Numerical simulation of coiled adiabatic capillary tubes in CO2 transcritical systems with separated flow model including metastable flow

A separated flow model of coiled adiabatic capillary tube for CO2 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 CO2. 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 CO2 increases about 7% when the coiled diameter changes from 40 mm to 600 mm.

► A separated flow model for CO2 coiled capillary considering metastable flow region.
► A modified friction factor for CO2 coiled capillary.
► Compare the four different models.
► Validate the model with the experiment data.