Instabilities in plasma spray jets can result in coatings with inconsistent properties. The arc root fluctuation and shear layer instability due to strong gradients are of foremost concern. The shear layer instabilities result from shear between the high velocity, low density hot core gas, the intermediate density and velocity boundary layer, and the high density quiescent environment. A cold-flow facility with density gradients similar to a plasma torch has been used for implementation of traditional fluid dynamics measurements such as hot-wire anemometry. Methods to control these instabilities are developed and tested using both the plasma torch and the cold flow facility. Through nozzle design modifications the instabilities resulting from arc root fluctuations and high density gradients have been reduced. The effectiveness of the control on the plasma jet is determined using in-flight particle characterization along with high speed imaging and photodiode measurements of the jet.

Instabilities of plasma spray jets have been a source of inconsistencies in coating properties. These instabilities can be minimized through the use of central injection torches or torches with fixed anode attachment. However, any low density ( < ~0.7) jet is globally unstable to small disturbances. Globally unstable jets are characterized by a short potential core, rapid spreading, and high entrainment, all of which are present in a plasma jet. Plasma jets have ratios of jet density to density of the surrounding gas on the order of 0.01, as well as rather low Reynolds numbers and thick boundary layers. In the present work, the instabilities are investigated through analysis of the disturbance growth in the shear layer between the plasma and the cold surrounding gas. These investigations are using two types of experiments, one consisting of a SG 100 spray torch with several optical diagnostic methods being applied to the shear layer analysis. The other experiment simulates the plasma jet at low temperatures by using a helium core jet exhausting into a sulfur hexafluoride (SF6) environment. The simulated plasma jet (SPJ) has a density ratio of 0.03. The simulated plasma jet (SPJ) allows controlled variation of the boundary layer through different fluid dynamic arrangements. It further allows use of diagnostics such as hot wire anemometry and PIV to clearly characterize the shear layer. Some of the characteristics of the shear layer are presented and comparisons between the plasma jet and the simulated plasma jet, and initial results on controlling the jet instability, are discussed. Abstract only; no full-text paper available.