Flow behavior of spiral capillary tube for CO2 transcritical cycle

Abstract

A numerical study has been carried out on the spiral capillary tube for the CO2 transcritical cycle. The model is based on the basic principles of conservation of mass, momentum, and energy. The results of this model are validated with earlier published test results. The effect of various geometric parameters like tube diameter, length, roughness, and pitch on the mass flow rate, cooling capacity, and COP has been calculated. The mass flow rate of the tube is mostly influenced by the internal tube diameter, as the diameter increases by 28%, the mass flow increases by 88%. A minor change in mass is observed with a change in pitch and surface roughness, as the pitch increases from 300 to 700 mm, and surface roughness increases by 14%, the mass flow rate and cooling capacity increase only by 2% and 1%, respectively. Similarly, the influence of various operating factors like gas cooler pressure, evaporator temperature, and gas cooler temperature is evaluated. A significant change in mass is observed with the change in gas cooler temperature, as the gas cooler temperature increases by 5%, the mass flow rate, cooling capacity, and COP decrease by 19%, 40%, and 33%, respectively. Relatively less variation in mass flow rate is observed with the change in evaporator temperature. As the evaporator temperature increases by 12%, the mass flow rate decreases by nearly 5.5%, and cooling capacity decreases by 18%. Energy and exergy analyses of a CO2 transcritical system are carried out.