Abstract
This study experimentally investigated the tube condenser of a loop thermosyphon by visualization and condenser resistance measurements under different heat loads, inclination angles, inner surface wettabilities of the condenser tube. These results can be used for the condenser design. In the evaporator, the thin-film evaporation was managed by adopting multi-layer copper mesh wick with hydrophilic treatment. This led to low evaporator resistances and stable evaporation without intense oscillation due to boiling in the evaporator. Visualization experiments were first conducted for glass condenser tubes. The static contact angle of a sessile water drop on the untreated glass was 18°, while it was 86° after a hydrophobic surface treatment. Film condensation was observed in the untreated glass tube, while dropwise condensation was observed with a cyclic process of rapid drop growth from very small size, coalescence of drops until a threshold size, drop falling to sweep off the drops below, and the regrowth from very small drops. Quantitative measurements of the condenser resistances were conducted for copper condensation tubes, with three pairs of thermocouples tightly attached to the outer tube wall by plastic insulation belts to measure the longitudinal temperature distributions along the outer wall. The condenser resistances were estimated by dividing the temperature differences between the condenser’s inlet vapor temperature and the outer wall temperatures by the heat load. The static contact angle of a sessile water drop was 0° and 116° on the copper tube with hydrophilic and hydrophobic treatment, respectively. Under various inclination angles, slightly lowered temperatures by thicker water films were measured near the lower end of the condenser, due to water accumulation and weakened or reversed vapor shear stress. Under inclination, the wall temperatures were slightly lower at the lower side of the tube due to the thicker water films. For the hydrophobic tube, the wall temperatures were nearly uniform, irrespective of the inclination angle. The condenser resistances were also similar for different inclination angles. In summary, an evaporator operating with thin-film evaporation provides lower evaporator resistances; hydrophobic condenser tubes leads to lower condenser resistances than hydrophilic tubes; and an inclination angle of about 45° yields lowest condenser resistances.