Analytical and Experimental Analysis of a High Temperature Mercury Thermosyphon

High temperature thermosyphons are devices designed to operate at temperatures above $400°C$⁠. They can be applied in many industrial applications, including heat recovery from high temperature air fluxes. After a short literature review, which shows a deficiency of models for liquid metal thermosyphons, an analytical model, developed to predict the temperature distribution and the overall thermal resistance, is shown. In this model, the thermosyphon is divided into seven regions: three regions for the condensed liquid, including the condenser, adiabatic region, and evaporator; one region for vapor; one for the liquid pool; one for the noncondensable gases; and another for the tube wall. The condensation phenomenon is modeled according to the Nusselt theory for condensation in vertical walls. Numerical methods are used to solve the resulting equations and to determine the temperature distribution in the tube wall. Ideal gas law is applied for the noncondensable gases inside the thermosyphon, while the evaporator and condenser heat transfer coefficients are obtained from literature correlations. Experimental tests are conducted for thermosyphon with mercury as working fluid, designed and constructed in the laboratory. The results for two thermosyphons with different geometry configurations are tested: one made of a finned tube in the condenser region and another of a smooth tube. The finned tube presents lower wall temperature levels when compared with the smooth tube. The experimental data are compared with the proposed model for two different liquid pool heat transfer coefficients. It is observed that the comparison between the experimental data and theoretical temperature profiles is good for the condenser region. For the evaporator, where two distinct regions are observed (liquid film and pool), the comparison is not so good, independent of the heat transfer coefficient used. In a general sense, the model has proved to be a useful tool for the design of liquid metal thermosyphons.