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.
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