Heat transfer coefficient and cross-sectional flow structure in condensation of steam in an inclined tube at a low mass flux

Condensation of steam at a low mass flux inside an inclined tube was experimentally investigated to aid the optimal design of air-cooled passive heat exchangers for advanced LWRs. While varying downward inclination angle from 0 to 90°, the heat transfer coefficient monotonically improved by more than 15% while previous studies that have used refrigerants have reported that the optimal angle is between 15 and 30°. To interpret the conflicting trends observed, the cross-sectional structures of the condensing flow were visualized. Stratified flow was the only type of flow noted; there was no transition to the annular flow typically observed during two-phase flow of refrigerants in a vertical tube. The observed variation in the condensation heat transfer of steam at low mass flux, with respect to the extent of downward inclination, was well predicted by a mechanistic condensation model for stratified flow that incorporates the cross-sectional stratification angles of condensates.