Characteristics of Shock Waves Generated by a Negative Pulsed Discharge in Supercritical Carbon Dioxide

The initiation and propagation process of shock waves in pressurized carbon dioxide including supercritical (SC) phase was observed by means of schlieren method. A pulsed laser light source was used for high resolution sequential flow visualization and an ultrahigh speed camera equipped with a flash lamp was used for time-resolved visualization. To generate shock waves, a negative pulsed voltage with a rise time of 90 ns and half-width of 410 ns was applied to a point electrode. After development of a bush-like streamer from electrode tip, a spherical shock wave was generated around the streamer. The shock wave velocities and Mach numbers were calculated from the schlieren images taken at gas, SC, and liquid phases. The largest Mach number was measured in SC phase, though shock waves velocity order, from weakest to strongest, was in the gas, SC, and liquid phases, respectively. The shock wave propagated almost linearly after 1 μs, while the shock front grew increasingly difficult after 6 μs to confirm. To examine the initial process of shock waves, the time-resolved high speed imaging setup was used instead of the pulsed laser optical setup. The measurements indicated that the shock wave sharply decayed within submicroseconds; and in comparison with streamer initiation, the shock wave generation was delayed. This delay time might depend on the medium conditions.