CFD Analysis of Unsteady Combustion Phenomena in the HyShot-II Scramjet Configuration

Abstract

Ground based testing of the generic HyShot II scramjet configuration was carried out in the High Enthalpy Shock Tunnel Gottingen, HEG, of the German Aerospace Center, DLR, and computational fluid dynamics (CFD) was applied to support the analysis of the experimental results. Previous numerical analysis was mainly focused on the steady-state design conditions at an equivalence ratio of approximately 0.3. Preliminary computations at large equivalence ratios showed that a change in flow topology corresponding to a transition from supersonic to subsonic combustion occurs. The formation of a subsonic combustion zone is seen to lead to the upstream propagation of an unsteady shock train. This phenomenon is also observed in experimental investigations. In the framework of scramjet engine design it is of particular interest to identify and to correctly predict the limiting operating conditions, e.g., engine unstart. Therfore, in conjunction with the test campaign performed in HEG, a parametric CFD study was performed utilizing the DLR TAU code to investigate the onset of unsteady combustor flow. Unsteady Reynolds averaged NavierStokes (URANS) computations were performed for off-design equivalence ratios above 0.5. I. Introduction mportant design issues for hypersonic propulsion systems in general, are the lack of ground based facilities capable of testing a full-sized engine at cruise flight conditions and the absence of general scaling laws for the extrapolation of wind tunnel data to large flight configurations. Therefore, there is a strong need for the development and validation of CFD tools to support the design process of scramjet-powered vehicles. Specific challenges for the applied CFD solvers include the accurate prediction of skin friction and heat transfer caused by turbulent boundary layers, the mixing and combustion of fuel in compressible turbulent shear layers, and the modeling of chemical nonequilibrium effects, which can be of significant importance for the prediction of engine performance. Due to the uncertainties associated with the modeling of these processes that are still associated with current CFD solvers and the limitations of experimental methods to comprehensively characterize the flow properties, a close interaction of CFD and experimental investigations is necessary to further improve the understanding of flow phenomena inside scramjet engines. I