Two-phase flow patterns in adiabatic refrigerant flow through capillary tubes

•The flow of R600a through capillary tubes was visualized with a high-speed camera.•The flow was found to be a predominantly inhomogeneous slug flow.•FEP and copper capillaries provide a similar pressure drop and mass flow rate.•A discrete jump in the vaporization point was achieved by creating imperfections.•Different surface textures could influence the homogeneity of the flow. This paper presents the results of an experimental study of the flow visualization of the refrigerant R600a through a capillary tube with an internal diameter of 0.8 mm made from a FEP (fluorinated ethylene propylene) polymer that was installed in a small-scale, vapor-compression refrigeration system. The main purpose of the study was to determine the flow patterns of the refrigerant flow in capillary tubes under different operating conditions and to verify whether the results obtained using a FEP tube are representative for the flow of a refrigerant in copper capillary tubes. The results of the study revealed that the two-phase refrigerant flow in the FEP capillary tubes consists predominantly of slug flow. These slugs continuously expand while traveling downstream. It was also found that different operating conditions can have a strong influence on the frequency of the vapor bubbles’ formation and their growth. In addition, an experimental study of the effect of local or global imperfections on the inner surface of the FEP tube was performed. In some cases the imperfections increased the number of vapor-bubble inception points, which caused a homogenization of the two-phase flow.