Experimental Investigation of a 3D Thermal Compression Scramjet Using Advanced Optical Techniques

Abstract

Scramjet engines are limited in performance due to engine starting problems at low hypersonic speeds. For an accelerating hypersonic vehicle, intake contraction ratios are typically fixed to the low speed starting requirements. This reduces engine performance at higher Mach numbers, where higher contraction ratios are required to efficiently combust flow within a practically-sized combustor.The use of a combustion process called "Thermal Compression" was suggested in the 1960s that could improve the performance of low intake contraction scramjet engines. In such an engine, the flow field is non-uniform within the combustor, with regions of locally high pressure and temperature even at low net intake contraction ratio. The flow properties in these regions are suitable for combustion of air-fuel mixtures, inducing autoignition. As heat is released, increased temperature and pressure are transmitted to the surrounding flow through pressure waves. Conditions in the surrounding flow become suitable for combustion, and this process continues until combustion completes throughout the entire flow field.This work presents results from experimental and numerical analysis of a three-dimensional, thermal compression scramjet engine at two high Mach number flight conditions. Experiments were conducted in the T4 Shock Tunnel at The University of Queensland. The tunnel test gas simulated flow that had been processed by an engine forebody at either 8.0• or 8.8• angle of attack, in an equivalent flight condition of Mach 10, 50 kPa dynamic pressure.An engine test model was developed to examine the effect of thermal compression on combustion-induced pressure rise at these inflow conditions. Non-uniform combustor flow was generated using a three-dimensional intake with a ramp angle that varied in the spanwise direction. This produced a low compression side and a high compression side throughout the scramjet engine. Fuel injection plenums were split such that different gases could be injected in either the low compression or high compression side. Combustion was induced by injecting hydrogen fuel, or suppressed by injecting helium.i Pressure measurements were taken along the streamwise length of the engine, in three spanwise locations. Optical techniques, including emission and laser-induced fluorescence (LIF) methods, were used to visualise the distribution of OH radicals throughout the combustor. These experimental results were then compared to fully-combusting, RANS CFD simulations.The influence of thermal compression was determined by comparing combustion-induced pressure rise in the fully-fuelled case to the sum of both cases where combustion was suppressed on each side of the engine. It was found that for the forebody angle of 8.8• case, thermal compression increased pressure rise by 14.4 ± 6.0 % and 6.7 ± 6.2 % for fuel-air equivalence ratios of φ = 0.8 and φ = 1.0 respectively. Emission from OH radicals, which is qualitatively indicative of combustion, also increased by 19 -54 % in the rear of the combustor. Numerica...

Temperature measurements in hypervelocity flows using thermally-assisted laser-induced fluorescence

“…Section 4.1 LIF system set-up in the T4 shock tunnel 40 to allow optical access to the scramjet model for LIF purposes. A detailed description of these modifications is given in the PhD thesis by Vanyai (2017).…”

“…Thermal compression is a method that takes advantage of the scramjet engine flow non-uniformities to encourage combustion throughout the entire flow. The present research was done in parallel with another PhD project (Vanyai, 2017) which focused on the study of the thermal compression engine. Preliminary numerical work by Bricalli et al (2012) showed promising results for a 3D "Thermal Compression" inlet, which encourages strong flow non-uniformity within the combustor.…”

“…Each plenum chamber, which had a volume of approximately 20 cm 3 , supplied fuel to two porthole injectors. This design of the plenum chambers was to allow separate fuels and fuelling rates to be used on the low and high compression sides of the injector plate, for example, in the study by Vanyai (2017). For the present study, all plenums provided with the same hydrogen fuel and same fuel-supply pressure.…”

Investigation of supersonic turbulent combustion in a Mach 10 scramjet engine

“…This was confirmed to match the doublet with a test scan where the laser is passed through a flame, and a photomultiplier is used to capture the fluorescence. This doublet was chosen as the populations in these states do not change considerably over the temperature range expected in the scramjet combustor [123].…”