Experimental investigation of low GWP refrigerants in-tube flow condensation for medium-temperature heat pump applications

The shift toward low global warming potential (low-GWP) refrigerants has increased interest in alternatives such as R-1234ze(E), R-1233zd(E), and R-1234yf for medium to high-temperature heat pump systems. However, experimental condensation data for these refrigerants at medium-to-high saturation temperatures remain limited, constraining the validation of heat transfer and pressure-drop correlations needed for system design. To address this gap , this study experimentally investigates the flow condensation heat transfer and pressure drop behavior of these refrigerants in a 2 mm inner diameter smooth horizontal copper tube. Tests were conducted at saturation temperatures from 40 °C to 55 °C, mass fluxes between 200 and 400 kg m −2 s −1 . R-1233zd(E) exhibits the highest condensation heat transfer coefficients (HTCs), followed by R-1234ze(E) and R-1234yf, primarily due to its high liquid-vapor density ratio, which enhances interfacial shear and turbulence. R-1234yf consistently shows the lowest pressure drops, while R-1233zd(E) records the highest because of its higher liquid viscosity and lower vapor density, with R-1234ze(E) offering a balanced compromise between heat transfer and flow resistance. Among the evaluated models, Shah (2009) provides the most accurate HTC predictions overall; however, while most correlations predict R-1234ze(E) and R-1234yf well, they systematically underpredict HTCs for R-1233zd(E). For pressure-drop prediction, Friedel's correlation performs best overall, providing very good pressure-drop predictions for R-1234ze(E) and R-1234yf, and demonstrating improved accuracy for R-1233zd(E) at higher mass fluxes compared to lower flow conditions. These results highlight the limitations of generalized correlations for high-viscosity and high liquid-vapor density-ratio refrigerants and emphasize the need for property-aware modeling approaches to ensure reliable predictions across a wide range of operating conditions. Furthermore, a combined performance evaluation using the Penalty Factor (PF) and Total Temperature Penalization (TTP) reveals that the HTC-based ranking is reversed when pressure drop is accounted for, with R-1234yf emerging as the best overall condenser performer (TTP = 1.69 K), followed by R-1234ze(E) (TTP = 2.11 K), while R-1233zd(E) ranks last (TTP = 4.00 K) despite its highest HTC. This work provides new experimental data and correlation assessments at medium-to-high temperature conditions that are not well represented in the existing literature, supporting more reliable design of next-generation heat pump and ORC systems.