Analysis of R515B condensing in microfin tubes at multiple inclinations

Refrigeration plays a vital role in modern society, with applications ranging from food preservation and biomedical systems to civil air conditioning and data center cooling. However, many conventional refrigerants, such as R134a, are hydrofluorocarbons (HFCs) with high global warming potential (GWP). To mitigate their environmental impact, regulations like the Kyoto Protocol and the European F-gas Regulation 2024/573 have mandated the progressive phaseout of HFCs, targeting a complete elimination by 2050 (Regulation (EU), 2024). This context has spurred the development of alternative refrigerants with lower environmental footprints. Hydrofluoroolefins (HFOs), particularly when blended with HFCs, offer a promising compromise between thermodynamic performance and environmental safety. Among these, R515B—a non-flammable azeotropic blend of 91.1% HFO-1234ze(E) and 8.9% HFC-227ea—has emerged as a viable replacement for R134a. It exhibits a low GWP of 293, good thermodynamic properties, and belongs to ASHRAE safety class A1 (Mateu-Royo et al., 2021; Yildirim et al., 2022; Tangri et al., 2023). Recent studies have explored R515B in various thermodynamic systems. Azzolin et al. (2022) examined its condensation performance in small-diameter channels and found that its heat transfer coefficients (HTCs) were similar to those of R1234ze(E), while frictional pressure drops (FPDs) were slightly higher. Liu et al. (2022) reported comparable HTC and FPD behavior between R515B and R1234ze(E) under flow boiling conditions. However, most studies focus on thermal performance metrics, with limited attention to flow visualization particularly inside enhanced tubes under inclined configurations. Inclination plays a fundamental role in two-phase flows by altering gravitational stratification, phase slip, and wall wetting. Micro-fin tubes, in particular, show amplified sensitivity to inclination due to the secondary flow structures induced by the fin geometry (Huang et al., 2022). To address this gap, the present study provides a systematic experimental investigation of the two-phase condensation flow patterns of R515B inside two helically micro-finned tubes (4 mm, 5 mm, and 7 mm OD), each tested at different five inclinations of −60°, −30°, 0°, 30°, and 60°. A high-speed camera was used to capture and classify flow regimes, enabling a deeper understanding of flow regimes and heat transfer coefficient (HTC) under varying gravitational alignment.