Unsteadiness and mixing in thermal plasma jets

Abstract

This study was undertaken to examine the mechanisms which produce the large entrainment measured at the exit of thermal plasma torches. The experiments studied a Metco 7MB plasma torch with a 706 (6.35 mm diameter) anode nozzle and swirled argon gas injection. The vortex structure produced in the shear layer of the plasma jet was visualized using a laser shadowgraph system with a short exposure lime (10⁻⁴ s). A high-speed video system provided information on the structure and unsteadiness of the hot potential core of the plume. Tile shear layer visualizations were compared to previous measurements of acoustical power spectra and indicate coherent vortex structure formation at low gas flowrates. At higher gas flowrates the shear layer rapidly broke down, producing relatively small scale turbulence. The visualizations of the hot potential core were compared to previous measurements of the torch voltage fluctuations caused by arc instabilities. At low flowrates the arc-produced voltage fluctuations were guile low card the phone was very steady. At higher flowrates the voltage fluctuations increased and produced “surging ” and “whipping” in the hot potential core.