This work presents an experimental and numerical study of Two-Phase Closed Thermosyphon (TPCT) heat pipe filledwith R134a as a base fluid. For this purpose, a test rig was designed and developed toperform a series of tests on the TPCT heat pipe by applying different heat inputs at the evaporator section. The surface temperature along the TCPT heat pipe and temperature changes of the cooling water across the condenser section were measured. The influence of the TCPT
heat pipe operating conditions on its thermal performance was reviewed. The considered parameters were; heat pipe transport capacity, conductance, both heat transfer coefficients of the evaporator and condenser sections and the thermal efficiency. Then, a CFD model was developed to investigate the two-phase flow and heat transfer mechanism during the transient and steady-state operation of the TPCT heat pipe. The results of the experimental work were used to validate the CFD model and acceptable agreements were noticed. The validated CFD model is utilized to predict features of the mass and heat transfer processes, as well as the nucleate pool boiling and the liquid film condensation phenomena during TPCT heat pipe operation.