Modelling of heat and mass transfer in a two-phase closed thermosyphon

A CFD model is developed to simulate the heat and mass transfer inside a two-phase closed thermosyphon. Based on the "volume of fluid" method, governing equations are solved using the OpenFOAM utilities. The involved complex phenomena like evaporation and condensation occurring in such a device and associated with twophase flow are investigated. A noticeable novelty of this model is that neither artificial, ad hoc nor specific nucleation means is needed to trigger evaporation along the heat source walls; an altogether unique phase change model is valid and uniformly used within the flow occurring inside the thermosyphon. The numerical results are compared to well-documented experimental data showing very good agreement: maximum deviation of 1 % and 2.7 % are obtained for mean temperature and condenser pressure, respectively. The axial thermal resistance values are also compared with the experimental data. Fair agreement was obtained in the overall equivalent resistance (26.4-32.5 %) and in the evaporator axial resistance (25-27.5 %). In addition, the evaporator slug flow pattern is successfully captured by the model. The liquid fraction distribution inside the thermosyphon is analyzed as time evolves. It shows, in particular, that bubble activation and growth have the same trend as previous experimental visualization results, namely denser activation close to the meniscus. The velocity distribution shows also recirculation in the top of the condenser zones and above the liquid meniscus in the evaporator zone, too, as a direct effect of buoyancy and natural convection. As activated bubbles rise and coalesce into larger bubbles, the occurrence of a slug flow is observed. Furthermore, the tested model had shown its efficiency to predict the main fluid flow and thermal characteristics of a thermosyphon when pseudo-steady state is reached.