Velocity surveys in a gas-flow spark gap switch

Abstract

A series of mean velocity and turbulence intensity surveys have been obtained for flow between the electrodes of a gas-flow spark gap switch over a range of gas pressures, gas species, and gap spacings for one electrode geometry consisting of two 3.81-cm-diam electrodes with domed tips placed transverse to the gas flow direction. Data include mean and fluctuating velocity profiles measured between the electrodes, both on the electrode axis and downstream of the electrodes. Pitot, total head, and one-and two-channel hot wire probes have been used. These data document flow conditions occurring in a gas-flow spark gap switch and provide insight into flow processes affecting recovery of such a switch when operated repetitively. Results have indicated that highly turbulent wakes are formed downstream of these electrodes; these wakes are believed to degrade switch electrical recovery due to trapping of heated debris from prior arcs. Also, it has been found that, for this electrode geometry, freestream turbulence levels do not have a great effect on recovery.Nomenclature d = interelectrode gap spacing / = longitudinal turbulence intensity, defined as rms x -velocity fluctuation divided by mean velocity p = gas pressure R = electrode radius U = streamwise mean velocity in x direction V = horizontal mean velocity in y direction x -horizontal coordinate, measured in direction of gas flow y = horizontal coordinate, measured transverse to direction of mean gas flow z = vertical coordinate, measured along electrode axis from midgap A = turbulence integral length scale (large, energycontaining eddy size) 1 = turbulence microscale (eddy size relating mean square strain rate fluctuation to mean square velocity fluctuation) T = switch recovery time for 70% recovery