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Contribution to evaluation of the interruption capability of CF3I/metal thermal plasmas: Preliminary study of the transport coefficients

In the field of the electrical installation protection, and more particularly for high voltage circuit breakers, one of the most important challenges is to find new gases to replace SF6 and reduce the greenhouse effects. Among the several candidates such as CO2, air, vacuum or g31, the recent studies on CF3I highlight interesting electrical and thermal properties2 for this gas. Unfortunately, these studies only consider pure CF3I gas or CF3I mixed with other gases such as N2, air, CO22. Yet, during the creation of an electrical arc between the two electrodes, several metallic vapors issued from the electrodes' erosion and mixed with pure CF3I gas strongly modify the electrical ad thermal properties of the initial gas. This work proposes to study the influence of the copper, iron or aluminum metallic vapors on the thermal properties of CF3I for a pressure of 6 bar, temperatures from 300K to 30000K and concentrations of vapors between 0% and 50%. The mass density and specific heat at constant pressure are directly deduced from the population number densities of the species, their partition functions and their enthalpies of formation. Electrical conductivity, thermal conductivity and viscosity are obtained according to the Chapman-Enskog method2 and using the collision integrals calculated from theoretical expressions or given in the literature. The results show that few proportions of metallic vapors (less than 1%) strongly increase the electrical conductivity at low temperature, modify the thermal conductivity around 5-7kK and the viscosity around 10kK for proportions higher than 20%. We compare the transport properties for the three metallic vapors and highlight the strong effect of the species having the lowest ionization potential on the electrical conductivity. Coupled with the electrical properties and the radiation losses, these database will enable us to estimate the interruption capability of CF3I in presence of metallic vapors and consequently, to conclude on the use of CF3I as a good alternative to replace SF6 in high voltage circuit breakers.