Direct numerical simulation of supersonic combustion with thermal nonequilibrium

Abstract

Flow inside the combustor of a scramjet engine is likely to be in thermal nonequilibrium due to upstream shock-based compression. Given the short flow-through timescales in such engines, the flow will not reach equilibrium in the combustor. Since the distribution of energy in the internal modes affects chemical reactions, nonequilibrium could have significant impact on ignition and flame stabilization inside scramjet combustors. In this study, direct numerical simulation (DNS) is used to understand the impact of thermal nonequilibrium on flame structure in a supersonic flow. The operating conditions correspond to the HyShot II scramjet experiment. A two-temperature model along with detailed chemical kinetics is used to model supersonic combustion. It is observed that when vibrational temperature is lower than translational temperature, there is significant delay in flame ignition. In addition, combustion reactions are weaker leading to reduced heat release. The vibrational temperature exhibits a broad dissipation structure compared to a conserved scalar, but contains thin dissipation elements near the shear layer between the fuel and oxidizer. There is a small improvement in turbulent mixing with nonequilibrium, primarily due to reduced viscosity arising from lower translational temperature in the domain. Overall, thermal equilibrium occurs over time scales longer than the integral scale, indicating that nonequilibrium will play a crucial role in the internal structure of the flame.