Effect of water conductivity on underwater microsecond pulsed streamer discharge type

Underwater streamer discharges have various potential applications in the fields of wastewater treatment, crop seed processing, etc. The underwater streamer discharge types have an important effect on the practical applications. In this work, the underwater microsecond pulsed streamer discharges are investigated by using an ultra-high-speed frame camera system at different water conductivities and applied voltages. It is found that there exist two different types of discharge under the same experimental conditions: the fan-shaped bush type and the long-single filament type. The water conductivity of 800 µS/cm marks the boundary point for the occurrence rates of the two discharge types: when the water conductivity is less than 800 µS/cm, the occurrence rate of the long-single filament type is 100%; when the water conductivity is larger than 800 µS/cm, the occurrence rate of the long-single filament type decreases, but the occurrence rate of the fan-shaped bush type increases with water conductivity increasing. When the water conductivity is larger than 1000 µS/cm, the dominant discharge type is the fan-shaped bush type, and the voltage required to reverse the appearance rates of the two discharge types increases as the water conductivity increases. The fan-shaped bush type streamer has a propagation velocity of ~1.7 km/s, and the long-single filament streamer has a propagation velocity of ~25 km/s in the early stage and a propagation velocity of ~0.8 km/s in the later stage. Neither of water conductivity and applied voltage has significant influence on the propagation velocities of the two types of streamers. The time lag of the fan-shaped bush-type discharge is about 8% larger than that of the long-single filament-type discharge. The injection energy per pulse of the fan-shaped bush-type discharge is about 20% smaller than that of the single filament-type discharge.