Entrainment mass transfer in annular flow regime of two-phase carbon dioxide

Abstract

Having more heat transfer rate and lower pressure drop, along with better environmental treatment, carbon dioxide has drawn the attention of many researchers. Most of the two-phase heat transfer analyses are involved in estimation of dry-out phenomenon in order to avoid heat transfer drops. Entrainment, because of having great influence on dry-out occurrence, is studied in this article. Most of the available models for entrainment are experiment-based and therefore applicable in restricted conditions. In the present study, a physical model is proposed for simulation of entrainment in annular flow regime of liquid-vapor in a vertical tube. The tube is supposed to be warming up by a constant and uniform heat flux applied on its lateral area. The proposed model is general and can be applied to all kind of fluids of different working conditions; it is verified with some experimental data available in the literature. Entrainment contribution of mass transfer is compared with evaporation contribution through introducing a factor which is called “Mass Transfer Ratio” (MTR). The magnitude of this factor demonstrates the significance of entrainment phenomenon. MTR factor is compared with some widely-used working fluids. Key words: Carbon dioxide, working fluid, entrainment, dry out, mass transfer.