An experimental study of refrigerant expansion inside coiled adiabatic capillary tubes and development of a general correlation

This article provides experimental results on the expansion of a refrigerant inside adiabatic helically coiled capillary tubes. The experimental data were obtained in capillary tubes of 0.787, 0.914 and 1.041 mm internal diameter, 2 and 3 m length, 83 mm coil diameter, mass flow rates between 2.7 and 14.3 kg/h, R134a as refrigerant fluid, and inlet subcooling between 6 and 47 °C. The uncertainties of the results for mass flow rate were below 4.5%. Ten prediction methods from the literature were compared with the experimental data, and the best prediction achieved 11.9% mean absolute error. A novel, general and simple dimensionless correlation for coiled and straight adiabatic capillary tubes based on a regression with an extended database containing eleven different refrigerant fluids is proposed. The correlation predicted the extended database with 5.7% mean absolute error and can be applied for subcooled, two-phase or supercritical inlet conditions.