Natural Convection of Subcooled Liquid Nitrogen in a Vertical Cavity

An experiment to measure the natural convection of subcooled liquid nitrogen between two vertical plates has been performed. The main objective of this study is to confirm the feasibility of our recently proposed design for an HTS power transformer cooled by natural convection of subcooled liquid nitrogen. A liquid nitrogen bath is cooled to nearly the freezing temperature (63 K) at atmospheric pressure by a vertical copper heat transfer plate thermally anchored to the coldhead of a GM cryocooler. A parallel copper plate generating uniform heat flux is placed at a distance so that liquid between the two plates may develop a circulating flow by natural convection. The vertical temperature distribution on both surfaces is measured in steady state, from which the heat transfer coefficient is calculated. The experimental data are compared with the existing correlations for a rectangular cavity where each vertical surface has a uniform temperature. The discrepancy between two data sets is examined by considering that the surface temperatures in this experiment decrease upwards as the cryocooler is located at the top. The formation of multi-cellular flow is qualitatively discussed in terms of the height-to-gap ratio of the cavity and the vertical temperature gradient as determined by the magnitude of heat flux.