Extrapolation of laminar ethylene/air flame speeds at elevated pressures with flame chemistry analysis

The laminar flame speeds of ethylene/air flames at various pressures (up to 10.0 atm) were measured using the expanding spherical flame. Three commonly used extrapolation models were employed, and the individual effects of pressure and flame radius range on the extrapolation uncertainty were investigated. It was found that the reduction in flame thickness reduces the extrapolation uncertainty at high pressures. At low pressures, large values of both lower and upper bounds of the extrapolation range in the flame radius range are preferred to reduce the extrapolation uncertainty. The measured laminar flame speeds were then compared with those from previous works and computations using USC and UCSD mechanisms. Sensitivity analysis was further conducted to illustrate the effects of pressure and equivalence ratio on flame chemistry and to reveal the controlling reactions for flame propagation. It was found that the laminar flame speed is more sensitive to flame chemistry at higher pressures, while the relative importance of individual chemical reactions is rarely affected by pressure. The C0 and C1 reactions were found to be the key for the laminar flame speed of ethylene/air flames under fuel-lean conditions, while the C2 reactions assume comparable importance with the C1 reactions on the fuel-rich side.