Operation regimes and heat transfer coefficients in sodium two-phase thermosyphons

•The filling ratio on sodium two-phase thermosyphon was studied. In all cases, the thermosyphons presented three operating regimes: fin, Geyser Boiling and ideal.•A new correlation is proposed for the heat transfer coefficient of sodium two-phase thermosyphon under nucleate pool boiling regime.•A new correlation is proposed for the film condensation heat transfer coefficient of sodium two-phase thermosyphon.•The results show that the proposed correlations allow to determine the heat transfer coefficient with reasonable precision in the three operating regimes.•The heat transfer coefficient in the Geyser Boiling regime is bonded by two limits: the low and the high performance, which is well predicted by the proposed model. The dry-out, cold tip and Geyser Boiling phenomena are highly undesirable in projects involving two-phase thermosyphons with sodium as working fluid. Therefore, for the application of this technology, mathematical models that allow for the prediction of the behavior of sodium two-phase thermosyphon are highly desirable. The present work aims to study the influence of the filling ratio on several phenomena, such as dry-out, cold tip, Geyser Boiling and evaporator wall overheating, so that the thermosyphon overall performance can be accessed. In addition, a correlation is proposed for the heat transfer coefficient of sodium under nucleate pool boiling regime. A correlation for the film condensation heat transfer coefficient, based on the experimental data obtained in this work, is also proposed. The results show that the proposed correlations allow for the determination, with reasonable accuracy, of the heat transfer coefficients in the evaporator and in the condenser, considering the undesired effects of cold tip and Geyser Boiling.