Studying the influence of refrigerant type on thermal efficiency of annular two-phase flows; mass transfer viewpoint

Abstract

Many parameters influence the thermal efficiency of two-phase systems; among them, the type of refrigerant employed in two-phase systems is of great importance. Carbon dioxide has been reintroduced as a possible R22 replacement, because of having more heat transfer rate and lower pressure drops, along with better environmental treatment compared with widely-used refrigerants. In the present article carbon dioxide is studied and compared with some thermophysically-different refrigerants from the viewpoint of probability of dry-out occurrence. Dry-out phenomenon in two-phase systems should be avoided as far as possible to prevent sudden drops in heat transfer. Dry-out occurrence is strongly influenced by entrainment mass transfer. In the present study a semi-empirical model is proposed for simulation of entrainment mass transfer in annular flow regime of liquid-vapor in a vertical tube. The significance of entrainment phenomenon in carbon dioxide is compared with that of some other refrigerants to figure out the probability of dryout occurrence in different refrigerants. It will be demonstrated that CO2 relative to other refrigerants has much lower amounts of entrainment. This issue along with other mentioned advantages shows the prominent effectiveness of carbon dioxide among other conventional refrigerants.