Numerical simulation of coiled adiabatic capillary tubes in CO ₂ transcritical systems with separated flow model including metastable flow

A separated flow model of coiled adiabatic capillary tube for CO ₂ considering metastable flow region is presented. In the case of straight capillary, different friction factors are compared with each other, the model using Churchill and Lin correlations gives a deviation of -1.8% compared with the experimental data for CO ₂. Three different friction factor correlations (Schmidt, M&N and M&N modified) are discussed for coiled capillary tube. The results show that the M&N modified friction factor gives the best reasonable prediction with a deviation of 4% compared with the experimental data for R22. The discrepancy between the capillary tube length calculated with the separated flow model and that calculated with homogeneous flow model is about 2-7% for the chosen conditions. The mass flow rate for CO ₂ increases about 7% when the coiled diameter changes from 40 mm to 600 mm.