Comparison between Homogeneous, Separated and Drift Flux Two Phase Flow Models for Adiabatic Capillary Tube Flow in a Transcritical CO_2 Heat Pump

摘要

A comparative study of flow characteristics of an adiabatic capillary tube in a transcritical CO_2 heat pump system have been investigated employing homogenous, separated and drift flux two phase flow models. The models are based on one-dimensional fundamental equations of mass, momentum and energy. Colebrook correlation in consideration of surface roughness is used for calculating the friction factor in the single phase region and Lin et al. correlation is used for calculating the two phase multiplier. Cicchitti correlation is used for mean viscosity of the two-phase mixture. Chisholm correlation is used for calculating slip ratio in separate flow model. The drift velocity of drift flux model is calculated by Wallis correlation for bubbly flow region, Zuber et al. correlation for slug flow region and Ishii et al. correlation for annular flow region. Comparison between predicted and measured mass flow rates at different operating conditions illustrated that 95% of the predicted mass flow rates lied within ±7% error band. Homogenous flow model gave better predictions than the other models.