Internal flow condensation of R515B and R1233zd(E) in smooth and structured round mini channels

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Flow condensation is a cornerstone of refrigeration systems, heat pumps, and electronics cooling technologies, and its enhancement is critical for improving efficiency and reducing heat exchanger size. Numerous enhancement strategies have been proposed for flow condensation, including micro- and nanostructured surfaces which have been investigated for their potential to promote condensate film redistribution. In this study, the coupled effects of tube diameter, refrigerant properties, and surface structuring on flow condensation performance are systematically investigated. Experiments are conducted in tubes with internal diameters ranging from 1.5 mm to 4.7 mm using two low-GWP refrigerants, R515B and R1233zd(E), on etched aluminum and etched copper surfaces. A counter-flow tube-in-tube facility is employed, with particular attention to the experimental challenges associated with small diameter flow condensation measurements, including flow oscillations, streamwise saturation temperature gradients, and uncertainty control. The results show that heat transfer enhancement is achieved only within a narrow vapor quality range governed by the interaction between condensate film thickness and surface feature size. Averaged over the highest vapor qualities (0.7–1), enhancements of up to 45% are obtained on etched aluminum tubes with a diameter of 2.4 mm, while etched copper tubes exhibit enhancements up to 55% with a diameter of 3.1 mm. Film thickness and flow regime analyses indicate that surface structures become flooded at vapor qualities as high as 0.8 ( x ∼ 0 . 8 ), suppressing capillary-driven film redistribution and eliminating enhancement occurring at lower vapor qualities. These findings establish critical thresholds for effective condensation enhancement in structured tubes.