Flow Patterns and Heat Transfer Characteristics of a Polymer Pulsating Heat Pipe Filled with Hydrofluoroether

Visualization experiments were conducted to clarify the operational characteristics of a polymer pulsating heat pipe (PHP). Hydrofluoroether (HFE)-7100 was used as a working fluid, and its filling ratio was 50% of the entire PHP channel. A semi-transparent PHP was fabricated using a transparent polycarbonate sheet and a plastic 3D printer, and the movements of liquid slugs and vapor plugs of the working fluid were captured with a high-speed camera. The video images were then analyzed to obtain the flow patterns in the PHP. The heat transfer characteristics of the PHP were discussed based on the flow patterns and temperature distributions obtained with thermocouples. Before starting heating, because of high wettability, large liquid slugs positioned at the evaporator section of the PHP. After starting heating, since the occurrence of boiling divided the large liquid slugs, oscillatory flow of smaller liquid slugs and vapor plugs was found in the PHP. Clear circulation flow of liquid slugs and vapor plugs was observed when the power input to the PHP was larger than 12.0 W. The flow patterns and temperature distributions confirmed that the circulation flow enhanced the heat transfer from the evaporator section to the condenser section of the PHP. In the circulation flow mode, large growth and contraction of vapor plugs were found one after another in all even-numbered PHP channels. However, the analysis of flow patterns clarified that the phase-change heat transfer rate by large growth and contraction of vapor plugs was 19% of the total heat transfer rate of the PHP. Although the generation of large vapor plugs was found in the PHP, most of the heat was transferred by the sensible heat of the working fluid.