An Investigation of Controlled Oscillations in a Plasma Torch for Combustion Enhancement

Abstract

An oscillating plasma torch was investigated as a device to produce an oscillating shock and resulting dynamic input to the supersonic combustion process. Several experiments were performed and the results were analyzed. The aim of the research was to thoroughly investigate the oscillation behavior of the plasma torch with the plan of controlling the oscillation at chosen frequencies. A modulating power system for dynamic control of the plasma torch oscillation was designed and tested in quiescent conditions (no flow), Mach 2.4 cold supersonic flow, and Mach 2 heated supersonic flow conditions. The oscillating plasma torch used nitrogen feedstock and was operated over a frequency range of 2Hz-4kHz. A dynamic torch model using the hybrid Mayr-Cassie electric arc model was developed to predict the plasma torch electric arc response at appropriate frequencies for interaction with supersonic combustion. In quiescent conditions, the dynamic response of the plasma torch power system and plasma jet were characterized using signal processing techniques and high speed video imaging. High speed Schlieren images were used to determine the behavior of the oscillating plasma jet in Mach 2.4 cross flow and its influence on the induced shock structure. The nitrogen-and air-fed torches were integrated with a flush-walled 4-hole aerodynamic ramp injector using hydrogen and hydrocarbon fuels, and was tested at the University of Virginia Aerospace Research Lab (ARL) in heated Mach 2 supersonic flow. Unsteady pressure variations from the oscillating shock produced by the plasma torch were recorded and measurements of the static pressure of the combustion produced by the steady and oscillating plasma torch were obtained. The oscillating torch system performed well over a range of different flow conditions. It will enable active control input to the combustion process. The controllable unsteady blockage can provide a type of shock interaction that has been shown to increase turbulence and mixing augmentation.