Numerical and experimental investigation of hypersonic streamwise vortices and their effect on mixing

Abstract

Scramjet propulsion is a promising technology that could lead to substantial improvements in cost and flexibility for access to space for satellite placement into Low Earth Orbit. However, scramjet development is hindered by different technological issues. Amongst these, fast and efficient fuel-air mixing, and heat management, are key areas. By increasing mixing rate, the combustor can be shortened while keeping a high combustion efficiency. This reduces drag and heat losses, highly improving the feasibility and viability of scramjet engine designs. Several techniques have been proposed to increase mixing rate in scramjets, such as hypermixers and strut injectors. However, these techniques improve mixing at the expense of increased losses and local heat loads. For this reason, the use of naturally occurring vortices in scramjet flowfields for mixing enhancement came into consideration. Most scramjet geometries inherently generate streamwise vortices. Therefore, there are no increased losses when using these vortices for mixing enhancement. However, these vortices are weaker than those generated by hypermixers or struts. Hence, the ability of these vortices to effectively enhance mixing rate has to be evaluated. This work addresses the study of the interaction between streamwise vortices in scramjet flowfields and fuel injected through an inclined porthole injector to establish optimum arrangements for scramjet performance. The analysis of this interaction is performed numerically and experimentally. Numerical RANS simulations are used as the principal tool to analyse and describe the vortex-injection interaction. In addition, experimental data was gathered to assess the validity of the numerical approach. Scramjet flows are highly complex. To study the effect of the vortex in isolation, a simplified, canonical geometry, consisting of a flat plate with a normal fin is used. The swept shock generated by the fin interacts with the flat plate boundary layer, inducing the formation of a vortex. The features of this vortex are equivalent to those of vortices present in real scramjet flowfields. Moreover, this geometry allows to control the vortex intensity by modifying the fin compression angle. The flat plate incorporates a porthole injector, from which Hydrogen fuel is injected into the vortex. The interaction between the injected fuel and the streamwise vortex was studied focusing on its effect on mixing. Moreover, its effects on maximum wall heat flux, and fuel combustion were also analysed. To evaluate the effect of different vortex and injection parameters, three different vortex intensities, three injector locations, and two injection pressures are combined in different test cases. The effect of each of these parameters on mixing, heat flux, and combustion is reported. To produce data relevant to scramjet applications, the flow is representative of the flow within the M12 REST engine during ground testing of a Mach 12 flight conditions along a 50kPa constant dynamic pressure trajectory. The presence of the vortex substantially increased mixing rate. The location of the injector within the vortex plays an important role in mixing rate improvement. Moreover, the sensitivity of mixing rate to injector location increases with increasing vortex intensity. The effect on mixing in the injector vicinity, and further downstream, are analysed separately. Mixing in the injector vicinity is negatively affected by the vortex. Nonetheless, the effect of the vortex further downstream highly increases mixing, inducing a notable increase in the global mixing efficiency. The best injector location for mixing enhancement is identified as the central point between the fin shock and the separation line. The effect of the interaction on heat flux is also investigated. The region just upstream of the injector, and the corner formed by the flat plate and fin are the regions most severely affected. Injector location plays a major role in heat flux peak value. Moreover, the effect of the fin angle on the heat flux near the injector varies substantially depending the location of the injector. The influence of the vortex in combustion is studied. The flow conditions investigated are representative of the flow in two different engine locations: inlet and combustor. Combustion in the inlet is undesirable, whereas fast and robust combustion in the combustor is beneficial. For inlet conditions, the presence of the vortex did not produce any significant combustion. In contrast, the vortex-injection interaction affects the ability of the fuel to ignite, and also the production of species precursor to combustion. The effect of injector location, fin angle, and injection pressure in facilitating the combustion of the fuel is described. To evaluate the validity of the numerical results, experimental tests were performed in the T4 reflected shock tunnel. Wall heat flux and Schlieren images were acquired. The experiments are reproduced numerically, and the experimental data is used as a benchmark to assess the accuracy of the numerical investigation. The outcomes of this investigation are summarized and compiled to provide a series of recommendations and considerations for scramjet fuelling design.