Investigation of Combustion Mode Control in a Mach 8 Shape-Transitioning Scramjet

Abstract

Accelerating scramjet engines for an access to space system should optimally anchor dual-mode combustion at one end of the trajectory and supersonic combustion at the other. Conventionally, dual-mode combustion engines flamehold with physical obstructions, causing prohibitive losses at high speed. Past experiments of an airframe-integrated scramjet engine at Mach 8 showed that dual-mode combustion was initiated and sustained with an unobstructed flowpath by varying the fueling rate. This flowfield is examined numerically to ascertain the fluid dynamic phenomena leading to the separated combustion modes. Two regimes of dual-mode combustion occurred above stoichiometric fueling: a separation-train-dominated flowfield downstream of the injectors and a flowfield dominated by a large separation upstream of the body-side injectors at higher fueling rates. The first mode occurs when two shocks combine and create a separation on the body-side wall that merges with the body-side injector wake separation, causing a train of separations. The second regime is from the upstream movement of combustion with increasing equivalence ratio: when combustion reaches the rear of the body-side injector, the near injector flow structure changes, allowing fuel and combustion to enter a large separation upstream. The performance of the different modes is explored.