Adiabatic capillary tube model for a carbon dioxide transcritical cycle

This paper presents a mathematical model for a capillary tube using CO2 as fluid in steady flow transcritical cycle. The capillary tube is divided into N volumes controls and the model is based on applying the equations of conservation of energy, mass and momentum in the fluid in each of these volumes controls. The model calculates the mass flow of the CO2 in the capillary tube as a function of CO2 pressures at the inlet and outlet of the capillary and the temperature of CO2 at the input of this device. The capillary tube is considered to be adiabatic, and the limit of operation due to blocked flow condition is also considered in the model. The validation of the model was performed with experimental data and the results showed that the model is capable of predicting the mass flow in the capillary tube with errors less than 10%. The model was also used to determine the minimum diameter of the capillary tube for various conditions of CO2 transcritical cycle.