Two Phase Heat Transfer Characteristics in a Vertical Small Diameter Tube at Sub Atmospheric Pressure

Two-phase heat transfer is experimentally examined through vertical small diameter tubes, D =1.45 and 2.8 mm using water under a pressure of 50 to 81 kPa and a natural circulation condition. The pool boiling correlation by Stephan-Abdelsalam and the thermosyphon boiling correlation by Imura, et al. predict the measured experimental data in the 2.8 mm tube with an error of -30%. A large heat transfer enhancement in the 1.45 mm tube is caused by the change of heat transfer regime from nucleate boiling to film evaporation. Predictions of heat transfer coefficients by conduction heat transfer through thin liquid film give good results at high heat flux region and the upper elevation of the test section but give lower values at the other elevations. A proposed correlation predicts the measured data of water in annular flow with convective heat transfer regime within an error of ?±25%. The correlation also predicts well the measured heat transfer coefficients of previous experiment with ethanol in the 1.45 mm diameter tube at a pressure of 99 kPa with an error of 25%.