Numerical study into thermal-compression scramjets

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 nonuniform compression scramjet flow path that was designed by blending a high-compression two-dimensional profile with a 18° ramp at the centreline with a low-compression two-dimensional profile with a 8° 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 nonuniform-compression 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 become corrugated; however, the same global combustion processes characterised in the premixed study persisted: the shock-induced boundary layer separation ignited the flow in the boundary layer and transported radicals from the high to low-compression side of the engine; and TC from combustion in the high-compression side was found to couple with the spanwise flow gradients to increase heat release in the engine. Additionally, radicals that formed upstream from the combustor within the shear layer of the fuel and air, also enhanced combustion in the engine. The uninstalled specific impulse in the nonuniform-compression engine improved by 100 s and 150 s, at the Mach 8 and 10 (an improvement of approximately 8% for each condition) relative to the equivalent uniform-compression engine with approximately the same inlet compression and contraction ratio. The improvement in performance was found to be from the TC effect. The results showed that the TC mechanism is likely to be reproduced when using other injection schemes which deliver a partially premixed flow with spanwise non-uniform-compression to the entrance of the combustor. The results presented in this work are for a simple, non-optimised geometry, however, the fundamental knowledge and insights gained can, and should be, exploited within scramjet designs. This was demonstrated through a swept nozzle investigation that showed how the flow nonuniformity delivered to the exit of the combustor can be used to reduce the combustor surface area, with no adverse affect on combustion or uninstalled specific impulse. The work successfully applies modern high fidelity computational tools to explore, extend and develop new knowledge and insight into the TC concept originally conceived by Ferri, and provides a fundamental basis upon which advanced engine concepts can now be explored and developed.