Power VCSEL driven Schlieren visualization for cascaded injection in supersonic flow

Extended abstract Results are presented of a study on utilising Vertical-Cavity-Surface-Emitting-Laser (VCSEL) driven Schlieren visualization of cascaded injection in a supersonic flow. The background of the study is fuel injection within a supersonic combustion ramjet (scramjet). The scramjet is a ramjet airbreathing jet engine in which combustion takes place in a supersonic air flow. Scramjets promise significant economic advantages over rocket-based flight travel. However, at hypersonic flight speeds the compressibility effects delay shear layer mixing. In order to maintain for scramjets, the fuel-air mixture required for high combustion efficiency, the combustor becomes relatively long 1. In the present case of cascaded injection, the downstream injector benefits from the shielding effect induced by the smaller upstream injector. This provides a reduction of the momentum in the flow, allowing better penetration of fuel in the air stream over a shorter length. Validation of theoretical and computational results for the flow in scramjets requires a high spatial and temporal resolution of the flow field. In the present study Schlieren visualisation is employed to investigate the flow field. In our previous studies pulsed LED-driven Schlieren visualization was employed. However, for a Mach 1.6 free stream, the limited pulse width of 130 ns of LEDs creates a motion blur of roughly a pixel per second. Therefore, LED-based Schlieren visualisation is not adequate for Schlieren imaging of flows at higher Mach numbers. Furthermore, using an appropriate knife-edge filter, the power of 6 W/mm 2 of the LED employed, is only just sufficient to obtain acceptable Schlieren images. Fig. 1 Left: Schematic of Schlieren setup. Right: Example of Schlieren image obtained for dual-jet injection in a Mach = 1.6 cross-flow. Diameter orifices 1 mm (upstream) and 2 mm (downstream). Visualised are the tandem jets, 20 mm apart, momentum ratio J = 1.37, each featuring a Mach barrel at their exit; the two bow shocks induced by the jets; the boundary layer along the walls and their interaction with the shocks. Also visible are Mach waves originating from small slope discontinuities of the walls of the wind tunnel Power VCSELs provide the high-pulse modulation speeds necessary for high temporal resolution Schlieren imaging of high-speed flow fields. VCSELs consist of very small, densely packed, laser diodes, ordered on a chip in a 2D array, emitting light perpendicular to the chip's surface. Each VCSEL is around 25 micrometres in size and although its Continuous Wave (CW) power is limited, in our case to 10 mW, pulsing the laser increases the power up to a factor of 10. Furthermore, by integrating 600 lasers per square millimetre, the