Study on the vapor density of the residual spark channel generated by microsecond-pulsed underwater spark discharges

Temporal evolution of the residual spark channel of a microsecond-pulsed underwater spark discharge was observed utilizing an ultra-high-speed camera system. The radius, the vapor density, the total vapor mass of the residual spark channel, and the energy consumed to produce a unit mass of vapor were studied. The effects of the water conductivity, the applied voltage, and the electrode gap distance on the radius, the vapor density, and the total vapor mass of the residual spark channel were investigated. Under various experimental conditions, the vapor density is 500–800 kg/m3, the radius is 0.7–1.5 mm, and the total vapor mass generated by a single discharge pulse is 14–46 mg. The vapor density and the radius are approximately uniform along the axial direction of the residual spark channel, and the expansion speed of the residual spark channel is ∼37.5 m/s. Within the range studied, the water conductivity, the applied voltage, and the electrode gap distance hardly affect the vapor density, the radius, and the expansion velocity of the residual spark channel. The energy consumed to produce a unit mass of vapor is 100–350 J/g, which is much less than the latent heat of liquid water.