The heat transfer in the condenser sections of conventional and annular two-phase closed thermosyphon tubes has been studied experimentally and analytically. In addition, the results of a series of experiments on the flooding phenomena of the same thermosyphons are reported. Freon 113 and acetone were used as working fluids. An improved correlation was developed to predict the performance limits of conventional thermosyphons using the present and previously existing experimental data for flooding with different working fluids. The prediction of the theoretical Nusselt number for the situations associated with measured heat transfer coefficients in the condenser section indicated that the effect of interfacial shear on the film flow is small. The increase of the experimental reflux condensation heat transfer coefficients over theoretical predictions is attributed to waves at the vapor–liquid interface.

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