Simulation Study On The Effects Of Bubble On Liquid Dielectric Insultion

Liquid dielectric with high electric breakdown strength, self-healing, low cost and easy operation is extensively used in intense electron-beam accelerators. But until now, liquid medium, because of its complex physical and chemical properties, didn't establish advanced breakdown model and mechanism as air dielectric. The bubble discharge theory of liquid breakdown, which successly explain the impact of pressure and conductivity on liquid electric breakdown strength, creates a new thought for us to study the liquid breakdown mechanism. Based on the bubble behaviors under the electric field, the pattern of bubble behaviors and its influence on liquid insulation are studied through the theoretical analysis and simulation in this paper.At first, the electric field distribution of parallel plane electrodes, immersing in transformer oil with the bubble existence, is calculated through the theoretical analysis and simulation. The results are shown that the electric field strength enhances inside the bubble, the enhancement factor reaches up to 1.4 for air bubbles in the transformer oil,; Increasing the external pressure is one of the effective methods to avoid the bubble discharge; according to Paschen law, the minimum of external pressure is 2.03 atm (electrode gap equals 1cm,electrode voltage equals 50kV,bubble radius equals 1mm).Based on bubble kinetic equations, the deformed mesh technology of software (Comsol Multiphysics 3.4a) is used to simulate the bubble motion under a typical non-uniform electric field (the electric field of the cylinder-to-plate electrode system) .The bubbles-bridge theory of liquid breakdown is preliminarily verified. The main concludes as follow:(1) Bubble motion includes three phases:First phase, the electric field gradient force plays a decisive role and bubble trends to move to the lower electric field strength areas,its velocity reaches the maximum very quickly. In the second phase, bubble velocity decreases under composition forces impaction and reaches to a region named as stagnation line. In the last phase, the forces on the bubble almost balance , bubble moves very slowly in the electrodes system.(2) The bubbles stay for a long time in the electrodes system, so it is likely to form a bubble bridge along its trajectory. With the addition of electric surface force in the built-in function of Comsol Multiphysics Level-set mode, the bubble deformation in the electric field is traced. The results as follow:(1)Bubble deformation along the electric field direction;its aspect ration (AW) is proportional to the initial radius and extra electric field strength,it means that bubble may be broken into multiple small bubbles if the applied electric field is too high or size of air bubble is too large.(2)The risk of the bubble breakdown increases for electric field enhancement inside the bubble and bubble deformation. To avoid the bubble breakdown, the extra pressure is a effective way and its minimum is proportional to bubble size and extra electric field strength.