MIXING IN PLASMA AND LOW-DENSITY JETS

This study was undertaken to examine the mechanisms which produce the large entrainment measured near the exit of thermal plasma torches. A research facility was constructed to examine low density jet behavior under similar dimensionless conditions as those produced by thermal pl asma spray torches; the Reynolds number based on jet diameter and aver age properties was 1000, and the ratio of jet to ambient density was 0 .07. This very low density jet produced organized vortex structures wh ich were partially responsible for the rapid entrainment of external a ir. The formation of these organized structures could be disrupted by introducing turbulence, but the rapid entrainment process was not sign ificantly affected. The structure of the jet produced by a commercial plasma torch was examined and compared to the low density research jet . At low gas flow rates the plasma jet also displayed the formation of coherent vortex structures, the passage frequency of which compared f avorably with that measured in the low density research jet. At higher gas flow rates the shear layer of the plasma jet rapidly broke down p roducing relatively small scale turbulence. Visualizations of the hot plasma core were compared against measurements of the torch voltage fl uctuations caused by arc instabilities. At low flow rates the arc volt age fluctuations were quite low and the plume was very steady. At high er flow rates the arc voltage fluctuations increased and produced ''su rging'' and ''whipping'' in the hot potential core. It is believed tha t this low frequency unsteadiness is partially responsible for the rap id entrainment measured in plasma torches.