Condensation, Heat transfer, Pressure drop, Microfin, Small-diameter, R1234ze(E), Fin geometries
In the refrigeration and air-conditioning fields, small-diameter microfin tubes with an outside diameter of 5 mm or less have been developed; these tubes enhance energy savings by improving the performance of the heat exchanger and by reducing the charge amount of the refrigerant for downsizing the heat exchanger. Small-diameter microfin tubes with 4 mm outer diameters have been utilized as a part of practical applications. It is important to investigate heat transfer and pressure drop characteristics at lower mass velocities. Furthermore, hydrofluoroolefin (HFO) refrigerants have been recently attracting attention as low global warming potential (GWP) refrigerants. However, only limited studies exist on condensation heat transfer of HFO refrigerants in small-diameter microfin tubes. This study experimentally investigated the condensation heat transfer and pressure drop characteristics of R1234ze(E) inside horizontal small-diameter 4.0 mm OD microfin tubes having three different types of fin geometries. The specifications of the three fin geometries were 40 fins with a fin height of 0.18 mm and a helix angle of 18°, 50 fins with a fin height of 0.15 mm and a helix angle of 12°, and 50 fins with a fin height of 0.12 mm and a helix angle of 25°. The experiments were carried out for a range of mass velocities, from 50 to 400 kgm-2s-1, and at a saturation temperature of 35 °C. The effects of fin geometries such as the number of fins, fin height, and helix angle on the heat transfer and pressure drop were investigated. The heat transfer coefficient increased as the number of fins increased for the lowest mass velocity. Fin height was most effective on heat transfer enhancement at higher mass velocities. The heat transfer coefficient and pressure drop of the microfin tubes were compared with those of smooth tubes and were evaluated in terms of the enhancement ratio of heat transfer and the increment ratio of pressure drop. The measured heat transfer coefficient and pressure drop were compared with previous correlations for small-diameter microfin tubes.