Abstract
Due to the lack of wick structures, Pulsating Heat Pipes (PHPs) have drawn the attention of many researchers for enhancing heat transfer through passive two-phase flow heat transfer mechanisms. The characteristics of flat-plate closed-loop pulsating heat pipes are recently applied on spreaders and are viewed as competitive passive heat transfer devices. In the present study, flat-plate PHP structures (rectangular cross-section area are 1.5mmx2mm, hydraulic diameter =1.714mm), which are directly machined onto an aluminum substrate (110mmx44mmx3mm) and are covered by an acrylic plate, are made for visualization study. The working fluid is DI water. By using a high-speed video camera, the effects of two-phase flow patterns on the evaporator temperature and the heat transfer mechanism for varied heat load are investigated. The optimal performance of heat transfer occurs when a steadily circulating flow pattern appears. To further examine the thermal performance, a flat-plate PHP made of copper(112mmx48mmx4mm) with the same groove dimensions as that for visualization study is made for the problem of interest. It is found that, with a filling ratio of 50% and a tilting angle of 90°, the optimal performance yields an overall thermal resistance of 0.52(°C/W) and a maximum heat capacity of 100W. The operation limit of the tilting angle is 20° below which oscillating two-phase flows disappear and the heat transfer mechanism approximately becomes a process of pure conduction. Finally, this thesis proposes some workable ways to improve the visualization study of flat-plate PHPs in the future.