Effect of thermal nonequilibrium on ignition in scramjet combustors

Abstract

The effect of thermal nonequilibrium on flame ignition and stabilization in supersonic scramjet combustors is studied using reaction-layer resolved high resolution numerical simulations. The flow configuration is based on the HyShot-II experiment, with a sonic H2 jet injected into a supersonic crossflow. A detailed H2/air chemical kinetics model is used to simulate the combustion process. Nonequilibrium is described using a multi-temperature model, where the vibrational population distribution of each species is treated using a species-specific temperature. Ab initio derived chemical rates are used for key initiation reactions, while a vibrationally-coupled model is used to describe other reactions under nonequilibrium conditions. The simulations reveal an interesting phenomenon, namely, temperature inversion driven flame stabilization. While conventional wisdom states that ignition should be delayed behind a compression shock due to underpopulation of vibrational levels, the results show that ignition occurs much earlier than in equilibrium flows. The main reason is the generation of a temperature inversion region due to cross-stream fuel injection. Here, the slower vibrational relaxation produces regions of higher vibrational temperatures with combustible mixtures, while the nonequilibrium-driven translational temperature is higher than the equilibrium temperature. The ab initio rates show that certain reactions are relatively insensitive to vibrational energy distribution. Hence, for the same total energy, the reaction rates are increased when the translational energy is higher, leading to stable combustion with enhanced dissociation very close to the nozzle exit. Further analysis shows much of the combustion heat release occurs in this zone where O2 vibrational temperatures are higher than translational temperatures. Even though the presence of H2O quenches nonequilibrium relatively quickly, the initial differences in temperatures created by the nonequilibrium flow is sufficient to substantially alter ignition location.