A theoretical estimation of the pre-breakdown-heating time in the underwater discharge acoustic source

Abstract

One of the common characteristics of the electrothermal breakdown in an underwater discharge acoustic source (UDAS) is the existence of a pre-breakdown-heating phase. In our experiment, two phenomena were observed: (1) the breakdown time that takes on high randomicity and obeys a "double-peak" stochastic distribution; (2) the higher salt concentration that reduces the residual voltage and causes 100% non-breakdown. The mechanism of electrothermal breakdown is analysed. To specify the end of the pre-breakdown-heating phase, a "border boiling" assumption is proposed, in which the breakdown time is assumed to be the time needed to heat the border water around the initial arc to 773 K. Based on this 'border boiling' assumption, the numerical simulation is performed to evaluate the effects of two heating mechanisms: the Joule heating from the ionic current, and the radiation heating from the initial arc. The simulation results verify the theoretical explanations to these two experiment phenomena: (1) the stochastic distribution of the radius of the initial arc results in the randomicity of the breakdown time; (2) the difference in efficiency between the radiation heating and the Joule heating determines that, in the case of higher salt concentration, more energy will be consumed in the pre-breakdown-heating phase.

A COMSOL modeling of the pre-breakdown heating phase in the electro-thermal breakdown of conductive water

“…The second phase, which shows an intense oscillation, is the post-breakdowndischarge phase. According to our previous study [6], when the gap interval is fixed, the intensity of the shockwave in electro-thermal breakdown mainly lies on the temperature of the discharge arc, which mainly lies on the remaining voltage after the pre-breakdown-heating phase (U B in Fig.2). Fig.1 The main setup of the underwater puse discharge device Fig.2 The evolution of the voltage on the underwater gap [7] A good electrode for the electro-thermal breakdown with a fixed gap interval, should reduces the energy leakage in its affiliate area and raise the remaining voltage.…”

“…In the pre-breakdownheating phase, an amount of energy must be deposited to the underwater gap from the external circuit, and the temperature of the gap water must be raised to a threshold value, which can be regarded as one of the necessary conditions for the succeeding breakdown. In our previous study [6], that threshold temperature is estimated to be 800K. However, that estimation is not accurate, because the figure of the electrode is out of consideration and the electric field in the underwater gap is assumed to be a homogeneous field.…”

“…According to our previous study [5], most of the underwater discharge devices are operated in a "electro-thermal breakdown" mode, which usually happens in conductive water under a weak electric field of 10 kV/cm order. The characteristic of the electro-thermal breakdown is that there exists a pre-breakdown-heating phase before breakdown, which may last 100μs~1ms.…”

Effect of Prebreakdown Time on Shock Wave Generation Characteristics of Underwater Plasma Sound Source

“…However, for the prebreakdown process, the main factors affecting the prebreakdown time include the synchronization of trigger switches, the distance between discharge electrodes, discharge voltage, discharge current, conductivity, and so on. Secondly, discharge electrode material, discharge electrode structure (tip-tip or tip-plate structure), water temperature, and other factors will also affect the prebreakdown time [20][21][22][23][24]. In the experimental study, it is assumed that the trigger switch has good synchronization performance, and the discharge electrode adopts a tip-to-tip structure, which can reduce the discreteness of plasma channel formation.…”

Electric characteristic and cavitation bubble dynamics using underwater pulsed discharge

“…When a voltage pulse is applied to the electrodes underwater, the water medium between two electrodes cannot be broken down immediately. At the first phase, the electrodes are surrounded with the border water, which is heated by the initial arc and the Joule heating form the ionic current [29]. When the temperature of the border water reaches a threshold value (773 K) [29], the thermal kinetic energy of the water molecules will be enough to overcome the hydrogen bonds.…”