Thermosyphons are effective devices for heat dissipation due to their high thermal conductance, compact design, low carbon emissions and non-reliance on electricity. This project developed a wickless, gravity-assisted, closed loop two phase thermosyphon. The use of a two-phase working fluid means that a larger pressure gradient can be obtained compared to single-phase natural circulation loops. A three-dimensional theoretical model for two-phase slip-flow was derived from the mass, momentum, and energy conservation equations assuming quasi-steady state behaviour and water as the working fluid. The model, along with semi-empirical correlations, were used in a semi-implicit transient thermal-hydraulic numerical simulation. A natural draft, air-cooled, copper thermosyphon using water as the working fluid was designed and built. Successful testing showed the system capable of transferring 90% of the required heat for several thermal loads. Sub-cooling of the return condensate and saturated operation assured mostly stable operation, a big concern with natural circulation loops. The simulation captured the thermodynamic behaviour of the working fluid within the thermosyphon well and can be used with reasonable certainty to determine the performance of the natural circulation loop for various thermal loads.