Current zero behaviour of an SF6 nozzle arc under shock conditions

The effects of arc-shock interaction on the behaviour of an SF6 arc burning in a supersonic nozzle during current zero period are numerically investigated for the experimental conditions of Frind and Rich. The Prandtl mixing length model is used to simulate the turbulent arc. The adverse pressure gradient associated with the shock causes flow separation in the nozzle which leads to the formation of vortices and the broadening of the arc cross-section. Compared with the shock in the absence of the arc, the shock centre moves upstream and the shock is broadened. When the current is linearly ramped down towards zero, the flow separation point moves towards the axis which creates a very stagnant flow. This results in slow thermal recovery in the arc-shock interaction region and a reduction in the effective arc length. Comparisons between the calculated and measured critical rate of rise of recovery voltages (RRRV) at three stagnation pressures (P0) and two rates of current decay before current zero (di/dt) show good agreement with the chosen value of the turbulence parameter. Attention is also paid to the correct prescription of the boundary conditions when the gas is sucked into the nozzle at the nozzle exit plane. (paper)