Simulation on the performance of adiabatic capillary tube in CO₂ transcritical refrigeration system

A one dimension homogeneous steady model of adiabatic capillary tubes in CO₂ transcritical refrigeration systems was established to evaluate the performance and the flow characteristics of capillary tubes. The simulation results by using three different friction factor correlations (Churchill, Colebrook, Bittle & Pate) were compared with each other. The distributions of temperature, pressure, enthalpy and entropy along the tube length were investigated. The influences of four parameters (inner diameter, inlet pressure, inlet temperature, back pressure) on the mass flow rate were explored, and the chocking phenomenon was analyzed. The results show that the Churchill correlation and the Colebrook correlation predict 92% of the refrigerant mass flow rates with an error of less than 10% while the Bittle & Pate correlation may be unsuitable for simulating the flow in CO₂ capillary tubes due to its neglect of the inner wall roughness of capillary tubes. The back pressure has slight influence on the mass flow rate. In addition, the rate of mass flow without chocking is approximately equal to that with chocking.