Experimental study of the heat transfer coefficient and pressure drop of R1234yf condensing flow in flattened smooth tubes

The present study investigates the heat transfer coefficient and pressure drop of refrigerant R1234yf condensing flow in flattened tubes. Three tubes with internal heights of 6.2, 5.1 and 2.8 mm (aspect ratios of 1.5, 2 and 4), made by flattening 8.2mm inner diameter round tubes, were used. The tests were conducted at a mass velocity range of 95-380 kg m(-2) s(-1) and vapor quality range of 0.15-0.8. The effects of tubes flattening, mass velocity and vapor quality on heat transfer coefficient and pressure drop were studied. The results indicated that using flattened tubes increases the heat transfer coefficient and pressure drop. However, at low mass velocities, flattening does not have a significant effect on the heat transfer coefficient. Based on the results, two correlations were proposed to predict the heat transfer coefficient and pressure drop in flattened tubes by modifying the existing correlations in the literature. The performance of flattened tubes was evaluated by defining the performance evaluation criteria (PEC), considering both heat transfer enhancement and pressure drop increase at different working conditions. The highest PEC was observed for the tube with an aspect ratio of 4 at the mass velocity of 380 kg m(-2) s(-1). (C) 2019 Elsevier Ltd and IIR. All rights reserved.