Jan Martinez Schramm

An Experimental Investigation of Steady and Unsteady Combustion Phenomena in the HyShot II Combustor

A series of experiments has been carried out in the HEG facility (High Enthalpy Shock Tunnel, Gottingen) to obtain detailed measurements on the HyShotII scramjet configuration under both steady and unsteady combustion conditions. Standard pressure measurements are performed, but a main focus of this campaign is the use of optical and visualization techniques: high-speed visualizations of OH* chemiluminescence together with pulsed-diode laser Schlieren imaging are employed to gain information about the approximate flame location as well as the interaction between the flow features and combustion characteristics. In particular, this allows an unprecedented level of insight into the transient combustioninduced phenomena present in the combustion chamber at high equivalence ratios. Measurements are compared with computational data obtained from the DLR TAU code. I. Introduction

Ground Testing of the HyShot II Scramjet Configuration in HEG

Ground based testing of the HyShot II supersonic combustion ramjet flight configuration was carried out in the High Enthalpy Shock Tunnel Gottingen, HEG, of the German Aerospace Center, DLR. The main focus of these investigations was to obtain surface pressure and surface heat transfer data on the intake and the cowl and body side surfaces of the combustion chamber and the nozzle. Further, high speed flow visualisation of the combustor flow was performed. Two HEG operating conditions which are related to 33km and 27km flight altitude were applied. Computational fluid dynamics (CFD) was applied to support the analysis of the experimental results and for the prediction of the free stream conditions in the HEG test section.

High Enthalpy Cylinder Flow in HEG: A Basis for CFD Validation

Basic aerodynamic configurations are well suited to study fundamental aspects of high enthalpy flows. A test campaign of the flow around a cylinder has been performed in the High Enthalpy Shock Tunnel Gottingen, HEG. Measurements of the density distribution in the shock layer as well as surface pressure and heat flux were taken. The flow in the shock layer is subjected to various relaxation phenomena and therefore the presented measurements provide a useful basis for the validation of physico-chemical models used in CFD codes. This paper presents results of the test campaign and comparisons of measurements to the numerical computations. The sensitivity of the CFD results to different parameters of physico-chemical models is also discussed.

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

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.

Application of Temperature and Pressure Sensitive Paints to DLR Hypersonic Facilities: “lessons learned”

PSP and TSP measurements have been carried out on the following DLR facilities: small test shock tube, High Enthalpy Shock Tunnel (both in Gottingen) and the Hypersonic Wind Tunnel (in Cologne). Based on the results from these measurements, various challenges and problem areas could be identified: the role played by these in hindering a quantitative application of PSP/TSP to the facilities is discussed, and ways and means are presented for either overcoming them or at least ameliorating their influence.

Investigation of Unsteady/Quasi-Steady Scramjet Behavior using High-Speed Visualization Techniques

Experiments are carried out in the HEG (High Enthalpy Shock Tunnel Gottingen) wind-tunnel to obtain detailed measurements on the HyShot II scramjet configuration at equivalence ratios close to the incipient choking point at simulated 28-km altitude flight conditions. Diagnostic techniques include time-resolved pressure measurements and simultaneous high-speed schlieren and OH* chemiluminescence imaging. Similar to previous experiments at higher equivalence ratios, the onset of choking is signaled by the formation of an unsteady shock train that initially propagates up the duct. The shock motion then slows, however, and a quasi-steady topology develops with the shock lodged at a position that depends strongly on the equivalence ratio (the position lying further upstream for higher equivalence ratios). This topology persists until the conclusion of the steady test time. Based on these measurements, a value for the critical choking equivalence ratio, i.e., that at which the shock train first appears, in the range of 0.38-0.39 is determined. High-speed temperature sensitive paint measurements are also carried out on the model intake ramp to provide global information on the boundary-layer transition behavior.

High Speed Flow Visualization at HEG

The High Enthalpy Shock Tunnel Gottingen (HEG) of the German Aerospace Center (DLR) is one of the largest free piston driven shock tunnels capable of simulating high temperature effects in re-entry flows. Full utilization of such a ground based high enthalpy impulse facility, with typical test times in the order of a few milliseconds, requires detailed knowledge of the test section flow, and its starting process. Further, the temporal development of the model flow must be known in order to assure that a steady flow field has developed during the test time window. Therefore, in addition of time resolved model surface property measurements such as pressure and heat transfer, a high speed shadowgraph and schlieren system was implemented at HEG. For the first tests with the new system a cylinder flow was chosen. The large shock stand-off distance of the cylinder flow allows high spatially resolved optical measurements. Due to the sensitivity of the shock stand-off distance to the gas composition, the measurements are also a valuable aid in detecting driver gas contamination in the test section. The results presented in this paper are related to HEG operating condition III (h 0= 13.4 MJ/kg, p 0= 48.3 MPa, Ma=8.7, u ∞=4776 m/s, T ∞=694 K, p ∞=687 Pa, ρ ∞ = 3.255 g/m3).

Shock tunnel and numerical studies of a large inlet-fuelled inward turning axisymmetric scramjet

A large axisymmetric inward turning inlet-fuelled scramjet flowpath has been tested at flight Mach 8 enthalpy in the High Enthalpy Shock Tunnel Gottingen (HEG) of the German Aerospace Center (Martinez Schramm et al (2012), as part of the international SCRAMSPACE program. Experiments have been conducted that explore the fuel-off flow structure and the effects of fuel injection, mixing and combustion over a range of Reynolds numbers (1.3×106 to 3.9×106 m-1) and fuelling rates (φ = 0.0 to 0.98). The test flows have been reconstructed with RANS CFD, employing the SST turbulence model and the Jachimowski (1992) hydrogen/air combustion model. The experimental results indicate a highly axisymmetric fuel-off flow, mild three-dimensional effects due to fuel injection, and a variety of effects of combustion heat release. CFD modeling produces excellent agreement with the experimental data, except at high pressure and high fuelling rates.

An Investigation into Internal and External Force Balance Configurations for Short Duration Wind Tunnels

An investigation of both an internal and an external stress wave force balance that measures forces and moments on a re-entry type configuration was undertaken. Simulations performed using a finite element code suggest superior recovery of the applied forces and moments with an external force balance configuration. This is hypothesized to be due to the increased balance stiffness and decoupling between force components that can be achieved with an external type configuration. The influence of an external force balance as compared to an internal force balance on the model loads was determined to be negligible.

Free Flight Testing of a Scramjet Engine in a Large Scale Shock Tunnel

The free flight force measurement technique is a very attractive tool to determine forces and moments in particular in short duration ground based test facilities. With test times in the order of a few milliseconds, conventional force balances cannot be applied here. The technique has been applied in a number of shock tunnels utilizing models up to approximately 300 mm in length and looking at external aerodynamics. In the present study the technique is applied using a complex 1.5 m long hypersonic integrated supersonic combustion ramjet (scramjet) engine consisting of intake, combustor and thrust nozzle. For scramjet engines the design objective is a combustor with efficient mixing and combustion within the shortest possible length, but still robust enough to operate in various operational conditions. In the framework of the EU co-funded project LAPCAT II, a M=7.4 scramjet powered small scale flight experiment (SSFE) configuration was designed. Since free jet testing of the complete combustion flow path is a mandatory step within the design roadmap of future engines, ground based testing of the SSFE engine was conducted in the High Enthalpy Shock Tunnel Gottingen (HEG) of the German Aerospace Center, DLR. This type of facility allows duplication of flight conditions in excess of M=8. Here tests were performed simulating Mach 7.4 flight conditions in approximately 28 km altitude. The numerically predicted thrust of the engine could be confirmed in HEG utilizing optical tracking of the free flight wind tunnel model. Combining these experimental results with computed aerodynamic data of the complete SSFE showed that for a selected flight condition a positive aero propulsive balance of the complete configuration could be achieved.