No abstract

An experimental investigation was performed with R22 and R410a for single-phase flow, evaporation and condensation inside five micro-fin tubes of various geometries to obtain pressure drop and heat transfer characteristics. The results suggest single-phase flow heat transfer coefficients are increased by 46% to 64% compared with smooth tubes values. Tube geometries that had higher evaporation heat transfer coefficients or higher condensation heat transfer coefficients were identified. Condensation pressure drop characteristics also varied with tube geometry. Based on experiment data, a new correlation which contains the characteristics of a liquid film in annular flow is established. The new correlation can predict the experimental data within an error band of 15% and, for 77% of the data from the literature, within an error band of 30%. The Choi et al. correlation can predict the present condensation pressure drop data within a 20% error band and the Yu and Koyama correlation can predict the present condensation heat transfer coefficient data within 25%.

An experimental investigation was performed for convective condensation of R410A inside four micro-fin tubes with the same outside diameter (OD) 5 mm and helix angle 18°. Data are for mass fluxes ranging from about 180 to 650 kg/m2s. The nominal saturation temperature is 320 K, with inlet and outlet qualities of 0.8 and 0.1, respectively. The results suggest that Tube 4 has the best thermal performance for its largest condensation heat transfer coefficient and relatively low pressure drop penalty. Condensation heat transfer coefficient decreases at first and then increases or flattens out gradually as G decreases. This complex mass-flux effect may be explained by the complex interactions between micro-fins and fluid. The heat transfer enhancement mechanism is mainly due to the surface area increase over the plain tube at large mass fluxes, while liquid drainage and interfacial turbulence play important roles in heat transfer enhancement at low mass fluxes. In addition, the experimental data was analyzed using seven existing pressure-drop and four heat-transfer models to verify their respective accuracies.