Jet Injection into Supersonic Crossflow: Flowfield and Mixing studies

Abstract

Supersonic air-breathing engines or scramjets are perhaps the most important technological hurdles towards the development of hypersonic transportation vehicles for both easy access to space and for missile related applications. In these engines, the air entering the combustor must remain supersonic, which significantly brings down the time available within the engine for fuel-air mixing and combustion. There are many possible strategies for fuel injection in such engines; the transverse jet injection into supersonic crossflow being one. The principle parameter in jet in crossflow studies is the momentum flux ratio (J), which is defined as the momentum flux ratio between the jet and the cross-flow. Compared to the subsonic crossflow case, the supersonic crossflow has additional complexities due to the presence of different shock structures in addition to the many different vortical structures. The aim of the present work is to experimentally study the flowfield and mixing associated with sonic jet injection into a supersonic crossflow. In addition to detailed flowfield and mixing studies of the basic steady jet configuration, enhancement of mixing using a newly developed passively modulated (injection) jet has also been studied. In the latter case, a large range of jet modulation frequencies ( f ) characterized by the ratio of the modulation frequency to the natural shear layer frequency ( f / fsl) have been investigated ( f / fsl = 0.12 to 1.31). In all cases, the flowfield is investigated using Particle Image Velocimetry (PIV) to characterize the different flow features and the penetration of the jet into the crossflow. Mixing studies have also been carried out using acetone Planar Laser Induced Fluorescence (PLIF), with the injected jet containing acetone vapours, which is then tracked after injection into the crossflow to quantify mixing. In summary, the modulated jet shows significant changes in jet penetration, unsteady motions of the jet, and the mixing of the jet into the crossflow. Among the large number of cases studied, the penetration and the mixing are found to be higher at modulation frequencies corresponding to the sub-harmonics of the shear layer frequency. The maximum penetration is seen close to f / fsl = 0.26, but the maximum mixedness of the injected jet is seen to occur at f / fsl = 0.46, where the penetration is higher than the base case, but not the highest