Abstract
This study experimentally investigates the tube condenser of a loop thermosyphon under different heat loads, inclination angles, and inner surface wettabilities. Thin-film evaporation is managed by adopting a copper-mesh-wick evaporator with super-hydrophilic treatment. This leads to low evaporator resistances, quiescent evaporation and stable condensation without oscillation due to boiling in the evaporator. Visualization experiments are first conducted for glass tubes. The static contact angle of the untreated glass surface is 18°, and it is 90°after hydrophobic treatment. Film condensation prevails in the untreated glass tube; while for the hydrophobic condition, dropwise condensation is observed with a cyclic process of rapid growth of small drops, drop coalescence to a critical size, drop falling to sweep off the downstream drops, and regrowth from very small drops. Condenser resistance measurements are conducted for hydrophilic and hydrophobic copper tubes with a static contact angle of 0°and 116°, respectively. For hydrophilic conditions, lower wall temperatures are measured at the downstream bottom side of non-vertical tubes, due to a thicker film of draining water. Non-uniform film distribution results in improved heat transfer with lower condenser resistances found at an inclination angle of 45°. For the hydrophobic condition, the wall temperatures are nearly uniform, irrespective of the inclination angle and heat load. Hydrophobic condensation yields lower condenser resistances than hydrophilic with a largest improvement of 50% for vertical tubes.