Effects of Materials on the Heat Transfer Coefficient During Condensation and Evaporation of R410A

An experimental investigation was conducted to demonstrate the effects of materials on the heat transfer characteristics of R410A during evaporation and condensation inside two horizontal plain tubes with the same inner diameter of 6 mm, but with two different materials of aluminum and stainless steel. The variation of vapor quality for the test section was kept in the range of 0.2-0.9, while mass velocities were allowed to vary from 100 to 400 kg/m(2)/s(1). First, a series of single-phase and repetitive experiments were conducted to verify the accuracy and reliability of the test rig. Results of the evaporation experiments show that the plain aluminum tube performs best for all tested mass velocities. Several different correlations were employed to predict the present data, and their predictive ability was compared and discussed. Results indicate that the Liu and Winterton correlation could accurately predict the present results except for low mass velocities. Roughness effects were accounted for employing a correction factor. The larger roughness of the stainless steel tube was supposed to make the stainless steel tube perform better if roughness effects were accounted for, so the better performance of the aluminum tube was mainly attributed to the material effects. The pool boiling heat transfer as predicted by the VDI model was compared with the experimental results, and more obvious material effects have been found for pool boiling conditions. The minor differences between the two tubes in this case may be explained by the nucleate boiling suppression and incomplete wetting. For the condensation experiments, little difference was found between the two tested tubes, which means that the material and roughness effects may have had little influence on the thermal performance during condensation.