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

• Arc-plasma-induced turbulence in an SF 6 flow was simulated successfully using an in-house code. • The turbulence was triggered by forward and backward flows in the arc plasma with significant thermal expansion due to dissociation of SF 6 molecules. • The apparent difference between the SF 6 and air flow systems was attributed to their thermal diffusivities and Reynolds numbers. • A considerable degree of turbulent mixing of the thermal energy occurs especially with the mass density fluctuation as the self-quenching. Thermofluid dynamic behaviours of SF 6 and air flows including arc plasma and non-ionized gas in a converging–diverging cylinder were studied numerically. The arc plasma in SF 6 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 SF 6 flow system, SF 6 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. High-temperature-gradient arc plasma — turbulent thermofluid field and self-quenching mechanism —.