Flow condensation in enhanced tubes: heat transfer and fiction

Abstract

To investigate the mechanism of condensation heat transfer enhancement of refrigerant R32 in helix micro-fin (HX) tubes, experimental studies were conducted on the heat transfer coefficient (HTC) and frictional pressure drop (FPD) of two HX tubes with different outer diameters (5 mm-HX#1 and 9.52 mm-HX#2) and their corresponding smooth (ST) tubes. The experimental conditions covered saturation temperatures from 36 °C to 48 °C, mass fluxes from 70 to 450 kg m−2 s−1, and vapor qualities from 0.1 to 0.9. The results indicate that the heat transfer enhancement factor (EF) of HX#1 and HX#2 reached its maximum at low mass flux (up to 2.93), with the enhancement primarily attributed to increased fluid turbulence and improved condensate drainage. The performance evaluation factor (PEF) also peaked at low mass flux (up to 2.7) and decreased with increasing mass flux. Moreover, the HTC of the HX tubes exhibited weak dependence on mass flux, and for HX#2, the HTC showed an overall decreasing trend as mass flux increased. Based on the experimental data, a new HTC correlation was developed by introducing dimensionless parameters that characterize both flow pattern transitions and geometric structure. The proposed correlation predicted the HTC for HX#1 and HX#2 with mean relative errors (MRE) of +0.7% and −3.2%, respectively, providing a theoretical foundation for the design of enhanced tubes.