Visualization experiments on flat-plate heat pipes with composite mesh-groove wick at different tilt angles

Visualization experiments are performed on flat-plate heat pipes with a sintered composite copper mesh-groove wick at different tilt angles. Deionized water is used as the working fluid. The composite wick consists of a layer of 200-mesh copper screen sintered over parallel semi-circular grooves with a width of 0.18 mm and depth of 0.09 mm. Also investigated are a groove wick with the same groove size and a sintered double-layer 200-mesh (2 × 200 mesh) wick. The effective length of the heat pipes is 101 mm. The heat pipe performance of the composite mesh-groove wick excels the other two wicks in the maximum heat load (Qmax) under the horizontal orientation and in the anti-gravity ability. Visualization reveals two evaporation stages for the horizontal orientation before full dryout in the heated zone of the composite wick. In the first stage with Q up to about 40 W, the wick at the heated zone is filled with water; in the second stage, partial dryout in the grooves occurs and expands with increasing heat load. Up to about 60 W, the heated zone becomes fully dried. In contrast, the Qmax of the 2 × 200 mesh wick is 21-25 W, and full dryout prevails at 14 W for the groove wick. When the tilt angle is between 30° and 90°, the Qmax for the composite wick may reach 39-49 W, followed by drastic increase in the evaporator resistance. The 2 × 200 mesh wick suffers serious dryout at Q = 14 W under α = 45°; the groove wick cannot operate under α ≥ 30°. No nucleate boiling is observed in all the tests for water.