Experimental study on the pool boiling heat transfer of slush nitrogen under triple point to atmospheric pressure

Abstract

Slush nitrogen is a solid-liquid two-phase cryogenic fluid, with temperature of about 63.15 K under pressure between the triple point of nitrogen and atmospheric pressure. High-temperature-superconductor (HTS) cables generally require the cryogenic coolant such as subcooled liquid nitrogen. Compared with the subcooled liquid nitrogen, slush nitrogen has lower temperature, higher density and higher heat capacity, and is considered a promising coolant for HTS cables. In order to intuitively understand the boiling heat transfer characteristics of slush nitrogen, a pool of slush nitrogen of 0-30% solid volume fraction was prepared as coolant and was filled into a dewar, and helium was charged to change pressure from triple point to atmospheric pressure. Experiments under different pressure conditions were carried out to obtain the variation of boiling curves. The critical heat flux (CHF) of slush nitrogen was found to be reduced by increasing solid volume fraction, because of denser solid nitrogen with lower heat transfer capacity within the main boiling field. By fitting the result to Kutateladze Correlation which predicts CHF, while considering the influence of subcooling, the improved coefficients applicable for slush nitrogen are obtained with an accuracy of ±10%.