No abstract

This work presents the first high-fidelity numerical study into thermal compression (TC) scramjets. Unlike conventional scramjet engine designs, TC scramjets are designed with an inlet that delivers purposely induced nonuniform-compression to the combustor. This approach was first proposed by Antonio Ferri in the 1960's, who claimed that TC can be used to improve the performance of fixed-geometry scramjets that must operate over a range of flight Mach numbers. The purpose of this work was to develop new knowledge and insight into TC with the use of high-fidelity computational fluid dynamics.In this study, we investigated how combustion in one region of the combustor ignited the flow in other regions of combustor. The study was performed on a generic nonuniformcompression scramjet flow path that was designed by blending a high-compression two-dimensional profile with a 18 o ramp at the centreline with a low-compression two-dimensional profile with a 8 o ramp at the side wall. The high-compression profile produced sufficient compression for combustion, while the low-compression side produced insufficient compression for combustion. The simple geometry allowed the complex propagation of combustion from the high-compression to the low-compression side to be studied in detail. The free stream flow properties corresponded to a Mach 10 flight condition with a dynamic pressure of 100 kPa.The study was conducted with CFD++ using the Reynolds-averaged Navier-Stokes equations. Turbulence was simulated using the SST turbulence model and combustion with the Jachimowski 13 species 33 reaction mechanism. Two numerical approaches were developed to decouple the fluid dynamic/combustion effects: firstly, combustion was disabled in certain regions with the flow field; and secondly, reacting and inert (nonreacting) fuel was injected into the engine at different fuelling sites.Both premixed and inlet-fuelled injection studies were performed on the generic nonuniformcompression engine. The premixed study provided insight into the combustion behaviour independent of any particular fuel injection method. Three flame propagation processes were identified in the study: three-dimensional flow features, emanating from the high-compression side which produced temperatures in the boundary layer that allowed the flow to ignited in the low-compression side of the engine; radical transport through a shock-induced boundary-layer separation, which enhanced combustion in the low-compression side; and TC from combustion in the high-compression side which coupled with the spanwise shock structure. All three flame propagation processes had a significant influence on igniting the flow in the low-compression side of the engine.The premixed work was extended in the inlet-fuelled injection study to evaluate how the added complexity from the flow structures generated from inlet port-hole injection influenced the flame propagation processes. The additional flow structures caused the three-dimensional shock-induced boundary-layer separation to becom...

A numerical study on the combustion behavior of an inlet-fueled three-dimensional nonuniform-compression scramjet is presented. This paper is an extension to previous work on the combustion processes in a premixed threedimensional nonuniform-compression scramjet, where thermal compression was shown to enhance combustion. This paper demonstrates how thermal compression can be used in a generic scramjet configuration with a realistic fuelinjection method to enhance performance at high flight Mach numbers. Such a scramjet offers an extra degree of freedom in the design process of fixed-geometry scramjets that must operate over a range of flight Mach numbers. In this study, how the combustion processes are affected is investigated, with the added realism of inlet porthole fuel injection. Ignition is established from within a shock-induced boundary-layer separation at the entrance to the combustor. Radicals that form upstream of the combustor within the inlet, from the injection method, enhance combustion. Coupling of the inlet-induced spanwise gradients and thermal compression improves combustion. The results highlight that, although the fuel-injection method imparts local changes to the flow structures, the global flow behavior does not change compared to previous premixed results. This combustion behavior will be reproduced when using other fueling methods that deliver partially premixed fuel and air to the combustor entrance.

Scramjet engines can decrease the cost of access-to-space systems. One-dimensional solvers can be used to rapidly analyse many scramjet engine configurations to maximise performance. This paper uses an inviscid, quasi-one-dimensional, chemical equilibrium solver to examine the effects of combusting using constant area, constant pressure, constant Mach number or constant temperature processes. Constant area combustion typically produces the highest specific impulse for given combustor entrance conditions. When a maximum engine pressure constraint is imposed the constant pressure process becomes more effective. This work shows that if combustion occurs at constant area until the maximum pressure is reached, then continued using a constant pressure process, the required intake compression and contraction ratios can be decreased by 53% and 35% respectively from the constant pressure case. This decrease results in improved starting characteristics for a negligible cost to performance. The effect of maximum engine pressure on performance is examined and it is shown that there is little benefit from increasing intake compression ratios above 60, or maximum engine pressure over a factor of 150 above the freestream pressure.

No abstract