Efficient Generation of Strong Shock Waves in Underwater Pulsed Spark Discharge

Abstract

There is considerable interest in the research of underwater pulsed discharge as a practical tool in environmental applications during the last two decades. Among the by-products generated by underwater pulsed discharge, shock wave is applied in various industrial fields. The numerous studies for increasing the strength of the shock wave have been researched because the application efficiency is higher with increasing the peak pressure. Therefore, this work investigates the mechanism of the shock wave formation in water and the effect of operating parameters on the peak pressure for finding the method of stronger shock wave formation using less energy. The UPSD is consisted of two phase such as pre-breakdown phase and breakdown phase. The leakage energy during the pre-breakdown phase exists certainly and does not contribute to the spark channel expansion and shock wave formation after the breakdown. To reduce this leakage energy, it is effective that the energy for water heating, which occupy a large portion of the energy consumed for the pre-breakdown phase, decreases by changing the electrode geometry such as reducing the anode area. And the initial resistance of the spark channel is high as increasing the charging voltage at lower gap distance. It means that the conductivity of the spark channel times channel cross section is a dominant factor rather than the length of the spark channel. To reduce the uncertainty, the experiments is carried out in the condition of narrow anode exposure area. Therefore, there will be no the leakage energy during the pre-breakdown phase. The shock wave formation of water can be understood by the fundamental mechanism based on the shock wave generated by an accelerating piston. The shock wave is a result of the accumulation of the compression waves until the spark channel expansion speed reaches the maximum value of the speed. Therefore, the delivered energy to the spark channel during the accelerating phase of the spark channel is a critical parameter to determine the peak pressure. To compute this energy, it is needed the time of maximum channel expansion speed obtained from the modified power balance equation. When the power is integrated over time of maximum channel expansion speed, the delivered energy to the spark channel during the acceleration phase (E*) is obtained. As a result of the experiments, the strength of the shock wave pressure measured at a specific position sufficiently far from the spark channel is proportional to E*. The resistance of the spark channel is important to determine the E*. Therefore, E* of the experiment compares with E* calculated by using channel resistance models. As a result, the channel resistance is not a constant value but time-varying waveform. As decreasing the channel resistance rapidly, calculated E* shows the low value. The shock wave formed by an underwater spark discharge is a highly transient and nonlinear phenomenon. The spark channel resistance and the delivered power to the spark channel mutually interact. For this reason, the self-consistent model is constructed. By using this model, the optimum condition of the strong shock wave formation is investigated. To generate the strong shock wave, it is important to increase the energy absorbed to the spark channel at early stage of the discharge. It is possible that this energy is adjusted effectively by selecting the appropriate operating parameters such as the capacitance, breakdown voltage and gap distance. As increasing the capacitance with fixed breakdown voltage and gap distance, the peak pressure and E* tends to saturate above specific capacitance. In case of variable breakdown voltage with fixed capacitance and gap distance, the peak pressure and E* increase with increasing the breakdown voltage because the power rising rate is faster with higher breakdown voltage while t* is saturated. Finally, the longer gap distance make the strong peak pressure at the fixed capacitance and the breakdown voltage, because the long gap distance make the initial resistance of the spark channel is high and it leads to increase the rising rate of the power, although the problem of the increasing breakdown voltage with increasing the gap distance. This research provides the better understanding of the shock wave formation mechanism and the fundamental operating parameters for obtaining the strong shock wave.