A systematic analysis of chemical mechanisms for ethylene oxidation and PAH formation

Accurate prediction of soot formation and evolution remains a formidable challenge due to the complex interaction between gas-phase composition and solid-phase particles. Recent studies have shown that the choice of gas-phase mechanisms is of primary importance in affecting predictability. In this work, a systematic analysis of ethylene (CH) combustion mechanisms denoted as KAUST, Stanford, Aachen, Polimi, ABF, DLR/UT, Naples, Caltech, and SJTU, which have been widely used in the soot community, is performed to investigate their differences in fundamental chemistry, polycyclic aromatic hydrocarbon (PAH) chemistry, and soot prediction. It is found that the nine mechanisms exhibit large differences even in predicting canonical combustion properties (e.g., ignition delay time, laminar flame speed, and extinction strain rate), indicating significant variations in the fundamental chemistry. This is due to the fact that although most mechanisms share very similar dominant fuel-oxidation reactions, there are notable differences in the rate coefficients of sensitive reactions used in these mechanisms. Owing to the uncertainties in the fundamental chemistry, the predictions of CH from the nine mechanisms show significant differences, which contributes to the differences in soot precursor prediction, in conjunction with the difference in benzene (A1) formation pathways demonstrated by the element flux analysis of the C atom. Furthermore, it is found that PAHs containing two rings play a dominant role in soot formation for most mechanisms. Moreover, it is observed that while Caltech and SJTU significantly under estimate soot formation, they reasonably reproduce its sensitivity to strain rate. These results indicate that despite substantial advances in the development of CH oxidation and PAH formation chemistry, the various existing mechanisms lead to significant differences in predicting soot concentrations which can be traced back to considerable differences in both fundamental chemistry and PAH chemistry. This suggests that the fundamental chemistry should be calibrated or improved before further development of PAH chemistry and soot models.