Development of subsonic electrical discharges in water and measurements of the associated pressure waves

Abstract

This paper first presents an experimental electrical and optical study of the development of an electrical discharge in water. The point–plane water gap is subjected to a 0.02 µs/350 µs impulse voltage. A Schlieren device associated with an image converter or a photomultiplier demonstrates that the discharge phenomenon requires heating of the water located around the extremity of the point. This thermal process leads to the formation of gas bubbles in which an electrical discharge propagates. In the experimental conditions a threshold value of 80 J is necessary to create bubbles. No UV or IR light emission is recorded before the presence of bubbles is detected. When the energy conditions are sufficient (⩾200 J), the volume of bubbles grows until the whole inter-electrode space is filled; then a breakdown of the gap occurs. When this happens, a high amplitude pressure shock wave is generated. In the second phase of this work the shock wave created by the gap breakdown was studied for energy levels up to 100 kJ. It is clearly pointed out that the pressure shock wave peak value depends on the energy remaining at breakdown time. For a constant remaining energy, the peak pressure value increases with increasing gap length.

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

“…The G is the underwater gap, which provides a pulse electric field for electro-thermal breakdown. In electro-thermal breakdown, the evolution of the voltage on the underwater gap is shown in Fig.2 [7]. From Fig.2, the discharge can mainly be divided to two 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. The most common electrode is shown in Fig.3, in which the insulator cone with a cone figure is to achieve a full release of the discharge shockwave.…”

Underwater Discharge Phenomena in Inhomogeneous Electric Fields Caused by Impulse Voltages

“…The filamentary shaped streamers are observable at the tip of the needle electrode, and correspond to sequences of low-current streamer pulses. Streamer coronas develop in the water when a very high electric field is suddenly applied to the water gap causing an electron avalanche to grow directly in the water, leading to the propagation of a water streamer [15]. If the applied voltage is insufficient for bridging the test gap, the streamer coronas can cease, as shown in Fig.…”

Electrical Properties Associated with Discharge Developments in Water Subjected to Impulse Voltages

“…An electron avalanche can occur immediately in water under a high electric field, leading to the propagation of the water streamer [7]. Streamer corona initiation and propagation mechanisms in some liquids have been reported, two models for streamer corona initiation are considered: one is the electronic process, which involves electron injection drift in the liquid at the cathode, the other covering the concept of microbubble formation due to vaporization and/or cavitation and electrical breakdown in the gas bubbles [8], [9].…”