DC nozzle arcs with mild wall ablation

Abstract

Recent experimental results (Smith et. al., 1980 and New land et. al., 1982) indicate that nozzle wall ablation can occur even before the arcing current reaches its thermal blocking limit. The wall ablation is caused by intense arc radiation. The evaporated wall material forms an ablation layer which reduces the effective flow area for the arc quenching gas. A theoretical model is established to study the complicated interaction between the ablation layer, the external gas flow and the arc. It has been found that for arcs burning in affinely related nozzles the arc behaviour is controlled by two nondimensional parameters, one of which is similar to the nozzle coefficient (Fang et al, Cowley and Chan). The other nondimensional coefficient, which characterises the effect of wall ablation, is determined by the thermodynamic properties of the nozzle material, the characteristic quantities of the gas and the radiation coefficient for a given upstream electrode material. Three nozzle materials, steel, copper and PTFE, have been studied and two arc quenching gases, SF6 and air, examined. It has been found that wall ablation greatly reduces the thermal blocking current but only has a marginal effect on the voltage-current characteristics. A comparison with the recent test results (Smith et al, New land et al) indicates that the theoretical model can adequately describe the nozzle arc behaviour in the presence of wall ablation.