Nanosecond pulsed discharge in a gas–liquid mixture produced by hydrodynamic cavitation using Venturi tube

Abstract

The nanosecond pulsed discharge in a gas–liquid mixture of a Venturi tube with the injection of oxygen was numerically and experimentally investigated. It was found that the density of the gas–liquid mixture close to the electrode at the water inlet is significantly lower than that at the water outlet. The lowest void fraction close to the water outlet is higher than 50%. When the oxygen flow rises from 0.5 SLPM to 3 SLPM, the flow mode changes from the bubble flow to the annular flow. If the applied voltage is kept at 45 kV in amplitude, the probability of the breakdown increases with the water flow and it reaches 100% as the water flow rises to 9.5 L/min. Since the pulsed breakdown voltage changes from shot to shot under the same experimental conditions, the distribution of 200 breakdown voltages was measured and the median in the distribution of the probability density, defined as U50, was determined. The curve of the U50 as a function of the oxygen flow takes the shape of “V,” similar to that of the Paschen curve obtained from the gas breakdown. An abnormal polarity effect in the breakdown voltage was observed, and the reason was given. By combining the results from the experiment with the numerical simulation, the spatial distribution of the reduced field at the time of breakdown was determined. The oxygen flows not higher than 0.5 SLPM are the better choice for getting a reduced field higher than 500 Td.