Investigation of thermal loads onto a cooled strut injector inside a scramjet combustion chamber

Abstract

For future aviation or space transportation systems, scramjets could provide a complement or even an alternative to conventional propulsion systems. However, due to the high-enthalpy flow environment, scramjet development still implies considerable technical challenges. One of the most relevant issues is the need for an efficient fuel injection and mixing system. It has to guarantee a stable and reliable combustion process, as the flow residence time inside the engine is only in the order of several milliseconds. Strut-based injection systems have proven to be a suitable choice due to their ability to provide fuel directly into the center of the flow. In contrast to wall-based injection systems, however, struts are exposed to the complete aerodynamic heat loads of the flow, which necessitates active cooling to avoid structural damages. As experimental facilities are hardly able to reproduce flight conditions over a long period of time, a numerical approach is inevitable to assess the heat loads onto a strut and to evaluate the internal cooling mechanism. Within the present thesis, a numerical solver for the conjugate simulation of heat transfer in supersonic flows was developed and integrated into the OpenFOAM software package. A thorough validation for a variety of data from both literature and in-house studies was conducted. The accurate prediction of different phenomena relevant for supersonic flows could be verified. The solver was then applied to the evaluation of an internally cooled strut injector. In a first step, the injector was investigated at moderate flow temperatures. Experimental data for different flow temperatures and coolants was obtained using infrared thermography of the injector surface. A comparison to numerical simulations led to the identification of characteristic well and poorly cooled zones along the injector surface, which could be explained by features of either the external or the internal flow field. Finally, the lobed strut injector was studied numerically at hot gas conditions representative for the ITLR model combustor, where no experimental data of the surface is available. Besides the leading edge, a second hot zone was identified towards the trailing edge of the strut, which was attributed to the impact of the reflected leading edge shock wave onto the surface. Activation of internal air cooling was found to lower the general temperature level, but to have only a small effect on the leading edge. Instead, heat conduction towards the cooled combustor side walls provided a considerable part of the cooling in this area. Switching to hydrogen as coolant led to a further reduction of the injector temperature at a considerably lower coolant mass flux, without changing the overall characteristics of the cooled injector. Changing to more realistic, hotter combustor side walls for a hydrogen-cooled strut caused a generally higher injector surface temperature. While the hottest injector regions were found to be near the side walls, the leading edge could still be partially cooled by the internal hydrogen flow.