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

Abstract: 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 an 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 an 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 a heterogenous flow composed of slugs, which continuously expand downstream. It was also found that at higher inlet pressures the frequency at which the vapor inception occurs is higher than at lower pressures. 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.