Complementary analytical, numerical, and experimental investigations of fluid flow and heat transfer in closed-loop thermosyphons with vertical heated and cooled sections are presented in this thesis. A new model is proposed to couple the local results of numerical simulations performed in the heated and cooled section of a thermosyphon with a one-dimensional analytical model. The numerical simulations are based on a finite-volume numerical method which was formulated for the solution of laminar mixed-convection flows in vertical pipes with (or without) conjugate conduction in the pipe wall. Experimentally, a closed-loop thermosyphon was specially designed and constructed for this study. In the closed-loop, the power input is supplied by a semi-transparent gold-film which provides a uniform wall heat flux while enabling flow visualization. The proposed model was successfully validated against experimental data obtained in this study. Results obtained with the proposed model also indicate that traditional one-dimensional models can significantly overpredict the average velocity in thermosyphons when strong mixed-convection effects are present in the heated and cooled sections. It is also shown that conduction in the pipe wall can significantly affect the velocity and temperature fields in mixed-convection flows. Photographs of flow visualization experiments in mixed-convection flows are also presented.

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