Turbulence and entrainment in plasma and heated jets

This thesis aimed at understanding the rapid mixing rates measured in thermal plasma jets in an effort to control the entrainment. A heated air jet was used to study the effect of exit conditions on low density jet structure and entrainment. Decreasing the jet density produced more regular vortex structure formation for laminar exit boundary layers. The structures scaled with the exit momentum thickness; a thicker boundary layer resulted in lower frequency and longer wavelength structures. Boundary layer turbulence disrupted the formation of organized vortex structures in the mixing layer and produced less entrainment for all of the density ratios investigated. The structure of a jet under similar conditions to the jet produced in thermal plasma torches was investigated using a heated helium stream exiting into air. The heated helium jet produced a potential core of approximately three exit diameters with rapid mixing downstream for both laminar and turbulent initial conditions.; The structure of the plasma jet produced by a commercial torch was also studied. At low gas flowrates the arc voltage fluctuations were low, producing a steady plume. The exiting boundary layer appeared laminar and coherent structures were apparent in shadowgraph visualizations. The turbulence level appeared to increase at higher gas flowrates as the shear layer rapidly broke down into small scale turbulence. However, at higher flowrates the voltage fluctuations increased due to arc instabilities, producing "whipping" and "surging" in the plume. Several schemes were investigated in an effort to produce a steady arc and therefore a steady plume. These schemes focussed on the principle of allowing the arc to attach to the anode without breaking through a cold anode boundary layer. The first anode modification incorporated boundary layer bleed holes to remove the cold boundary layer. This design produced low voltage fluctuation levels but also insufficient cooling and relatively rapid anode wear at the attachment. A second design incorporated tungsten prongs which protruded through the boundary layer. The arc attached on the tungsten prongs but high voltage fluctuations were produced when the arc moved from one prong to another.