Evaporation heat transfer coefficients for R-22 in micro-fin tubes of different configurations
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Evaporation heat transfer coefficients and pressure drops were measured for HCFC-22 in a series of mechanically expanded micro-fin tubes of 9.52 mm (3/8 in.) and 7.94 mm (5/16 in.) diameter. Specifically, five different 9.52 mm (3/8 in.) expanded micro-fin tubes with different fin cross sections were tested, while three of these micro-fin tube geometries were also tested with 7.94 mm (5/16 in.) diameters. In order to create realistic tube diameters and fin-tip conditions, all of the tubes tested were mechanically expanded by the procedures and methods used during assembly of air-conditioning evaporator coils. Tests were conducted at evaporation temperatures of 2C and 7C over a range of mass fluxes. For the 9.52 mm (3/8 in.) tubes, a mass flux range of 150 kg/m2s to 350 kg/m2s was used, while the 7.94 mm (5/16 in.) tubes were tested over a mass flux range of 135 kg/m2-s to 400 kg/m2-s. Inlet qualities of 8% to 15% and outlet qualities of 80% to 88% were used for all tests. The heat transfer coefficients and pressure drops that were measured were averages over the 3.67 m long test tube. Comparing the average heat transfer coefficients for the different 9.52 mm (3/8 in.) tubes showed that some differences did exist in the performances of the different fin designs. However, the differences were not large. Over the range of conditions tested, the 3/8 in tube results varied by less than 25% and the 7.94 mm (5/16 in.) tubes by less than 17%. Heat transfer enhancement factors for the micro-fin tube, defined as the heat transfer coefficient for the micro-fin tube divided by the heat transfer coefficient of a smooth tube at similar conditions, varied from 2.5 to 1.8 for these micro-fin tubes.
