Experimental study on flow film boiling heat transfer over a vertical cylinder

摘要

The knowledge of subcooled film boiling heat transfer is important as the basis of understanding the reflooding phenomenon during the emergency cooling in a nuclear reactor under a loss of coolant accident. In this study, forced convection film boiling heat transfer from a vertical cylinder in Freon-113 flowing upwards along the cylinder was measured for the flow velocities ranging from 0 to 1.3 m/s, and liquid subcoolings ranging from 0 to 20 K at pressures near atmospheric. A platinum heater with a diameter of 3 mm was heated by electric current. The heat transfer coefficients obtained are almost independent of vertical positions on the cylinder. The heat transfer coefficients are almost independent of velocity for the velocities lower than about 1 m/s and become higher for the velocities higher than 1 m/s. The heat transfer coefficients at each velocity are higher for higher liquid subcoolings. Improvement of film boiling heat transfer from the vertical cylinder with the increase in flow velocity is much less than that of horizontal cylinder in cross flow previously reported by the authors. This is mainly due to the difference of heat transfer enhancement mechanism; the former is the drug force on vapor flow acted by a liquid flow and the latter is the pressure gradient near the front stagnation point caused by external potential flow.