Homogeneous versus separated two phase flow models: Adiabatic capillary tube flow in a transcritical CO2 heat pump

A comparative study of flow characteristics of an adiabatic capillary tube in a transcritical CO2 heat pump system have been investigated employing separated and homogeneous two phase flow models. Separated flow model is employed considering the annular flow pattern. The models are based on fundamental equations of mass, momentum and energy which are solved simultaneously. Two friction factor empirical correlations (Churchill, Lin et al.) and McAdams viscosity model are used. Chisholm correlation is used to calculate slip ratio while void fraction is calculated based on Premoli correlation. Sub-critical and super-critical thermodynamic and transport properties of CO2 are calculated employing a precision in-house property code.The results indicate that both two phase flow models predict reasonably well. Discrepancy between the separated flow model and homogeneous flow model has a maximum about 8 to 11%. Void fraction is influenced by vapour quality and follows the same trend as vapour quality. Liquid velocity and vapour velocity difference is relatively lower compared to R22.