Numerical simulation of the effects of evaporation on the n-heptane/air auto-ignition process under different initial air temperatures

Abstract

Numerical simulations are carried out to study the effects of fuel evaporation and mixing on autoignition process of n-heptane/air with complex chemistry. By assuming that n-heptane spray evaporation and mixing are infinitely fast under initial air temperature Tair of 740–1200 K and pressure of 1–24 atm, homogeneous gaseous mixture of n-heptane/air with different equivalence ratio can be obtained. Three types of the most reactive mixture fraction ηMR are identified in mixture fraction space: ηMR,LTand ηMR,ig are associated with the shortest first-stage ignition by low temperature chemistry (LTC) and the total two-stage ignition delay time respectively, while ηMR,HT corresponds to the shortest single-stage ignition delay time by the high temperature chemistry (HTC). With the increasing initial air temperature Tair, ηMR,ig and ηMR,LT will increase, but the corresponding ignition delay times decrease. When Tair is high enough to make the temperature of gaseous mixture higher than the upper turnover temperature of NTC region in very lean mixture, ηMR,HT will appear with shortest ignition delay time. Moreover, the ignition delay time at ηMR,HT could be less than that at ηMR,LT in the spray autoignition process under high initial air temperature Tair=1200K and low initial pressure, 1 atm and 6 atm. This finding suggests that in spray autoignition process, due to the cooling effect of evaporation, LTC reaction and HTC reaction may coexist if the interval between the ignition delay times of ηMR,HT and ηMR,LT is quite small, and also that the high temperature ignition may precede the low temperature ignition. In addition, sensitivity and reaction pathway analysis are performed to disclose the controlling chemical kinetics at the three types of the most reactive mixture fraction ηMR of the case with high initial air temperature 1200 K and pressure 6 atm.