Local heat transfer measurements during in-tube condensation of refrigerants in a new test section made by additive manufacturing

Condensation process inside small diameter channels is nowadays of considerable interest in refrigeration, heat pumps and cooling applications. The reduced cross-sectional area of the channels enables compact heat exchangers design but it makes local heat transfer coefficient measurements more difficult during condensation. High-quality data collection is essential for developing and validating predictive models for heat exchangers’ design. To address this challenge, a novel test section having a 2.76 mm inner diameter channel was built by additive manufacturing for accurate local heat transfer coefficient measurements with refrigerants and using water as secondary fluid for heat rejection. The design, optimized by computational fluid dynamics (CFD) simulations, resulted in a complex finned geometry of the water side. The fabrication of the test section was achieved using laser powder bed fusion with AlSi10Mg alloy. Given the limited number of experimental data available for the low-GWP (Global Warming Potential) refrigerants in small-diameter channels, condensation tests were performed with R1234ze(E) and R1233zd(E) at 40 ◦C saturation temperature and mass flux ranging from 30 to 350 kg m-2 s-1. Results at 0.5 vapour quality show that, R1233zd(E) achieves 62 % higher heat transfer coefficients than R1234ze(E) at high mass flux (330–350 kg m-2 s-1) and 18 % higher at low mass flux (60 kg m-2 s -1). High-speed videos revealed distinct flow patterns that can be linked to the heat transfer coefficient trends. Finally, the accuracy of selected heat transfer coefficient correlations was assessed, showing good predictive capability for R1234ze(E) but lower accuracy in the case of R1233zd(E).