane, propene, propadiene, and 1,3-butadiene), premixing monotonically decreased soot production. All of these results could be explained in terms of the differences between the pure pyrolysis and oxidative pyrolysis decomposition mechanisms of the fuels and the relative significance of different pyrolysis products as benzene precursors. Measurements in a 10% ethane/90% ethylene mixture showed that small amounts of alkanes are sufficient to prevent an increase in soot production for ethylene. Thus, overall the results indicate that increases in soot production due to premixing are unlikely to occur in practical systems.