Thermal non-equilibrium in dispersed flow film boiling in a vertical tube

Abstract

The departure from thermal equilibrium between a dispersed liquid phase and its vapor at high quality during film boiling is investigated, The departure from equilibruim is manifested by the high resistance to heat transfer between the dispersed and continuous phases, which result in much higher vapor temperatures and a defect in the amount of vapor generated. The effect on the overall heat transfer is to raise the tube wall temperature, and incomplete evaporation occurs within the tubes. Film boiling tests with liquid nitrogen (70,0 90 SG 190,000 lbm/hr/ft2 and 5000sq/A!25,000 Btu/hr/ft ) were made with 0.228, 0,323, and 0.462 inch ID tubes, 4 and 8 foot long. Visual observations showed that complete evaporation occurs at heat inputs much greater than the required heat of evaporation based on thermal equilibruim (A Hinput >Hfg); in terms of quality, the heat inp t was as large as 300% quality for G = 70,000 lbm/hr/ft . The departure from equilibruim is principally a function of the total mass velocity, being less at higher mass velocities. The non-equilibruim quality was measured experimentally by a helium tracer ggs technique; reliable quality data at G = 70,000 lbm/hr/ft was found to be in agreement with the departure from equilibruim calculated by applying a modified single phase heat transfer coefficient to the film boiling data. A kinematic-heat transfer analysis of the core flow, which takes into account the acceleration, evaporation and breakup of a droplet, confirmed the trends in the departure from equilibrium. A Weber number criterion (Wec = 7.5) was found to adequately describe the breakup of droplets over a partial range of test conditions. Film boiling pressure drop is also reported.