Abstract

Capillary tubes are components in refrigerating systems which act as expansion and flow-rate controlling devices. The proper design of capillary tubes is an important factor in performance efficiency of these systems. An experimental test loop was designed and built for testing the capillary tube. This test loop simulated the actual refrigeration system that normally supplied refrigerant with or without oil to the capillary tube, while providing easily controlled boundary conditions. Pressures and temperatures along the capillary tube and suction line were also instrumented. The capillary tube was tested for a wide range of conditions including: subcooled inlet, diabatic and adiabatic capillary tube, refrigerant HFC-134a flow with and without oil. The process of refrigerant flow through a capillary tube is a flashing process, in which the state of the refrigerant changes from subcooled liquid to a vapor-liquid mixture. Metastable phenomena in the flow, i.e. a thermal nonequilibrium phenomena, may exist within a flashing process. Whereas a literature survey showed that the metastable flow of refrigerant was only observed in adiabatic capillary tubes, the experiment demonstrated metastable flow through the diabatic capillary tube for the first time. It is found from the experiment that the metastable flow will vanish when $\rm G\sb{c}/G\sb{s}<77.8.$ A correlation of the underpressure of vaporization (a characteristic quantity for metastable flow) for a diabatic capillary tube, based on homogeneous nucleation theory, was developed to consolidate this new result. The flow in the capillary tube was numerically modeled by dividing the flow into two regions: single-phase and two-phase. The liquid single-phase flow region was described by four governing differential equations and boundary conditions. The numerical model for the two-phase flow region used one-dimensional nonequilibrium two-fluid equations which accounted for the relative velocity and the temperature difference between the vapor and liquid phases. Results of the numerical models compared satisfactorily with measured experimental data. The results indicated that the vaporization occurred near the capillary tube exit with a strong heat transfer between the capillary tube and suction line; the mass flux of refrigerant in the capillary tube without oil was about 2.2% higher than that with oil