The effects of oxygen index on soot formation of ethylene, propane and their mixtures in coflow diffusion flames

Oxygen index (OI) effects on soot formation and oxidation of ethylene, propane and their mixtures were investigated in coflow diffusion flames. Experimental results show that the peak soot volume fraction (SVF) firstly increases and then reaches the plateau region as OI increases from 0.21 to 0.42 in ethylene and 90%ethylene/10%propane mixture flames. However, the peak SVF firstly increases and then decreases with the increase of OI in propane flame. All these three cases present a consistent trend in terms of maximum soot yield which increases first and then decreases as OI increases. Synergistic effect was captured in ethylene/propane mixtures while it was firstly enhanced and then mitigated when OI increases in the investigated OI range. Numerical simulations were also conducted. A significant relationship was found among flame temperature, O₂ mole fraction and SVF in terms of their spatial gradients. The non-linear trends of SVF and soot yield with OI can be attributed to the competition between soot formation and oxidation. Such competition was also indicated from polycyclic aromatic hydrocarbons (PAHs) growth. Numerical analyses revealed that small radicals including O, CH₂ and C₆H₅ contribute to the non-linear changes of PAHs in the synergistic effect.