Numerical Calculation of Mass Flow Rate in Capillary Tubes Using â⇬˜Artâ⇬◢, an Advanced Simulation Software

A capillary tube is a common expansion device used in small sized refrigeration and air-conditioning systems. This paper present a numerical method used to calculate mass flow rate in a capillary tube. The proposed method solves the conservation laws (continuity, momentum and energy) in a 1D mesh. A iterative process is performed for a guessed value of the mass flow rate, and it is followed until critical flow conditions are obtained, the resulting length is compared with the capillary tube length and a new guess of the mass flow rate is imposed, and then, the iteration is repeated until convergence. In two phase flow, a separated flow model is assumed. Both, two-phase friction factor and viscosity models were determined by Lin correlation, and void fraction from Zivi correlation. This model is included in ‘IMST-ART’, a software for simulation and design of refrigeration equipment. The addition of capillary tube model allows to calculate the superheat at the evaporator giving the capillary tube geometry. A simulation with different operative conditions and capillary tube geometry is presented and the results are compared with those given by ASHRAE correlation.