Effect of mixing methane, ethane, propane, and propene on the synergistic effect of PAH and soot formation in ethylene-base counterflow diffusion flames

The synergistic effect of the enhancement of PAH and soot formation by the fuel mixing of ethylene and propane has been previously investigated, and the effect has been explained based on the competition between the H-abstraction-C₂H₂-addition reaction and the incipient ring formation through propargyl recombination, and subsequently polycyclic aromatic hydrocarbon (PAH) growth through odd-carbon chemistries. To further elucidate the effect of fuel mixing on the formation of PAHs and soot, four different types of fuels - methane, ethane, propane, and propene - have been mixed to counterflow ethylene-base diffusion flames. Planar laser-induced incandescence and laser-induced fluorescence techniques were employed to measure relative soot volume fractions and PAH concentrations, respectively. Results showed that a small amount of ethane mixing in ethylene diffusion flame also exhibits the synergistic effect on PAH and soot formation. The mixing of propane in ethylene diffusion flame has a more pronounced synergistic effect as compared to the propene mixing when the mixture ratio is small. Since the decomposition reactions of ethane or propane can be the source of methyl radicals, such behaviors emphasize the role of methyl radicals in the formation of PAHs and soot. Propargyl formation could be additionally enhanced by the reactions related to methyl radical, which can be the reason for the synergistic effect in the mixture flames of ethylene with propane or ethane. Numerical results on the concentrations of CH₃, C₂H₂, and C₃H₃ substantiated the importance of methyl radical in the formation of propargyl and thereby the synergistic effect on the formation of PAHs and subsequent soot growth.