Mixing and combustion enhancement in a Mach 12 shape-transitioning scramjet engine

Despite being an active topic of research for over 50 years, scramjet technology has only recently matured to a point where flight tests are being successfully carried out at the lower end of the hypersonic regime. While this progress is encouraging, a renewed interest in low-cost, reliable, and environmentally responsible access to space has identified scramjets capable of accelerating to speeds as high as Mach 12 as desirable. One class of scramjets thought to be capable of hypervelocity performance are those that employ three-dimensional streamtraced compression inlets to efficiently compress captured air. One promising example of this type of scramjet is the Mach 12 Rectangular-to-Elliptical Shape-Transitioning (REST) engine. The aims of this study are to investigate and characterize the flow physics behind the Mach 12 REST engine’s current performance, and then attempt to improve its combustion performance by tailoring the engine’s fuel injection to its internal flow field without otherwise modifying the engine geometry. To meet these aims, the engine was studied both numerically and experimentally. The first-ever combusting simulations of a REST scramjet operating at Mach 12 conditions were performed for the Mach 12 REST engine using the CFD research code US3D. The simulations covered a range of conditions, including: unfuelled engine flow, inlet-fuelled flow, and various combined inlet/combustor fuelling configurations. The simulations were found to match well with the experiments they were designed to reproduce and be compared against. A comparison of simulations with experimental inflow conditions and their equivalent flight conditions on an otherwise identical engine showed that experiments in the T4 Stalker tube reproduce engine pressure and heat flux distributions well. The tunnel condition tends to capture less incoming flow than the engine at flight conditions, which leads to the ground-tested engine over-predicting the engine equivalence ratio. The Mach 12 REST inlet was found to produce a thick “bubble-shaped” boundary layer along its bodyside compression surface, due to the compression effects of the inlet sidewalls acting on a thick turbulent boundary layer ingested from the vehicle forebody. This thick boundary layer forces the majority of inlet-captured air into a high-density, high Mach number flow region along the engine cowlside wall . The inlet also produces a symmetric pair of high-temperature swept separations that enter the engine isolator along the sidewalls of the engine. When fuel is injected from the bodyside surface of the inlet, it remains trapped inside the thick, turbulent boundary layer, where it becomes well-mixed and begins to burn just upstream of the inlet throat.