Enthalpy-flow limitation of gas-blasted SF6 arcs

Experimental measurements of arc voltage and dielectric-recovery strength after current zero have been made for gas-blasted SF6 arcs in a small polycarbonate (insulating) nozzle, to determine the arc power input which exceeds the enthalpy-removal capability of the gas flow. The experiments were conducted with a 1.9cm electrode spacing, 400¿s duration square current pulses of 100 A and 350 A, and an upstream arc-chamber pressure of 4atm, which ensured that critical pressure and sonic velocity occurred near the throat. The results have been analysed using an enthalpy-flow arc theory, which incorporates the concept of an expanding arc in the direction of the gas flow, as arc energy transported radially heats, and thus indirectly accelerates, the cooler gas surrounding the arc. The maximum enthalpy flow that can be cleared through the nozzle by the SF6 gas flow is about 200 kW/cm2 atm, which corresponds to an axial arc temperature in the nozzle throat of about 25000K.