Plasma-assisted combustion in a supersonic flow

In this study, a nanosecond pulsed plasma discharge is used to ignite jet flames (hydrogen and ethylene) in supersonic crossflows. The nonequilibrium plasma is produced by repetitive pulses of 15 kV peak voltage, 20 ns pulse width and 50 kHz repetition rate. Sonic or subsonic fuel jets are injected into an air or a pure oxygen supersonic free stream flow of Mach numbers Ma 1.7 to Ma 3.0. The flow pattern and shockwaves, induced by the fuel jets and flow disturbances originating from the surface geometric alterations of the test model are characterized by Schlieren imaging. Planar laser induced fluorescence and emission spectroscopy are employed for imaging the distribution of OH radicals depicting fuel/oxidizer reaction regions.;Two geometric configurations of the test model are utilized with the application of the pulsed plasma, which are a cavity model and a flat wall model. The cavity provides a recirculation region where cavity flames are ignited and sustained. The cavity flame is found to be enhanced by the application of the pulsed plasma in the cavity. An investigation of the time evolution of the cavity flame reveals that the flame enhancement is primarily caused by the reduction of ignition delay time by the plasma. In the flat wall model experiment, the fuel injection nozzles and electrodes are mounted flush with the surface of a flat wall, oriented to be parallel to the flow to minimize stagnation pressure losses associated with generated shockwaves. A configuration combining an upstream subsonic oblique jet and a downstream sonic transverse jet is shown to provide an adequate flow condition for jet flame ignition. The OH fluorescence images of the region in the vicinity of the discharge confirms jet flame ignition by the plasma. Similar trends are observed in both of hydrogen and ethylene fuel injection experiments with the two test models.;The experimental results with the hydrogen fuel jets are validated using a numerical approach. The pulsed plasma is modeled as a radical source providing radicals to a flammable gas mixture periodically. The reactions following the radical production are simulated by a MATLAB based code, Cantera. The reduction of the ignition delay and the jet flame ignition by the plasma on the flat wall are successfully validated by the method. In addition, the effects of the plasma operation conditions (e.g., plasma power and frequency) are investigated. The results suggest that higher plasma power further reduces the ignition delay time and the radical production by the pulsed plasma of a fixed power varies inversely or proportionally with the plasma frequency depending on the initial temperature of gas mixture.