Evaporative Heat Transfer and Pressure Drop Performance of Internally-Finned Tubes with Refrigerant 22

Abstract

Heat transfer and pressure drop measurements were performed on three integral spiralled inner-fin tubes (12.7–15.9 mm OD, 30–32 fins, fin height 0.5–0.6 mm) with two-phase flow of refrigerant 22 under evaporating conditions. The data were compared with the performance of smooth tubes with and without a star-shaped insert. Based on the same length of heated test section (0.80 and 2.44 m), change in refrigerant quality (0.2 and 0.7) and mass velocity range (65,000 to 270,000 g/s · m2): (1) The enhancements in heat transfer coefficient for the internally-finned tubes over those for the smooth tubes ranged from 30 to 760 percent, and typically increased with mass velocity. Tighter fin spiralling significantly increased heat transfer. (2) The enhancements in heat transfer coefficient for the smooth tube with the star-shaped insert ranged from 40 to 370 percent, but decreased with mass velocity. (3) The increases in pressure drop for the internally-finned tubes over those for the smooth tubes ranged from 10 to 290 percent, while those for the smooth tube with the star-shaped insert were 300 to over 2000 percent. The factors enhancing the performance of the internally-finned tubes include the low fins which result in only a small reduction in cross-sectional flow area, and the tight spiral which increases the corner length per unit length of tube available for nucleation of vapor bubbles.