Plasma-Assisted Ignition in Scramjets

Abstract

This study assesses the prospect of main-fuel ignition with plasma-generating devices in a supersonic flow. Progress from this study has established baseline conditions for operation, such as the required operational time of a device to initiate a combustion shock train as predicted by computational fluid dynamics computations. Two plasma torches were investigated: a direct current constricted-arc design and an alternating current unconstricted-arc design based on a modified spark plug. Both plasma torches are realistic in size and operate within the same current and voltage constraints, although differing substantially in orifice geometry. To compare the potential of each concept, the flow physics of each part of the igniter/fuel-injector/combustor system was studied. To understand the constraints involved with the ignition process of a hydrocarbon fuel jet, an experimental effort to study gaseous and liquid hydrocarbons was conducted, involving the testing of ethylene and JP-7 fuels with nitrogen and air plasmas. Results from individual igniter studies have shown plasma igniters to produce hot pockets of highly excited gas with peak temperatures up to 5000 K at only 2 kW total input power. In addition, ethylene and JP-7 flames with a significant level of the hydroxyl radical, as determined by planar laser-induced fluorescence, were also produced in a Mach 2 supersonic flow with a total temperature and pressure of 590 K and 5.4 atm. Information from these experiments is being applied to the generation of constraints and the development of a configuration with perceived high ignition potential in full scramjet combustor testing.