Experimental Analysis of Supercritical Ethylene Injection Into Supersonic Cross-Flow Using Optical Techniques

A supercritical ethylene fuel injection system was tested in the T4 free-piston shock tunnel facility at the University of Queensland. Ethylene fuel was injected at supercritical conditions into a rectangular duct with Mach 3 cross-flow under flight-like scramjet combustor conditions. Schlieren and Nitric-Oxide Planar-Laser-Induced-Fluorescence (NO PLIF) optical techniques were used to observe the oscillatory vortex shedding and rapid fluid phase change immediately downstream of injection. Analysis of the 730 kHz Schlieren using Spectral Proper Orthogonal Decomposition (SPOD) showed dominant vortex shedding frequencies to be coupled to the pressure and temperature of the supercritical plenum conditions. NO PLIF images show rapid phase-change immediately downstream of the injector, where the supercritical ethylene flashes into exceedingly small liquid droplets sheathed in a gaseous envelope. Rayleigh scattering from these liquid droplets allow for the exclusive capture of the liquid core and show it dissipating downstream within 10 injector diameters.