Separation technology has been widely developed to improve the performance of thermodynamic cycles, such as refrigeration, heat pump and power cycle. As a promising separator, T-junction has attracted widespread attention, due to the characteristics of simple geometry, low cost and compact size. Previous researches on the phase separation of air-water and steam-water have indicated that the separation performance depends heavily on the T-junction geometry. Therefore, in this work, geometric effects on phase separation of refrigerant were experimentally studied with different configurations of branching T-junction. In this study, all of the T-junctions had horizontal inlet and outlet tubes. The internal diameter of inlet tube was fixed at 8.0 mm, and the diameter ratio of the branch to the inlet was set to be 0.75 and 1.0. Furthermore, three branch angles, 45°, 90° and 135°, were considered. More than 156 experimental runs were conducted using refrigerant R-600a with inlet mass flux and vapor quality being varied from 200 to 300 kg·m−2·s−1 and from 0.1 to 0.9, respectively. Meanwhile, the mass flux of the branch was regulated by keeping the mass flow ratios in three levels: 0.3, 0.5 and 0.7. Flow regimes prior to the T-junction were identified and characterized. Based on these generated data, phase separation performance was analyzed in terms of mass flow ratio under given inlet vapor quality and mass flux. Effects of diameter ratio and branch angle were also investigated. From the test results, it can be concluded that the outlet quality of branch is always higher than the inlet quality. The fraction of vapor extracted into the branch decreases with the increase of inlet vapor quality. Under the same experimental conditions, the higher the mass flow ratio is, the larger the vapor fraction, while the inlet mass flux has little influence on the phase separation of refrigerant. As for the geometric effects, more vapors usually prefer to flow into the branch with the smaller diameter, when the mass flow ratio is relatively large. For the effect of branch angle, at low vapor quality, vapor fraction for the angle 45° is lower than those for angle 90° and 135° under the mass flow ratio of 0.5 and 0.7
