The critic liquid-gas phase transition between liquid nitrogen and YBCO HTS bulks: from FEM modeling to its experimental study for ZFC levitation devices

Abstract

This paper presents an experimentally verified finite-element multiphysics model developed to simulate the coupled electro and thermodynamic processes (electromagnetic, thermal, and fluid-flow with liquid-gas phase transition) occurring between liquid nitrogen (LN 2 ) and HTS YBCO bulks. The model allows estimating: 1) the thermal time constants related to bulk HTS losing/gaining its superconducting state, and mainly 2) the LN 2 liquid-to-gas flow rate due to superconductor Joule losses caused by electromagnetic induction heating of it. Model accuracy is evaluated, for example, by comparing the cooling time needed to zero-field-cool the YBCO bulk, and the quantity of LN 2 vaporized as the superconductor cool. Other important issues identified were time evolution and spatial distribution of temperature in the superconductor, heat flux, nitrogen liquid flow, and also nitrogen gas concentration for specific cooling scenarios. Full-scale experimental tests in our laboratory allowed us to validate with success the model and its effectiveness predictions.