Super-adiabatic temperature in homogenous ignition of CH4/O2/N2 mixtures

Methane-oxygen burning has promising application in future rocket engines used in low-cost space launch systems. In this study, the phenomenon of super-adiabatic temperature (SAT) in homogenous ignition process of methane-oxygen with and without nitrogen dilution is investigated. Simulations considering detailed chemistry are conducted for homogeneous ignition processes and premixed flames in CH4/O2/N2 mixtures. The objectives are to assess the effects of equivalence ratio, oxygen content in oxidizer, initial temperature and pressure on SAT and to identify the key elementary reactions involved in SAT for methane. It is found that the SAT changes non-monotonically with the equivalence ratio and there exists two regimes for SAT. The contribution of each elementary reaction to the post-ignition temperature decrement is quantified and the key reactions responsible for the occurrence of SAT are determined. It is found that the SAT in the homogenous ignition of CH4/O2/N2 mixtures is mainly caused by the endothermic dissociation reactions of H2O, H2, CO2 after thermal runaway. With the increase of initial temperature and oxygen content and decrease of pressure, the dissociation of H2O, H2 and CO2 becomes stronger and thereby SAT is larger. Moreover, the chemistry involved in the SAT for homogeneous ignition and premixed flames is compared. It is found that the same chemical kinetics is involved in the SAT for homogenous ignition process and premixed flames.