Simulation and visualization experiment on the liquid nitrogen boiling behaviors during the quenching and recovery process of YBCO tapes

Abstract

The resistance Superconducting Fault Current Limiter (RSFCL) using YBCO tape is supposed to be of great potential in the high voltage power grids. The fast recovery rate after quenching is the safety guarantee of the electric system to prevent the subsequent fault current shocks. However, the transient heat transfer and local boiling bubble behaviors of liquid nitrogen (LN2) highly affecting the recovery rate are seldom revealed, especially for the disc structure RSFCL with tiny channels. In this work, the boiling heat transfer characteristics during the quenching and recovery process of a YBCO tape in LN2 bath were studied. A 3-D CFD model based on a local part of a typical RSFCL with disc structure was first built to reveal the transient heat transfer coefficient, cooling curve, gas phase volume and temperature distribution that cannot be measured in experiments. The visualization experiment for verification and observation on bubble distribution was conducted with a high-speed camera. Results show that the boiling bubbles generate immediately as quenching starts, film boiling can form quickly within in 0.035 s on the tape surface. The bubble motion behaviour is severely affected by the layout of the Teflon framework. The bubbles detachment from the upper surface of the tape is more frequent than that from the lower surface of the tape, causing inhomogeneous temperature distribution along the tape. Since the recovery is mainly under the film boiling with small heat flux, even a single YBCO tape needs about 2.5 s to return to 77.7 K from 162 K, which means it is impossible to reclose the YBCO tape within 2 s after the quenching cutting off, especially for practical large RSFCL systems with longer recovery time. Besides, there are bubbles still remaining in the system after 5 s when the temperature has recovered to 77.7 K. The remaining gas would change the dielectric field and induce a risk of electric breakdown under the next large impulse current, which should be considered in the safe operation of RSFCL.