Turbulent self-quenching mechanism of high-temperature-gradient arc plasma in a converging–diverging cylinder: a numerical study

Abstract

Thermofluid dynamic behaviours of SF₆ and air flows including arc plasma and non-ionized gas in a converging–diverging cylinder were studied numerically. The arc plasma in SF₆ gas was constricted with high temperatures exceeding 20 000 K. It formed a slightly compressible and subsonic flow, whereas the ambient non-ionized gas had temperatures below 1000 K and a compressible flow with island-like supersonic flow zones. The arc plasma in air gas had temperatures lower than 12 000 K and a blurred fringe with a low gradient. The entire fluid region including the plasma formed an incompressible and subsonic flow. The different temperature distributions were attributed to differences of their thermal diffusivities and Reynolds numbers. For the SF₆ flow system, SF₆ molecules dissociated in the arc plasma to seven atoms. They were ionized to their ions and electrons, causing considerably greater volume expansion and generating both forward and backward flows locally. Impingements of those flows occurred locally. The generated vortices conveyed the thermal energy outward. Great degrees of turbulent mixing of the thermal energy occurred with the mass density fluctuation, which produced outward heat flux much stronger than that produced by either convection or conduction. The arc plasma was quenched effectively.