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

Abstract

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.i 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...

Tailored Fuel Injection for Performance Enhancement in a Mach 12 Scramjet Engine

“…As such, no further mention of the heat transfer gauges will be made, but further information is available in Ref. [32].…”

Modeling of Supersonic Combustion Systems for Sustained Hypersonic Flight

“…The selected compression system is designed to decelerate the operating Mach 8 flow to Mach 2.5, where the boundary conditions used for the combustion chamber testing are based on the concept aircraft scramjet engine design (Table 2). High Mach number entry flow is investigated further in Reference [10] in recent research. The scramjet inlet was designed using methodology from studies carried out at Queensland University [11].…”

Optimising scramjet combustor geometry

“…The Mach 12 REST engine was considered for analysis, using initial conditions from [22] and geometry from [21]; the mixing efficiency curve was developed and used to describe the mass flow rate differential. The frictionless case was first compared to the case with friction losses; it was found that the inclusion of friction decreases combustion efficiency.…”

“…The initial combustor conditions, without fuel injection in the inlet of the engine, are = 0.1023 / , = 1560 , = 44.502 , = 3027 / and = 4.02 [22]. Though the real engine includes fuel injection in the inlet, and this changes the combustor inlet conditions, the case being considered here is without this fuel addition.…”