Vapor bubble and streamer initiation in water under long duration impulses

The generation of vapor bubbles in water under high voltage impulses of long duration (500 μs) is investigated experimentally. In most cases, the appearance of bubbles is followed by the inception of discharges occurring within the vapor, eventually transforming the bubble into a propagating streamer. Time delays to bubble and discharge inception are measured versus conductivity and voltage, using fast visualization and optical detection of bubbles. Before bubble inception, the liquid heating and the onset of an electro-hydro-dynamic (EHD) motion can be observed by Schlieren visualization. Electrical measurements show a decrease of the gap resistance versus time, attributable to the heating of water. When conductivity and voltage are changed, the onset of bubble inception is correlated to a nearly constant resistance drop, suggesting that a constant temperature is present when bubbles appear. At short times, the energy dissipation required to observing bubbles is independent of conductivity and applied voltage. Experiments show that vapor bubble generation results from a competition between liquid heating by Joule dissipation, and forced cooling induced by the EHD motion.