Simultaneous Condensation-Evaporation Heat Transfer on A Vertical Tube, II-Theoretical Study.(Dept.M)

Abstract

In the present work, simultaneous condensation-evaporation heat transfer in thin films on a vertical tube is, experimentally, investigated. Condensation takes place on the outer surface of the vertical tube and the evaporation occurs inside it. The condensed steam outside the tested tube imparts its later heat to evaporate the thin liquid film, which flows inside the tube. Throughout this work, the range of Reynolds number for water film flow is from 50 to 800, while that of condensate outside the tube, takes values less than 100. The studied operating parameters, which affect the condensation-evaporation processes are water flow rate, its degree of subcool, the flow rate of the dry saturated heating steam and its pressure. The flow rate of the falling heated water is 3-50 kg/hr at constant pressure of 1.1 bar, while the heating steam flow rate is up to 5 kg/hr at a pressure which varies from 1.2 to 2 bar. The results show that, decreasing the Reynolds number of the falling water film, up to dry out a the end of the venical tube, causes an increase of heat transfer coefficient for different operating parameters. In addition, the Nusselt number for different values of Reynolds number are calculated and in accordance, an empirical correlation for the condensation - evap oration Nusselt number as a function of the studied operating parameters is proposed.