Studies On The Discharge Characteristics In A Large Volume Underwater Corona Discharge Reactor

The electrical discharge could be generated in the gap of the electrodes in water applied with a high voltage pulsed voltage. Generally, the underwater electrical discharges initiate both physical and chemical processes, such as electrical field, ultraviolet radiation, bubble, shockwave and the formation of free radical OH and reactive molecular species of H2O2, O2, O3 etc. Its chemical processes is most notable, the underwater electrical discharges have received extensive attention in wastewater treatment recently. Due to the higher permittivity and density of water, a high electrical field in the order of megavolts per centimeter is necessary to initiate the electrical discharge in water. A point-to-plate configuration is employed to generate the underwater discharges in most of studies, but this electrode configuration isn't suitable for large-scale industrial application. In order to push the underwater discharge into practical applications in wastewater treatment, it is very meaningful to develop a discharge reactor for generating a large-volume underwater electrical discharge with a relatively low voltage. Our lab developed a coaxial rod-to-cylinder electrode, which is capable of initiating a large volume streamer discharge. The rod electrode was made of a stainless steel screw of 3 mm diameter, the screw was covered by a series of insulator rings of 2.5 mm in length and 0.5 mm in wall thickness. By using the optical and electrical diagnostics, the characteristics of the pulsed corona discharge have been investigated in the coaxial rod-to-cylinder electrode, and the effect of water conductivity on the corona discharge was studied. The main findings of the paper are stated as follows:(1) The conductivity of water has significant effect on the electrical characteristics of the discharge. The amplitude of the measured current and the dissipated energy increase with increase of water conductivity while the ignition voltage slightly decreases with increase of conductivity. As the water conductivity increases from 50 to 200μS/cm, the maximum of the measured current increases from-12 to-22 A at the applied voltage of-40kV; the dissipated energy per pulse increases from-0.14 to-0.24 J; but the ignition voltage of the discharge slightly decreases from-26.9 kV to-23.9 kV.(2) The discharges were generated from the circular seam between two adjoining insulator rings. During one pulse duration, multiple circular seams produced the streamer discharge. In one discharge seam, one or more initial sites appeared. An initial site produced several streamers filamentary. With increase of water conductivity from 50 to 200μS/cm, the maximum streamer length decreases from-9 mm to-6 mm, and the average number of the ignition seams decreases from five to three.(3) Based on the temporal evolution images of the discharge, the velocity of the streamer propagation in water was measured. The results show that the streamer velocity is to be constant during the propagation and is about 30km/s; the water conductivity has no influence on the propagation velocity of the streamer.