CFD Analysis of Unstart Characteristics of the HyShot-II Scramjet Configuration in the HEG Shock Tunnel

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. In the present article, the extension of previous numerical analyses which focused on the HyShot II design condition (equivalence ratio of approximately 0.3) to unsteady flow situations in the combustor is discussed. With increasing fuel equivalence ratio, a transition from steady to unsteady flow was observed experimentally. 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 Navier-Stokes (URANS) computations were performed for off-design equivalence ratios above 0.6. I. Introduction ue to the complexity of the flows in air-breathing propulsion systems of future hypersonic flight vehicles and inherent limitations of related ground-based and flight experiments, the application of CFD is mandatory to gain further insight into internal and external flow properties of such vehicles. Detailed flow analyses as well as load and performance predictions obtained by CFD analysis are required to support the design process. However, CFD predictions of compressible turbulent and reacting flows as present in scramjets require the application of complex physical and chemical models, and involve uncertainties which must be quantified.. Therefore, a strong link between CFD, ground based and flight experiments is needed to improve the understanding of the relevant flow physics and to further validate and improve the applied CFD tools. The objective of the work presented in the present article is to apply the DLR TAU code for the numerical analysis of experimental investigations of the HyShot II scramjet configuration which were performed in the free piston driven shock tunnel HEG of the German Aerospace Center, DLR. The experimental data was used as a benchmark to assess the accuracy and reliability of different CFD modeling strategies for internal scramjet flows. In turn, complementary CFD analysis has shown to provide valuable additional information for better interpretation of the experiments. Following previous detailed analysis 3,4 of the engine performance at the design conditions for a flight altitude of 27 km and at an equivalence ratio of approximately 0.3, the results which are presented in the present article focus on the prediction of the unstart characteristics and the operation limits of the HyShot II combustor at large (off-design) equivalence ratios.