Enhancement of boiling heat transfer by T-shaped fins during rapid cooling in liquid nitrogen

Abstract

Rapid cooling via boiling in liquid nitrogen is widely used in high-temperature superconducting (HTS) applications, such as resistive-type superconducting fault current limiters. However, the cooling performance is degraded by film boiling, which results in a low heat transfer coefficient. This study proposes a T-shaped fin with low thermal conductivity to retain the coolant under film-boiling conditions and enhance boiling heat transfer. We examined the effect of the height of T-shaped fins (0.5, 1.0, and 1.5 mm) on cooling performance. The results show that the optimal height depends on the temperature of the heated surface. In the relatively low-temperature regime, the fin height had minimal effect on the cooling performance. However, at higher temperatures a 1.0-mm-height fin yielded the highest cooling performance. The 1.5-mm-height fin enabled the generated vapor to seamlessly penetrate the liquid retention space, reducing heat transfer efficiency. With the 0.5-mm-height fin, which is the smallest height, the cooling performance significantly decreased after a particular temperature of the heated surface, as vapor covers the entire surface of the fin, indicating that film boiling occurs even on the fin surface. On the other hand, in the case of resin fin having quite low thermal conductivity, the cooling performance is higher than that of metal fin under high temperature conditions. This indicates that boiling on the fin surface is suppressed and liquid retention effect can be maintained even in the high temperature regime. To improve both the fin effect and liquid retention effect, we also propose a two-stage T-shaped fin design. This approach aims to maintain coolant retention and suppress the steep decline in cooling performance under high-temperature conditions. The study’s findings offer practical guidance for optimizing T-shaped fin designs to enhance cooling performance in HTS applications, reduce liquid nitrogen consumption, and ensure reliable thermal management under varying temperature conditions.