Generation of Microbubbles for Study of Prebreakdown Processes in LiquidsS.M.Korobeynikov*, A.V. Melekhov**, V.M. Antonov**, V.G. Posukh**
Bubbles play important role in prebreakdown processes in liquid insulating materials. Usually it is bubbles that randomly appears in liquid during strong electric field action. Another variant is random preexisting bubbles on electrodes or in the bulk of liquid.
The aim of this paper is the generation of microbubbles in the field that will be stressed by strong electrical field. There are usually two ways of bubbling: homogeneous nucleation and heterogeneous nucleation. The first one is realized in case of very fast input of energy. The second one is based on bubble’s nuclei that permanently exist in liquids. In our case it is important to obtain small bubbles of given sizes in the given place. It is proposed to apply current pulses through thin wire electrode.
The generation of microbubbles is carried out by pulse heating of quasi-needle electrode representing U-formed loop from nichrome wire by a diameter 0.2 mm, with radius of a bend of 0.5 mm. The wire is put to earth after the pulse of a current, opposite high-voltage electrode is a plane of a cell. Duration of pulses of a current by amplitude up to 3 A varies with the help of the relay of time in a range 0.1-5 s. After an establishment of temperature balance near to wire (~ 1 s), or after some period of expectation necessary for reduction of the sizes too large bubble, to electrode system “quasi-needle - plane” presses a pulse of a voltage. The intensity of an electrical field near to wire is necessary specially to compute, since for used electrode system there are no analytical decisions.
Experimentally and by computing it is shown, that 100-200mm bubbles are appeared at the surface when the temperature near wire surface is more than boiling temperature. Then size of bubble decreases very sharply during one second. After that at the surface of the quasi-needle there is long-lived slightly decreased microbubble by the size 10-100 microns. This bubble is ready for prebreakdown experiments.