Experimental investigation of external flow condensation heat transfer in horizontal tube-in-tube configuration

• Flow condensation setup with local heat transfer and flow visualization capability. • Examined the effects of PF-5060 and water inlet mass fluxes on local heat transfer. • Operating pressure affects the flow condensation heat transfer behavior. • Available correlations underpredict the external flow heat transfer coefficients. • High speed images revealed unique external flow condensation flow patterns. Flow condensation is an important process used to achieve heat rejection across thermal power and energy systems. Studies on tube condensation have concentrated on the condensing fluid flowing in the inner tube. However, common heat exchanger configurations like the shell-and-tube types see the condensing fluid on the outer surface of the tubes. To address this gap, in this study, we investigate the local and channel-averaged heat transfer characteristics of flow condensation happening on the exterior of a horizontal tube in the tube-in-tube configuration. An external flow condensation module is developed and tested to obtain heat transfer and flow visualization data, with PF-5060 as the condensing fluid flowing outside the tube and de-ionized water as the cooling fluid flowing inside the tube in the counter-current direction. Densely arranged thermocouples are installed on the exterior surface of the 12.7-mm outer-diameter tube and embedded within the water flow to measure variations in wall and water temperatures respectively, which determines the local heat transfer characteristics along a 683.6-mm condensation path. Flow visualization is achieved using a transparent polycarbonate plate that serves as the PF-5060 flow channel. The test conditions cover PF-5060 inlet mass velocities of 26.66 – 58.67 kg/m²·s, water mass velocities of 330.9 – 463.26 kg/m²·s, PF-5060 inlet pressures of 124.76 – 155.24 kPa, and PF-5060 inlet superheats of 4.39 – 5.63 °C. The local condensation heat transfer coefficient is highest near the upstream region and decreases monotonically in the downstream direction due to the thickening of liquid film and transition of flow regimes along the condensation path. Further, the heat transfer coefficient increases with both PF-5060 and water flow rates, with the PF-5060 showing a more pronounced effect. Pressure effects are also examined, showing the heat transfer coefficient decreases with the increase in operating pressure. Further, common correlations for internal flow condensation show underprediction in measured heat transfer coefficient for external flow condensation. Finally, flow visualization of external flow condensation reveals continuous detachment of liquid film at tube's underside, highlighting a clear distinction from the internal flow condensation.