Investigation of supersonic turbulent combustion in a Mach 10 scramjet engine

Abstract

The development of efficient scramjet engines to power access-to-space vehicles requires fundamental understanding of all its processes. It is estimated a combustion efficiency of at least 80\% is required for a scramjet-powered stage in a multi-stage access-to-space vehicle. For lower Mach numbers, trial and error approaches, supported by empirical data, are useful in the design of scramjet engines that reach this efficiency threshold. This technique, however, has not been successful in designing vehicles capable of generating enough thrust to cruise at Mach numbers equal or greater than 10. Therefore, better understanding of the mechanisms behind supersonic turbulent combustion are necessary for the development of high-performance engines from the ground up. This is no easy task: turbulent combustion remains an unresolved problem and the simulation models that have been developed, while extremely useful for subsonic turbulent combustion, have limited usefulness when applied to tackle the added complexities of its supersonic variant. Previous research using numerical simulations has indicated that scramjet combustion happens in multiple regimes. But, given the uncertainty of applicability of conventional combustion models used in numerical studies, experimental data is necessary to validate these results and provide further insight into the physics behind the supersonic combustion process. To this intent, an experiment has been developed to characterise supersonic turbulent combustion in a complete scramjet engine. One of the main characteristics of scramjet flow is its non-uniformity, driven by the intake shocks, which interact and reflect to form a complex shock train structure in the engine combustor, which needs to be recreated in order to capture the real effects present in a scramjet engine. An engine was designed with a symmetrical intake composed of \ang{6} and \ang{15} ramps to generate this non-uniform flow. It is an inlet-fuelled scramjet, equipped with a single injector located in the centre plane, 80 mm upstream of the combustor throat in the \ang{15} ramp on one side of the engine. This produces a fuel plume that can interact with the shock waves, which should enhance mixing and drive ignition of the fuel, just like in a real application. It also guarantees the fuel plume remains isolated from the side walls and the wall opposite the injector, making it easier to simulate with high-fidelity Large-Eddy Simulations, reducing computational cost. This engine was tested in the T4 Reflected Shock Tunnel at The University of Queensland with an inflow condition of Mach 7, using a Mach-10 flow enthalpy of 4.5 MJ/kg to reproduce the combustion conditions in a vehicle operating at Mach 10. The turbulent flame was directly observed in the flow using planar laser-induced fluorescence of \ch{OH} radicals. Complementing these results with Large-Eddy Simulations of the experimental conditions, it has been shown that the combustion process in scramjets is multi-mode, where neither premixed nor non-premixed combustion dominate, and both substantially contribute to heat release. Combustion is confirmed to happen over multiple regimes throughout the engine, particularly in the boundary between the regimes. Furthermore, simulations were performed of the same geometry, with a Mach-10 inflow, using oxygen enrichment, in which oxygen is injected, premixed with the fuel. These simulations showed that even though combustion efficiency and heat release are greatly increased by the injection of premixed fuel, there is little effect on the distribution of combustion regimes. In fact, the distribution of regimes, for the cases and ranges analysed, is insensitive to fuelling conditions. Finally, the thermofluidic compression effect was investigated with Reynolds-Averaged Navier-Stokes Simulations. The thermofluidic compression effect is the effect by which the increase in injected mass in an inlet-fuelled scramjet contributes to enhanced combustion by increasing compression and causing earlier ignition. Oxygen enrichment and thermofluidic compression both contribute to increased combustion efficiency and can be used, independently or in combination, to achieve higher scramjet performance. They can potentially be used to increase the operational envelope of a scramjet engine to higher altitude and Mach numbers, or to reduce combustor length, reducing the overall drag.n