Computational Study of Turbulence Generation Around Arc Plasma Sustained in a Converging-Diverging Cylinder*

Numerical simulation was performed to elucidate the mechanism of turbulence generation around arc plasma sustained in a converging-diverging cylinder. To reproduce a turbulent-like flow in an SF6 arc-plasma-discharge system, the inhouse simulation code PLASTIPC (Plasma All-Speed Turbulence with Implicit Pressure Code) was used. The simulation showed that arc plasma was constricted with high temperature over 20,000 K, while the ambient nonionized gas had low temperature below 1,000 K. The arc plasma had small Mach numbers, which meant that the arc plasma was a slightly compressible and subsonic flow, whereas the ambient nonionized gas formed a compressible flow in Mach number > 0.3 with island-like supersonic flow zones in Mach number > 1.0. The mass density decreased in the arc plasma remarkably, because dissociation of SF6 expands gas itself. Consequently, higher-speed backward and forward flows were generated inside the arc plasma to satisfy the mass conservation. The high-speed backward and forward flows inside the arc plasma impinged and pushed a radial flow out. This radial flow from the arc plasma encountered the low-temperature high-density SF6 gas flow, which generated a vortex. This process took place locally at several positions. As a result, vortices were produced intermittently and repeatedly at the arc plasma fringe. Those vortices interact and form a complex flow structure as turbulence.