Thermal Non-Equilibrium in Dispersed Flow Film Boiling in a Vertical Tube

摘要

The departure from thermal equilibrium between a dispersed liquid phase and its vapor at high quality during film boiling is investigated. The departure from equilibrium 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 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 equilibrium; in terms of quality, the heat input was as large as 300% quality for G = 70,000 lbm/hr/sq. ft. The departure from equilibrium is principally a function of the total mass velocity, being less at higher mass velocities. The non-equilibrium quality was measured experimentally. Film boiling pressure drop is also reported. (Author)