Comparative thermo-fluidic analysis of condensation characteristics inside smooth and enhanced tubes

Abstract

Three-dimensional simulations are performed to investigate the thermo-fluidic characteristics during the flow condensation of representative R134a refrigerant inside smooth and enhanced tubes. Along with a perfectly smooth surface, four different surface structures, i.e., hemispherical ribs, conical fins, axial, and circumferential continuous protrusions (tunnels and huddles) on the inner surface of the tube are tried to understand the heat transfer enhancement mechanism. Simulations are carried out for the mass flux ranging from 100 to 200 kg m−2 s−1 and inlet vapor quality of 0.8 at a phase change temperature of 40 °C. The effect of structures on the flow behavior is analyzed, and the presence of directional condensate drainage near the protrusions is observed. The qualitative and quantitative examinations of interfacial structures at different axial and longitudinal sections are also presented to better understand the distinctive condensation phenomenon for smooth and enhanced tubes. The spatial and time-averaged vapor fraction and liquid film thickness show lower values in the case of enhanced tubes compared to the smooth surface for all tested operating conditions. Furthermore, the hemispherical rib structure showed the highest heat transfer coefficient among the tested structures, whereas a tube with circumferential protrusions (huddles) results in maximum pressure drop during flow condensation. The benefits of heat transfer enhancement appear to be more than the pressure drop penalty for tubes with a conical fin structure and axial tunnels.