Experimental study of local heat transfer during downward flow condensation in vertical tube-in-tube annulus channel

Flow condensation is an important configuration in thermal management due to its efficiency in heat dissipation offered by both convective and phase-change heat transfers. Prior studies on flow condensation in literature focus on condensate flowing inside a tube, featuring the in-tube heat exchanger configuration. However, many common heat exchangers used across applications such as the shell-and-tube type see the condensate flowing on the outside the tube, featuring the outer-tube heat exchanger configuration. In this study, we experimentally investigate the local heat transfer behavior of the outer-tube downward flow condensation in a vertical tube-in-tube condensation module. The test module features a vertical outer-tube heat exchanger with downward-flowing PF-5060 (a clear, colorless, fully-fluorinated dielectric fluid for heat transfer applications manufactured by 3M^TM) condensing outside the circular tube and upward-flowing deionized water inside the tube flowing counter-currently with the PF-5060 flow. The tests include PF-5060 mass velocity from 26.5 – 58.9 kg/m²s, water mass velocity from 330.4 – 472.8 kg/m²s, inlet pressure from 139.6 – 168.2 kPa, and inlet superheated temperature from 4.2 – 5.8°C. Fine temperature measurements are made on the exterior of the tube wall and within the water flow along the module, which are used to determine the local heat transfer coefficients along the condensation path. The result shows that heat transfer coefficient decreases sharply upstream near the inlet and then gradually declines as we move further downstream. Further, the heat transfer coefficient increases along axial locations with PF-5060 mass velocity, while it rises upstream but shows mixed trends downstream with increasing water mass velocity. Correspondingly, the channel-averaged heat transfer coefficient increases with both PF-5060 and water mass velocities, with PF-5060 showing a much stronger impact. Pressure effects are also examined, revealing that the heat transfer coefficient fluctuates, decreasing upstream and showing mixed trends downstream. Finally, common correlations for in-tube flow condensation generally underpredict the experimental heat transfer coefficients but the approaches of Nie et al., Akers et al., and Shah show better predicting capability. The results highlight distinct differences in entrained liquid distribution and associated heat transfer between in-tube and outer-tube downflow condensation.