Thermal and fluid-flow characteristics were numerically analyzed for ultra-thin heat pipes. Many studies have been conducted for ultra-thin heat pipes with a centered wick structure, but this study focused on separated wick structures to increase the evaporation/condensation surface areas within the heat pipe and to reduce the concentration of heat flux within the wick structure. A mathematical heat-pipe model was made in the three-dimensional coordinate system, and the model consisted of three regions: a vapor channel, liquid-wick, and container wall regions. The conservation equations for mass, momentum, and energy were solved numerically with boundary conditions by using a code developed by one of the authors. The numerical results with the separated wick structures were compared with those with the centered, which confirmed the effectiveness of the separation of the wick structure. However, the effectiveness of the separation was affected by the position of the separated wick structure. A simple equation was presented to determine the optimum position of the separated wick structures. Numerical analyses were also conducted when the width of the heat pipe was increased with the cooled section, which clarified that the increase in the cooled-section width with the addition of wick structures was more effective than the increase in the cooled-section length. A 44% reduction in the total temperature difference of the heat pipe was obtained under the present numerical conditions. Furthermore, a comparison was made between experimental results and numerical results.

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.

An ultra-thin flattened heat pipe has been developed with a centered wick structure. This structure is essential to make the heat pipe thinner. However, the centered wick structure reduces the evaporation and condensation surface areas of the wick structure because it is sandwiched between heat pipe walls. In this study, because detailed discussion has not been made, heat transfer experiments were conducted for the wick structure sandwiched between two solid walls. This study focused on the evaporation heat transfer characteristics from the sandwiched wick structure. The experiments were conducted with three wick structures, that is, strip-shaped sintered copper powders with thicknesses of 0.5, 1.0, and 1.5 mm. Water was used as working fluid. The capillary pumping performance, that is, the liquid lifting velocities of the three wick structures were the same. The experimental results of the three wick structures were compared regarding the relation between the evaporation heat transfer rate and the superheat of the working fluid. The heat transfer experiments were also conducted when one of the solid walls was removed from the wick structure. It was confirmed that even if the wick structure was sandwiched between the solid walls, sufficient evaporation of the working fluid occurred from the thin sides of the wick structure.