Effects of oxygen and propane addition on soot formation in counterflow ethylene flames and the role of C₃ chemistry

The influence of oxygen addition and propane addition on soot formation is investigated experimentally in counterflow ethylene diffusion flames, and numerical calculations with detailed chemical reaction mechanisms are conducted to interpret experimental results. Soot volume fractions and polycyclic aromatic hydrocarbon (PAH) concentrations increase with a small amount of oxygen addition for ethylene diffusion flames but not for propane diffusion flames. This result supports that the reactions between O atoms and C₂H ₂ produce CH₂ and eventually form C₃H ₃, which are responsible for the enhancement of incipient rings and thereby PAH and soot formation through C₃H₃ recombination reaction. When a small amount of propane is added to ethylene diffusion flames, PAH concentrations and soot volume fractions also increase. However, the simultaneous addition of oxygen and propane in ethylene diffusion flames shows that the observed soot-formation enhancement with propane addition to ethylene diffusion flames diminishes with oxygen addition to fuel stream and eventually disappears when sufficient amount of oxygen is added. This implies that the mechanisms responsible for the soot-formation enhancement with propane and oxygen addition to ethylene flames can be explained based on the same chemistry. Numerical results on C₃H₃ concentrations can explain successfully both the effects of oxygen and propane additioninethylene flames, through which the importance of C₃ chemistry on soot formation has been demonstrated.