Thermo-Hydraulic Analysis of a Horizontal HTS ZFC Levitating Bearing Concerning Its Autonomy Safety Service Time

Abstract

Electromagnetic heating of high-temperature superconductors (HTSs) and non-perfect thermal insulation of cryostats cause liquid nitrogen (LN2) vaporization and consumption. This paper presents results from the thermo-hydraulic analysis of the maximum safety service time of the prototype of a horizontal axis radial levitation bearing with zero-field cooled (ZFC) HTS bulks. It first studies the temperature sensitivity of yttrium barium copper oxide (YBCO) bulks to the LN2 level, particularly when partially immersed. This, when the main component of heat flux is according to the c-axis of the bulk crystal structure, that is the case of the studied bearing prototype and in most of all cryostats of maglev vehicles. Measurements used two platinum resistance thermometers leaning the top and side surfaces of a non-magnetized ZFC YBCO bulk. Correlations between the temperatures measured by the two platinum resistors are made to determine the average temperature of YBCO bulk. The thermal-time constant associated with a YBCO bulk losing contact with LN2 is also determined for two conditions: 1) initial ZFC and 2) initial field-cooled (FC) under the influence of typical magnetic field values generated by the studied bearing permanent magnet (PM) rotor. Results indicate that the YBCO temperature evolution is not notably affected by Joule losses from initial magnetization currents for this bearing. Due to continuous PM rings, the magnetic field perceived by the YBCO bulks does not change in the ideal case of spinning with no vibrations. Another experiment shows that the heating by induction from vibration dynamics does not contribute notably to LN2 consumption. Finally, the safety service time is verified from measured LN2 mass and level evolution in the HTS bearing's stator, due mainly to defective thermal insulation from the room temperature. The process consisted of calculating the times that different leveled YBCO bulks experiments reached the critical temperature and using the conclusions taken from the initial study about the temperature sensitivity of single bulks to the LN2 level. 3D finite element simulations reinforce our experimental results.