Plasma Torch Atomizer-Igniter for Supersonic Combustion of Liquid Hydrocarbon Fuels

Abstract

To realize supersonic combustion of hydrocarbons, an effective atomizer-igniter combination with the capabilities of fuel preheating, atomization, penetration, mixing, ignition and flameholding is desired. An original design concept incorporating these capabilities was built and tested at Virginia Tech, and was found to provide good penetration, effective atomization, and robust ignition and flameholding. Quiescent testing with kerosene and JP-7 provided initial performance data. The atomizer-injector design was then modified for insertion into a supersonic wind tunnel, and tested with kerosene in an unheated Mach 2.4 flow with typical freestream conditions of To = 280 K and Po = 360 kPa. Water injection was utilized in both cases for comparison and to analyze atomization behavior. In the quiescent environment, the regeneratively cooled plasma torch igniter was found to significantly increase electrode life while heating, atomizing, and igniting the liquid fuel. Jet breakup length was measured and characterized, and mean droplet size was estimated using an existing correlation. Several qualitative observations regarding quiescent combustion were made, including torch power effects and the process of flame formation. In the supersonic environment, the effect of fuel injection direction was analyzed. Best results were obtained when fuel was injected with a velocity component opposite to the direction of main tunnel flow. Repeatable ignition occurred in the supersonic boundary layer at the fuel stagnation location near the plasma torch plume. Direct, filtered, shadowgraph, and schlieren photographs, temperature measurements, and visible emission spectroscopy provided evidence of combustion and the details of the flame structure. The new atomizer-igniter design provided robust and reliable ignition and flameholding of liquid hydrocarbon fuels in an unheated supersonic flow at M=2.4, with no ramp, step, or other physical penetration into the flowpath.