Numerical Simulations in Scramjet Combustion with Boundary-Layer Bleeding

Airframe-integrated scramjet engines swallow the boundary layers that develop on the forebody of space planes. The scramjet engine easily falls into engine stall (engine unstart) as a result of the boundary-layer separation resulting from combustion. In this study, to investigate the unstart characteristics, numerical simulations of a whole scramjet engine with boundary-layer bleeding are performed by using a reacting flow code, and the physics determining the engine performance is examined. Our computations well reproduce the engine combustion tests results with bleeding. Bleeding of 0.65 % in a captured airflow suppresses the separation of the ingested boundary layer and extends the start limit from the fuel equivalence ratio of 0.5 to 1.0. The numerical results predict small discrete circular flames anchored around individual fuel jets near the injectors. These discrete flames merge to form a large envelope diffusion flame in the downstream portion of the combustor, as a result of the secondary flow produced by high pressure of the cowl shock and intensive combustion. This merged structure causes a large mass of unburned fuel and restricts the combustion efficiency and the thrust performance.