Colloquia

Summary

In supersonic aviation, it is possible to use the ‘ram effect’ to compress the flow in air-breathing engines instead of a compressor. At high Mach numbers (M>5), the flow cannot be decelerated to a subsonic speed; the fuel will be injected and ignited into a supersonic crossflow. This type of engine is called a ‘scramjet’. To enhance the combustion efficiency, rapid penetration and mixing of the fuel into the supersonic crossflow is required, which can be challenging due to the short residence time. Research has shown that tandem jet injection, where two jets are aligned in streamwise direction, leads to an increase in mixing and penetration performance.

Current experimental research on tandem jet injection has been conducted only using air as injection gas into an air crossflow. Numerical research was done on hydrogen injection, but the influence of the jet properties on the jet plume behaviour was not studied. For single jet injection, most studies agree that the injectant affects the eddy structures in the jet plume, but there is no consensus on whether it influences the penetration depth. The effect of the injectant on the jet behaviour is of great importance to understand whether experiments using inert gases are an adequate representation of the injection of fuels like hydrogen or ethylene. Therefore, the effect of a different injection fluid on the jet penetration and large-scale structures is experimentally investigated in this study. Using schlieren imaging, helium, argon and air jets into a supersonic air crossflow are compared, for a range of jet-to-crossflow momentum flux ratios and orifice distances.

A dimensional analysis was done to obtain better insight into the possible ways the injection fluid choice may influence the jet plume behaviour. It was found that, in agreement with literature, high compressibility of the injectant leads to more coherent large-scale structures in the plume. With a semi-automated algorithm, a set of images was analysed for each experiment to locate the upper shear layer of the plume. It was found that the helium jet penetrates deeper into the crossflow compared to the argon and air jet with the same momentum flux ratio. This is most likely due to the velocity of the sonic jet, which is much higher in case of helium. The air and argon jets behave more similarly, but a slightly deeper jet penetration and higher Mach disk was observed for air.