Fluid Dynamic Characterization and Control of Turbulent Plasma Jets

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The objective of this research is to derive a detailed physical description of the crucial parameters in turbulent plasma jets and to devise means for controlling the turbulence. Modern measurement techniques developed for the relatively benign environment of combustion are exploited to unravel the complicated flowfield of the plasma jet. The strategy is to use laser scattering techniques to perform detailed measurements of three-dimensional velocity distributions with high time resolution in jets that simulate the plasma jet situation with respect to density ratios between jet fluid and environment and with respect to flow forcing due to arc attachment dynamics. The appropriate density field in the simulated jet is achieved using a novel facility that supplies helium as the core jet gas and argon as a boundary layer gas. This variable-density jet exhausts into an ambient environment of sulfur hexafluoride. The plasma instabilities are simulated by forcing upstream pressure and velocity fluctuations to produce conditions ranging from chaotic to periodic. These studies are supplemented by measurements on plasma spray torches, first to derive appropriate forcing functions, then to verify the velocity distributions measured under cold-flow conditions by performing similar measurements in the plasma jet fringes where the temperatures are lower. The jet characterization leads to an improved understanding of the causes of the jet instabilities, which in turn allows the design of fluid-dynamic means to control the large-scale turbulence of the plasma jet, such as arrays of surrounding microjets, passive nozzle shrouds, or shear-layer control using coflow.Broader impactLarge-scale turbulence in atmospheric pressure plasma jets remains a cause of irreproducible behavior in a number of plasma processes, such as plasma spraying and cutting, powder synthesis including plasma synthesis of nanoscale structures, and metallurgical processing. It is qualitatively understood that these turbulent fluctuations are caused by (a) the extreme density differences between the hot plasma and the surrounding gas, and (b) the upstream fluctuations of the flow due to varying arc-anode attachments. However, a detailed characterization has not been achieved so far because of the difficulty of obtaining data such as local velocities with high time resolution and sufficient signal-to-noise ratio under plasma conditions. Improved understanding of turbulence in atmospheric-pressure plasmas will result from the synergistic combination of expertise in combustion fluid dynamics and plasma science and technology. It is expected that the results impact not only a number of thermal-plasma processes through improved plasma-torch designs, but also that the knowledge is applicable to other jets with similar density gradients such as in aerospace propulsion systems. Furthermore, the expected improved turbulence control can lead to significant reductions in noise levels both for plasma-processing equipment as well as for propulsion systems.