Visualization and Evaporation Resistance Measurement for Groove-Wicked Flat Plate Heat Pipes

Abstract

This work experimentally studied the evaporation characteristics in groove-wicked flat-plate heat pipes. The parallel, U-shaped grooves have a width of 0.25 mm and a depth of 0.16 mm. Uniform heating was applied to the copper base plate near one end, and a cooling water jacket was connected at the other end. The evaporation resistance was calculated based on the difference of the plate temperature and the vapor temperature respectively under and above the center of the heated zone. Water was used as the working fluid. With stepwise increase of heat load, the behavior of the working fluid in the grooves was visualized, and the evaporation resistances were measured. Above a certain heat load, longitudinal liquid recession can be visualized with a steep-sloped liquid front. Behind the short liquid front is the accommodation region where the meniscus appeared to anchor on the top corners of the groove walls. Under a thermally stable situation, longitudinal oscillations of the liquid front existed in many grooves. Also, the liquid motion in different grooves seemed independent, forming a constantly varying zigzag front line. With increasing heat load, the liquid fronts gradually left the heated zone, accompanied by increasing plate temperatures. The evaporation resistance data appeared larger and more scattered than those associated with mesh or powder wicks in our published experiments, presumably due to the relatively large groove size and surface roughness from etching. No boiling was observed in all present tests. The evaporation resistances for groove wicks increase monotonically in response to the gradually enlarged dryout region with increasing heat load.