Post-Test Analysis of the LAPCAT-II Subscale Scramjet

Abstract

A subscale flight experiment configuration (SSFE) of a Mach 8 scramjet vehicle was designed within the framework of the EU-FP7-project LAPCAT II. Its purpose was to demonstrate - by wind tunnel experiments - the ability of the proposed scramjet engine to produce adequate thrust for hypersonic level flight of a complete subscale vehicle. The design of a small engine is particularly difficult. This is due to the unfavorable ratio of wetted surface to volume and the relatively long combustor needed for complete fuel consumption. To compensate for these adverse effects, the SSFE scramjet operates near an equivalence ratio of one and fuel mixing was carefully optimized by a two stage multi-strut injection concept. The thrust measurements for the 1.5 m long vehicle were carried out in DLR’s High Enthalpy Shock Tunnel Göttingen for a flight condition of Mach 7.4 at 27 km altitude. The experiments confirmed precedent CFD predictions of the total thrust and demonstrated the operability of the complete subscale concept. Yet, significant discrepancies between the CFD analyses, which were performed to design the vehicle, and subsequent detailed measurements of the pressure distribution in the combustor were observed und could not be resolved. This paper focuses on a further analysis of these residual discrepancies. The influence of different modelling parameters such as boundary layer transition locations and the application of different turbulence models are discussed. This is supported by recently available measurements of the intake surface heat flux distribution by temperature sensitive paint. These results allow for the first time to apply realistic assumptions on the laminar to turbulent boundary layer transition to numerical simulations. CFD predictions of the detailed flow properties inside the combustion chamber are assessed based on available high resolution pressure measurements. Main characteristics of the combustion process and the flow structure inside the combustor are postulated based on a combination of CFD results and available experimental data. It was found that the CFD predictions of the combustor pressure distribution are sensitive to the modelling of the intake boundary layer. This modelling influences cross-flow structures on the intake which are able to trigger different combustion modes in the upstream region of the combustor. The effect on total vehicle performance and the pressure distribution in the downstream combustor and thrust nozzles remains limited. Yet, a significant impact on the structure and magnitude of the surface pressure distribution was observed. I.e., the large combustor peak pressures, which occur in the experiment, can be explained by the occurrence of a strong shock train in the vicinity of the combustor wall.