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 in-flow 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. As much as 50% of this fuel is burned by the time it enters the engine isolator, while its injection and burning increases the inlet's drag by less than 5%. This burning bodyside flow region thermally compresses the remaining air flow within the engine isolator, and provides a source of heat and combustion radicals for the ignition of fuel injected further downstream. Overall combustion efficiency of inlet-injected fuel at the engine exhaust plane was found to be nearly 80% at high equivalence ratios.
Flow within the Mach 12 REST combustor is strongly shock-dominated. This is caused by both the cowl closure shock train transmitted from the inlet, and a strong recompression shock generated at the start of the combustor. This recompression shock is generated by the flow passing over a backward step at the entrance to the combustor, and is reinforced on the cowlside of the engine by compression caused by the engine flow path turning to realign with the nominal direction of flight. Fuel injected from the face of the combustor step was combined with inlet injection in an attempt to reduce skin friction drag through boundary layer combustion. This was found to be ineffective: the individual fuel jets never coalesced into a single continuous flow structure, and the fuel layer was quickly disrupted by the pressure gradients induced by the engine turn. The step-injected fuel was quickly ignited by the bodyside combustion region spreading circumferentially around the combustor wall, and its final combustion efficiency was found to be 82.9%.
Boundary layer injection from the step was replaced with tailored fuel injection, in which fuel was injected directly into the cowlside core flow, and the swept separation regions along the engine sidewall. This was found to improve combustion efficiency to 84.9%, verifying the validity of the tailoring approach. The tailored-injection Mach 12 REST engine was found to produce the same level of thrust as the boundary layer injection case despite having a lower fuel mass flow rate. The uninstalled specific impulse of the tailored injection engine was 5% higher than that of the step-injection configuration. Improvements to the tailored fuel injection geometry, combined with small modifications to the engine’s geometry and operation may be sufficient to boost thrust and reduce internal viscous drag enough to allow a flight-model scramjet to achieve net thrust.