Subcooled and low quality film boiling of water in vertical flow at atmospheric pressure

Subcooled and low quality film boiling is usually encountered in safety analyses of nuclear reactors. In most of the previous subcooled film boiling studies, cryogenic fluids were used either in a stagnant pool or a forced convective set-up. These data cannot be applied to reactor safety analysis without excessive conservatism or skepticism. In this study, a unique method is used to establish flow film boiling of water in a vertical tube at atmospheric pressure. The data cover a mass flux range of 50 to 500 kg.m-2.s-1 and an inlet subcooling range of 5 to 700C. It is found that the heat transfer coefficient depends on the mass flux, inlet subcooling and the axial distance from the point where film boiling first starts. A physical model is developed to predict the wall temperature of a tube during inverted annular film boiling. It considers the thermal boundary layers in the subcooled liquid core and in the superheated vapor film. The predicted wall temperatures and void fractions compare well with the measurements