An experimental study of pool boiling and falling film vaporization on horizontal tubes in R-245fa

This study investigates heat transfer performance on horizontal copper tubes in refrigerant R-245fa, having zero ozone-depleting potential. Pool boiling and falling film vaporization experiments were conducted at saturation temperatures of 5 °C (a typical saturation temperature of evaporators in commercial chiller units) and 20 °C, while heat fluxes varied from 4.5 to 48.5 kW/m2. A smooth tube, a fin tube of 0.4 mm fin height, 60 FPI (Fins Per Inch), and a new boiling enhanced tube (mesh tube) were tested. Falling film vaporization heat transfer coefficient of the fin tube is greater than that in pool boiling, but the opposite results were found for the mesh tube. The mesh tube yielded up to 5 and 7 fold heat transfer enhancement in falling film vaporization and pool boiling, respectively. Under the same test conditions, R-245fa resulted in slightly higher heat transfer coefficients than R-123. New correlations of falling film vaporization on smooth tubes with and without boiling are proposed. The falling film evaporation (non-boiling) correlation predicted the data within ±16%. The falling film vaporization correlation accounting for nucleate boiling predicted heat transfer coefficients with ±20% uncertainty for data of smooth tube in the range of Ref = 115–372, BoF = 0.044–0.473, WeF = 0.000165–0.00168 and Prf = 6.26–7.15.