Experimental investigation and numerical simulation of choked refrigerant flow through helical adiabatic capillary tube

This paper presents a drift flux model and experimental study of choked refrigerant flow through both straight and helical adiabatic capillary tubes. The conservation equations of mass, energy, and momentum are solved using the fourth order Runge-Kutta method. This model is validated by previously published experimental data and also by test results performed and presented in this work for R-134a with average error of 5.5%. The effect of capillary tube inner diameter, length, relative roughness and coil diameter, and also various test conditions such as inlet pressure, inlet temperature, and sub-cooling degree of refrigerants are investigated. Critical mass flux variation, pressure distribution and temperature variation are obtained experimentally as well as vapor quality, vapor velocity and void fraction variation by numerical simulation. The results show that mass flux reaches a maximum amount at a specific value of evaporator pressure in choked conditions and also it is decreased by increasing the length of capillary tube. Moreover, critical mass flux increases by increasing of the tube inner diameter, condensation temperature and refrigerant degree of sub-cooling.