CURRENT-ZERO BEHAVIOR OF AN SF(6) GAS-BLAST ARC .2. TURBULENT-FLOW

The conservation equations of turbulent gas-blast nozzle arcs based on local thermal equilibrium and on boundary layer assumptions have been solved numerically. A Prandtl mixing length model of turbulence has been adopted because of its simplicity and its success when applied to turbulent rounded jets. The investigation is aimed at an understanding of the arc in a gas-blast circuit-breaker, where the key problem is the arc behaviour during the current-zero period. Emphasis has been placed on quantitative analysis of turbulence-enhanced cooling and its influence on arc behaviour during the current-zero period. The dominant energy transport processes and the critical region for thermal interruption within the nozzle have been identified. The critical rate of rise of recovery voltage as a function of stagnation pressure has been studied and the physical mechanism responsible for this functional relationship discussed. Good agreement between the prediction and experimental measurements has been achieved for the two nozzle geometries investigated.