Modelling the Electrostrictive-force induced Cavitation Formation Process in Dielectric Liquids

Pre-breakdown phenomenon in dielectric liquids, also called streamer, has been widely investigated to understand the breakdown mechanism of dielectric liquids. The possibility of streamer initiation due to electrostrictive force induced cavitation formation under fast-rising voltage waveform is studied in this paper. The finite element method (FEM) based modelling is conducted by coupling Poisson equation and Navier-stoke equations in COMSOL Multiphysics. The pressure threshold of cavitation formation in cyclohexane and rapeseed oil is estimated to be -6 MPa and -9.31 MPa, respectively, which are 3-5 times smaller than that in deionized water (-30 MPa). This is mainly due to smaller surface tension coefficients of oils than deionized water. A detailed description on the dynamics of cavitation formation in deionized water is first presented and then compared with those in cyclohexane and rapeseed oil. Results showed that the electrostrictive pressure increases with the increasing applied voltage, which pushes local liquid towards the needle tip at tens of m/s and leads to large negative pressure in the vicinity of needle tip. Cavitation appears in the area where local negative pressure reaches the threshold. The electrostrictive pressure in deionized water is around 100 and 60 times larger than that in cyclohexane and rapeseed oil, respectively, under the same conditions due to the larger relative permittivity of deionized water. Therefore, it is much harder to initiate cavitation in cyclohexane and rapeseed oil than in deionized water. In addition, the voltage magnitude thresholds for cavitation formation are comparable between cyclohexane and rapeseed oil under different voltage rising rates.