Algebraic solution and experimental validation for adiabatic coiled capillary tubes operating in transcritical CO2 cycle

Abstract In this paper, an algebraic solution to study adiabatic coiled capillary tubes in transcritical CO2 cycle was proposed. This solution evaluated three different friction factors and three possible k factors (related to the specific volume), with a total of nine combinations. A correlation to estimate the sand grain roughness was also proposed. To validate the algebraic solution, and to be useful for future studies, experimental tests were performed with two different capillary tubes, in a total of 60 points. The results showed that 95% and 98.3% of the predicted mass flow rates fell within ± 10% and ± 15% error bands, respectively, with average deviation (AD) and absolute average deviation (AAD) of 0.1% and 4.4%, respectively. Overall, the C-M&N (M&N modified) friction factor performed better, however, the Schmidt friction factor was the only able to predict 100% of data within a ± 12% error band. An algebraic equation and a dimensionless correlation for straight capillary tubes were also adapted for the coiled shape, presenting a satisfactory performance. It could be concluded that the proposed solution was found to be an adequate tool for this application. To the best of the authors knowledge, an algebraic solution and experimental data for coiled capillary tubes in transcritical CO2 cycle are presented for the first time.