Thermosyphons are passive devices used to transfer heat via a liquid-vapor phase change cycle. By adding a secondary non-condensable gas (NCG) to the vapor phase of the thermosyphon it is possible to provide some control over the evaporator temperature as heat load and or ambient temperature vary. The objective of this paper is to perform measurements on the vapor phase in the condenser region of a gas-loaded thermosyphon using particle image velocimetry (PIV). An optically-accessible water/air thermosyphon with a 34.5 mm diameter was designed, constructed and characterized for a range of fill ratios, heat loads, coolant temperatures and masses of NCG. Smoke particles, added with the NCG, were used to seed the flow. The flow field in the condenser was found to be unsteady except with low masses of NCG. It was found that there was no discernable vapor/NCG front present. A large amount of mixing of the vapor and NCG and recirculation of the NCG was observed. Due to the complex 3-dimensional non-axisymmetric nature of the flow it is very difficult to obtain useful quantitative measurements, however, PIV was found to be a very useful tool in visualizing the flow providing an insight in to the flow structures in the condenser.

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