Visualization study on boiling of nitrogen during quench for fault current limiter applications

For the development of a 13.2 kV/630 A bifilar winding type high-Tc superconducting fault current limiter (SFCL), a visualization study has been conducted to clarify boiling characteristics during the quench of the high-Tc SFCL. A series of experiments was carried out using a stainless steel strip on a G10 plate as a heating element to simulate the quench state of the high-Tc SFCL. A pulse of DC power input was applied to the strip in saturated and subcooled liquid nitrogen. The magnitude of the heat generation was varied from 10 W/cm(2) to 170 W/cm(2) and the period of the heat impulse was fixed at approximately 100 ms. Bubble behavior was observed by a high-speed camera through view ports of a cryostat. The boiling phenomena, the temperature rising of the strip and the recovery time were compared for different power densities and liquid nitrogen operating conditions. The bubble suppression was clearly observed with respect to the degree of subcooling.