Modelling of Electrical Breakdown in Supercritical CO2 with Molecular Clusters Formation

A theoretical model is developed to describe the mechanism of electrical breakdown in supercritical fluids (SCFs) with molecular cluster formation. The model first calculates the collisional cross sections of supercritical clusters by taking into consideration of energy losses of electrons inside clusters. Then, Boltzmann analyses are performed to obtain the electron energy distribution function (EEDF) and estimate the dielectric breakdown characteristics of supercritical CO2 by the ionization (alpha) and attachment (eta) coefficients. The results show that the clustering effect in SCFs has a significant effect on both collisional cross sections and dielectric properties. The density-reduced critical electric field of SCFs with clusters formation is lower than that of the corresponding gas. Moreover, as the density increases, the average size of clusters gets bigger, the cross sections are more affected by the clustering effect. As a result, the breakdown voltage tends to saturate in the supercritical region. For supercritical CO2 at 311 K and 8.4 MPa, the density-reduced critical electric field is 66 Td, while for CO2 gas, it is 97 Td. The results in this paper are in good agreement with our experimental results.