Effect of Vibrational Nonequilibrium on Isolator Shock Structure

Analyses of dual-model scramjet engines often rely on the assumption of thermally perfect gas, for which the internal modes of molecular motion are assumed to be in thermal equilibrium. With an increase in enthalpy and in the presence of shocks and expansion waves, the equilibrium assumption does not hold within a scramjet inlet-isolator section. For the flight Mach numbers considered here, the molecular vibrational modes are affected, leading to not only a change in the gas properties but a redistribution of the total energy. In this work, a multitemperature model is used to describe thermal nonequilibrium of flow through a square duct with an inlet Mach number of 1.9. The highly resolved simulations show that the pseudoshock structure is altered due to nonequilibrium, with the leading edge of the shock structure moving upstream. Moreover, the length of the pseudoshock is considerably increased. Both of these effects are shown to be caused by slow relaxation of the molecules. Furthermore, even at the isolator’s exit, equilibrium is not established, which is manifested as a pressure defect. If and when the fluid reaches thermal equilibrium, inside the downstream combustor, there is a potential for a sudden surge in local pressure that could endanger scramjet operation.