Experimental investigation of mixing and ignition of transverse jets in supersonic crossflows

Near-field mixing and ignition characteristics of sonic transverse jets injected into supersonic crossflows were studied experimentally. The use of an impulse facility made it feasible to achieve high temperature and high velocity conditions relevant to a realistic supersonic combustor environment. The application of supersonic flow visualization at ultra-fast-framing rates enabled a detailed study of the temporal evolution of jets.; In the first part of the dissertation, hydrogen and ethylene fuel jets were studied because of their relevance to supersonic combustors. Ethylene transverse injection demonstrated higher penetration depth and larger jet shear layer growth as compared to the hydrogen case for the same momentum flux ratio. Previously the momentum flux ratio was suggested to be the main controlling parameter of the jet penetration; the results here demonstrated the existence of a “tilting-stretching-tearing” mechanism that alters the growth rate and the mixing properties of the jet shear layer.; In the second part, we investigated the stability of the jet shear layer at various speed ratios and density ratios via schlieren. The high shear stresses induced by the large velocity difference across the jet shear layer had a large effect on the structure of the layer. For the unstable case, we noticed: (1) loss of Kelvin-Helmholtz structures; (2) increased growth rates with decreasing values of jet-to-free-stream velocity ratio; (3) large intrusions of crossflow in between the eddies, and (4) additional shock waves and distortion of the bow shock around the large eddies. Stable layers showed well-defined Kelvin-Helmholtz rollers. The results plotted in a density-velocity ratio diagram demonstrated two separate regions of stable and unstable jet shear layers with a separation line at a critical velocity ratio.; The last part of the thesis studied the ignition capability of hydrogen and ethylene jets. Planar Laser-Induced Fluorescence (PLIF) of OH demonstrated self-ignition in the near-field of both fuels. While OH-fluorescence in the hydrogen case was only detected along the jet shear layer periphery in a continuous and a very thin filament, OH-radicals in the ethylene case could be detected in a wide region distributed across the jet. These results support the tearing mechanism suggested to enhance the near-field mixing properties of the ethylene jet.