UNSTEADINESS AND MIXING IN THERMAL PLASMA JETS

This study was undertaken io examine the mechanisms which produce the large entertainment measured at the exit of thermal plasma torches. Th e experiments studied a Metco 7MB plasma torch with a 706 (6.35 mm dia meter) anode nozzle and swirled argon gas injection. The vortex struct ure produced in the shear layer of the plasma jet was visualized using a laser shadowgraph system with a short exposure time (10(-4) s). A h igh-speed video system provided information on the structure and unste adiness of the hot potential core of the plume. The shear layer visual izations were compared to previous measurements of acoustical power sp ectra and indicate coherent vortex structure formation at low gas flow rates. At higher gas flowrates the shear layer rapidly broke down, pro ducing relatively small scale turbulence. The visualizations of the ho t potential core were compared to previous measurements of the torch v oltage fluctuations caused by are instabilities. At low flowrates the arc-produced voltage fluctuations were quite low and the plume was ver y steady. At higher flowrates the voltage fluctuations increased and p roduced ''surging'' and ''whipping'' in the hot potential core.