Modeling the effect of flame structure on the formation of aromatics in opposed flow diffusion flames

The effect of flame structure on the mole fractions of species included in the Appel, Bockhorn, and Frenklach C₂ mechanism was investigated numerically in an opposed flow ethylene diffusion flame using OPPDIF. All flames were produced at the adiabatic flame temperature of the neat ethylene-air flame (2370 K) with similar strain rates. Numerous experimental results show that soot formation can be reduced and even eliminated while maintaining constant temperature by removing inert from the oxidizer stream and diluting the fuel. This study shows that when comparing the extreme cases of a heavily sooting neat ethylene/air flame and a non-sooting 8%-C₂H₄-N₂/100%-O₂ flame with identical adiabatic flame temperatures the peak benzene mole fraction decreases by only a factor of eight, which would appear to be insufficient for complete soot suppression. The results also show that even though the ethylene mole fraction was reduced by a factor of 12 in the non-sooting flame, the peak mole fractions of n-C₄H₃, n-C₄H₅, and the propargyl radical (C₃H₃) decrease by only a factor of two, and acetylene (C₂H₂) decreases by a factor of four. In addition there is a slight increase in the peak mole fraction of the vinyl radical (C₂H₃) and a dramatic increase in the C₂H₃O peak mole fraction by nearly two orders of magnitude. These results indicate that other species in addition to benzene, acetylene, propargyl, n-C₄H₃, and n-C4H5 may be important in the path to PAH and soot particle formation for diffusion flames.