Effect of methane doping on nascent soot formation in ethylene-based laminar premixed flames

•The particle size distribution functions of soot particles in methane-doped ethylene flames behave bimodality.•The number densities in downstream zone of the methane-doped flames are much higher than pure flame.•Small ratio of methane doping can promote the volume fractions, while in large ratio cannot.

The impact of methane doping on sooting behavior of ethylene-base burner stabilized stagnation (BSS) flames was investigated by following the evolution of particle size distribution functions (PSDFs) of nascent soot. The M-series of flames with the equivalence ratio of 2.07 was doped by methane with the mixture ratio of 5%, 10% and 40%. All the methane-doping flames showed obvious bimodality in the PSDFs, and more particles in nucleation stage than the C3 reference flame. The synergistic effect appeared in small ratio (5% and 10%) methane doping cases, leading to the increase of the soot number density and soot mobility volume fractions. While the synergistic effect was not found under large methane doping condition (40%), in which the mobility volume fraction was less than that in C3 flame. By calculating the key species and analyzing the reaction pathways of pyrene formation with KM2-Mech, we found that small ratio of methane doping benefits C3H3 and pyrene (A4) formation, which strengthened the sooting tendency. However too much methane doping reduced the yield of C2H2 that is a critical intermediate of PAHs formation, leading to a lower sooting tendency compared with the pure ethylene flame.