Simulation of forced ignition in an accelerator scramjet operating at Mach 5

Abstract: Scramjets are air-breathing engines that operate at hypersonic speeds, where the Mach number is greater than five. At these speeds, they significantly outperform rockets in terms of specific impulse by utilizing oxygen from the atmosphere. One potential application of scramjets is to power the second stage of a partially air-breathing, reusable space launch system that has been modelled to deliver payloads to orbit more efficiently than a pure rocket system. In such a system, the scramjet accelerates from Mach 5 to 10 and, to reliably survive the harsh aerothermal environment, is envisioned to have fixed geometry. This introduces design challenges: to produce useful thrust at high Mach numbers (9-10), the engine must have a low compression inlet and be unobstructed by drag generating features such as ramp or cavity flame-holders, all of which run contrary to typical Mach 5-6 scramjet design approaches. Thus at Mach 5, the flow in the accelerator engine does not autoignite and there is no physical flameholder as a natural site for forced ignition. In this work, we investigate forced ignition in the accelerator scramjet combustor during Mach 5 operation using a hybrid RANS-LES approach: the outflow from a RANS simulation of the inlet is fed into a simulation of the combustor that transitions to Wall-Modelled LES mode as unsteadiness develops in the jet/cross-flow interactions. We study the dynamics of forced ignition by impulsive energy deposition in designed shock-induced boundary layer separations in the wake of fuel injectors. Keywords: Hypersonics, Scramjets, Large-Eddy Simulation, Combustion