The thermal performance of two thermosyphons with different geometries was experimentally investigated in this study. The first thermosyphon utilized a 310 mm long vertical evaporator and a 385 mm long condenser section that was inclined at 5 degrees from the vertical. The second was an elbow configuration with a 140 mm long vertical evaporator and a 190 mm long condenser oriented 8 degrees from the horizontal. Both thermosyphons were made of internally grooved copper tubing with an outer diameter of 15.87 mm, wall thickness of 0.5 mm and a nominal groove height of 0.3 mm. Tests were performed over a range of input heat fluxes where the condenser was cooled by flowing water around the condenser with inlet temperature of 10°C, 20°C, and 35°C. The effects of incrementally increasing and decreasing heat flux was investigated for the elbow thermosyphon. Temperature measurements along the thermosyphon were taken when incrementally changing the heat flux from 0.5 to 11 W/m2 for the first thermosyphon and 0.3 to 6 W/m2 for the second thermosyphon. Internal flow regimes were characterized using temperature transient profiles and compared to existing flow regime maps for closed thermosyphons suggested by Smith et al. (2018: Part a and Part b) and Terdoon et al. (1997). The temperature transients along the evaporator for the first thermosyphon settled to a more uniform profile as heat flux was increased. For the second thermosyphon the temperature profiles suggested a change to a more dynamic flow in the evaporator at heat flux of approximately 6 W/m2. The elbow thermosyphon showed evidence of a significant hysteresis in the evaporator performance at moderate heat fluxes between 2 and 8 W/cm2. Comparisons were made between the two thermosyphons to study the effects of inclination angle and the feasibility of angle corrections to the Nusselt film condensation model from Guichet and Jouhara (2020). A modification to the Rohsenow condensation model from Guichet and Jouhara (2020) was recommended for the first thermosyphon showing good representation of the condenser performance. The evaporator performance results were compared to existing models from the literature. / Thesis / Master of Applied Science (MASc)