Metastable flow of R-410A in capillary tubes

This work presents the results of an experimental study on the metastable flow of R-410A through adiabatic capillary tubes. Capillary tubes with internal diameter of 1.089, 1.381 and 1.643 mm, and length of 1500 mm, were tested for condensation temperatures ranging from 45 °C to 55 °C and subcooling degrees between 4 °C and 10 °C. Underpressure of vaporisation and metastable region length were evaluated from temperature profiles, inlet pressure, mass flow rate and friction factor equations for each capillary tube. A correlation based on experimental data was developed for predicting underpressure of vaporisation. Such correlation presented an average error of 21.8%, and predicted 82% of the experimental values within a ±30% range. The correlation was included in a capillary tube simulation model previously developed by the authors, and the validation indicated that, for smaller diameters, the introduction of the metastability effect virtually eliminates the difference between mass flow rate experimental values and simulation results, with small remaining differences that can be credited to numerical errors and measurement uncertainties. For the larger tube diameter, wherein the metastability phenomenon is less intense, the simulation model shows mass flow rate differences ranging from +2.2 to +5.3% for 85% of the cases.