Abstract
The behaviour of ablation-dominated AC arcs in supersonic nozzle has been analysed. For a given gas and nozzle material, the dynamic behaviour of arcs burning in affinely related nozzles is fully determined by three non-dimensional coefficients: the nozzle coefficient N; the ablation coefficient Ab; and the non-dimensional frequency Ω. For circuit-breaker operations, the values of these three non-dimensional parameters are such that flow reversal (i.e. from the nozzle to the upstream tank), caused by inner nozzle wall ablation, cannot be avoided. When this happens, the arc and flow conditions at current-zero are dependent on N (i.e. peak current for a fixed stagnation pressure). The temperature of the gas in the external flow can be much higher than that in the upstream tank. The peak current therefore has a direct influence on the thermal and dielectric recovery processes inside the nozzle. Arc modelling cannot in general be divided into a quasi-steady-state (hence DC) phase and a current-zero period.
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