CONDENSATION OF R-12 IN SMALL HYDRAULIC DIAMETER EXTRUDED ALUMINUM TUBES WITH AND WITHOUT MICRO-FINS

This manuscript provides heat transfer data for R-12 condensation and subcooled liquid in small hydraulic diameter, flat extruded aluminum t ubes. The tube outside dimensions are 16 mm x 3 mm (high) x 0.5 mm (wa ll thickness). The tubes contain three internal membranes, which separ ate 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 Wils on plot method used to determine the heat transfer coefficient for wat er-side flow in the annulus. Then, the tube-side condensation coeffici ent was extracted from the measured UA-value. The data show that the c ondensation coefficient increases with heat flux to the 0.20 power. Th e subcooled heat transfer coefficient for both geometries is well pred icted 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 Ake rs correlation (based on hydraulic diameter) for vapor qualities great er than 0.5. The authors propose that surface tension force is effecti ve in enhancing the condensation coefficient for vapor quality greater than 0.5. The proposed surface tension enhancement is particularly st rong at the lower mass velocities.