CONDENSATION AND EVAPORATION HEAT TRANSFER CHARACTERISTICS IN HORIZONTAL SMOOTH AND THREE-DIMENSIONAL ENHANCED TUBES

摘要

An experimental investigation of condensation and evaporation heat transfer characteristics was performed in a smooth tube and two enhanced tubes (1EHT, 3EHT) using RI34A as refrigerant. All tested tubes have the same inner diameter of 14.68mm and outer diameter of 15.88mm. The enhanced surface of the 1EHT tube is made up by dimples and a series of petal arrays, while that of 3EHT tube is rectangular cavities. The test runs are performed at a saturation temperature of 45°C over the quality range of 0.8-0.2 for condensation and at a saturation temperature of 6°C over the quality range of 0.1-0.6 for evaporation. For evaporation, the heat transfer coefficient ratio (h_(ETH)/h_s) is approximately 1.2-1.4 for mass from 50-90 kg/m~2s. For condensation, the 1EHT tube provides the best condensation heat transfer performance. This is mainly due to the dimples and rectangular cavities that increase heat transfer surface area and interface turbulence, producing additional disturbances, secondary flows near the boundary and flow separation.