Condensation of R-12 in small hydraulic diameter extruded aluminum tubes with and without micro-fins

Abstract

This manuscript provides heat transfer data for R-12 condensation and subcooled liquid in small hydraulic diameter, flat extruded aluminum tubes. The tube outside dimensions are 16 mm × 3 mm (high) × 0.5 rrm (wall thickness). The tubes contain three internal membranes, which separate the flow into four parallel channels. Two internal geometries were tested: one had a plain inner surface and the other had micro-fins, 0.2 mm high. Data are presented for the following range of variables: vapor qualities (12–97%), mass velocity (400–1400 kg s−1 m−2), and heat flux (4–12 kW m−2). The overall heat transfer coefficient was measured for water-to-refrigerant heat transfer, and the modified Wilson plot method used to determine the heat transfer coefficient for water-side flow in the annulus. Then, the tube-side condensation coefficient was extracted from the measured UA-value. The data show that the condensation coefficient increases with heat flux to the 0.20 power. The subcooled heat transfer coefficient for both geometries is well predicted using the Petukhov equation with hydraulic diameter. At low mass velocity, the Akers correlation agrees well with the plain tube data, and overpredicts the data 10–20% at high mass velocity. The micro-fin tube shows significantly higher performance than predicted by the Akers correlation (based on hydraulic diameter) for vapor qualities greater than 0.5. The authors propose that surface tension force is effective in enhancing the condensation coefficient for vapor quality greater than 0.5. The proposed surface tension enhancement is particularly strong at the lower mass velocities.