Influence of pressure ratios on cold gas flow field in HV SF ₆ circuit breaker nozzles

Cold SF ₆ gas flow field is calculated and analyzed for three kinds of Laval nozzle structures to guide thermal gas flow field simulation in high voltage circuit breakers. The influences of different nozzle length, upstream and downstream pressures are investigated by comparing the variety characteristics of the pressure and the Mach number. The critical pressure values of the three nozzle structures are confirmed. The relationship between shock initiation and pressure ratio is analyzed, and the calculation error caused by available plasma properties data is discussed. The results show that different length nozzle structures are with different critical pressure values. The shocks appear when the pressure ratio is lower than the critical value, otherwise no shock arising. Shock position varies with the pressure ratio and nozzle length. The outlet pressure is independent on the setting value and increases with the inlet pressure when the gas flow velocity gets always supersonic along in the nozzle downstream. The error in the simulation can be reduced with the effective implementation of the gas properties, which may improve the accuracy of the calculation results.