A theoretical model for predicting adiabatic capillary tube performance with alternative refrigerants

A theoretical model for predicting adiabatic capillary tube performance is presented. Model predictions of flow rate were compared to measured data with R-134a, R-22, R-152a, and R-410A over a flow rate range of 3 to 350 lb(m)/h (1.4 to 158.8 kg/h), which corresponded to a mass flux range of 2.39 to 16.89 lb(m)/s . in.(2) (0.17 to 1.19 kg/s . cm(2)). The range of comparison for capillary tube diameter and length was 0.026 to 0.090 in. (0.66 to 2.29 mm) and 20 to 200 in. (508 to 5,080 mm), respectively, while the range of comparison for inlet conditions was 30 degrees F (16.7 degrees C) subcooled to 30% quality. The comparison of the model predictions to the breadth of experimental data provided a comprehensive assessment of the model's accuracy. For a subcooled inlet condition and a mass flux less than approximately 6 lb(m)/s . in.(2) (0.42 kg/s . cm(2)), the model predictions were within +/-5% of measured values. However, at mass fluxes greater than 6 lb(m)/s . in.(2) (0.42 kg/s . cm(2)), the model began to consistently underpredict the measured values by up to 6%. For the quality inlet condition, the model accuracy was shown to be +/-10%. Several key modeling issues are addressed: a single-phase friction factor equation was developed for capillary tubes based on measured data from two independent sources, three two-phase viscosity models were evaluated and a recommendation of the best model was made, and a metastable flow correlation previously reported in the literature was evaluated and the limitations determined.