Analytical comparison of condensing flows inside tubes under Earth-gravity and space environments

The heat transfer behavior of flow condensation inside horizontal tubes under zero-gravity and Earth-gravity conditions is modelled and analyzed. For Earth conditions for wetting fluids the annular flow region changes to a stratified flow pattern owing to gravity drainage of the condensate from the upper portion of the tube. The stratified condensate layer is considered inactive in the heat transfer process; its magnitude is determined along the tube length using the analytical results of Rufer and Kezios. Under zero-gravity conditions no such gravity drainage occurs and so the flow is considered to be annular along the complete length of the tube. The analytical approach of Bae was used to evaluate the heat transfer rates under zero gravity conditions. The results indicate a substantially poorer condensing performance under zero-gravity conditions. These results can be simply explained in terms of the smaller condensate film thickness over the upper portion of the tube periphery at any axial location under earth-gravity conditions because of gravity drainage of the condensate.