Flow visualizationswith background-oriented schlieren in a transpiration-cooled model scramjet combustor

The Institute of Space Propulsion of the German Aerospace Center (DLR) has developed and set up a research test bench to investigate the applicability of transpiration cooling systems to scram- jets and to examine the resulting phenomena in the supersonic flow. It consists of a chemical hy- drogen/oxygen air vitiator with attached Scramjet model combustion chamber with an optional half-wedge-shaped shock generator. The used porous media consists of high-temperature resistant stainless steel and Inconel with different porosities. Gaseous nitrogen and gaseous hydrogen were used as coolant with different pre-pressures. Optical access allows the use of a schlieren system and background-oriented schlieren (BOS) for flow investigations. Phenomena such as hot spots on the porous wall, inefficient cooling, and shock-boundary-layer interaction (SBLI) were observed in the experiments. With the BOS system, thickening of the boundary layer/boundary layer separation was demonstrated. Furthermore, the development of shock trains forced by the introduction of a secondary coolant flow can be illustrated, leading to an increased physical blockage up to the chok- ing case. This publication summarizes the SBLI results gained by application of BOS setup and compares them with results obtained by the help of a classic schlieren setup. BOS emerged in the experiments to be less affected by changes in the refractive index of the quartz glass windows due to strong heating compared to the classic schlieren method. Challenges in optimizing the BOS systems’ performance and subsequent changes in the test setup are discussed.