| This work addresses condensation and single-phase heat transfer to R-12 and R-134a at 65$spcirc$C saturation temperature in small hydraulic diameter, flat extruded aluminum tubes. The tests include (a) subcooled liquid pressure drop, (b) subcooled liquid cooling heat transfer coefficient, (c) two-phase flow pressure drop, and (d) condensation heat transfer coefficient. All pressure drop data were measured for adiabatic flow. The tubes tested include two plain tubes with hydraulic diameters of 1.33 mm and 2.64 mm, and two micro-fin tubes with hydraulic diameters 1.56 mm and 1.41 mm. The fin heights for the two micro-fin tubes are 0.2 mm and 0.3 mm respectively. The data span vapor qualities of 0.12-0.97, mass velocities of 400-1400 kg/s-m$sp2,$ and heat fluxes of 4-12 kW/m$sp2.$ The condensation data show that the pressure drop is dominated by vapor shear in both plain and micro-fin tubes. However, the surface tension force is effective in enhancing the condensation coefficient at vapor qualities approximately above 0.5. The enhancement is increased as mass velocity is reduced. A semi-empirical model that includes vapor shear and surface tension effects was proposed to predict the condensation coefficient in micro-fin tubes. The proposed model predicts 95% of the data within ${pm16%}.$. |
