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This study presents an approximation for determining an optimized thickness of a concentric heated rectangular plate and derives an analytical solution for spreading resistance of a spreader having orthotropic conductivities. The solution for the orthotropic plate is obtained by separation of variables, and the optimized thickness is determined by taking the derivative of the thermal resistance with respect to the spreader thickness. According to the calculated results, an enhanced in-plane spreading effect can reduce the spreading resistance. The spreading resistance dominates the overall resistance of thin plates, whereas the one-dimensional conduction resistance becomes important for thick plates. However, the predicted optimized thickness from the approximation shows a disparity from the analytical results, while the aspect ratio between a spreader and heat source is less than 0.2. Even so, the thermal resistance corresponding to the predicted thickness is still in good agreement with the analytical solution. The proposed approximation will be useful for practical thermal design of heat sinks by predetermining the spreader thickness.

Two Si-based micro pulsating heat pipes (µPHPs) charged using HFE-7100 were either horizontally or vertically oriented and were tested using several heating powers. The width of each channel was 0.8 mm in one µPHP containing uniform channels, and the channel width was 1.0 mm or 0.6 mm in the other µPHP, which did not contain uniform channels. The depth of each channel was 0.25 mm. The overall size of each µPHP was 60 × 10 × 1.25 mm. Visual observation and temperature measurement of the µPHPs under various conditions were performed and the results were analyzed. The results indicated that when the µPHPs were operated horizontally at a heating power ranging from 1 to 7 W, the pulsating two-phase flow in the channels of the µPHPs could not begin, except when the µPHP containing nonuniform channels was tested at a heating power of 7 W. With a heating power less than 5 W, the frequency of the sine-like oscillating displacement of the vapor slug increased and the displacement of the vapor slug reduced in either vertically oriented μPHP, as the heating power increased With a heating power higher than 5 W, periodic "start-stop" behaviors were observed in the vertical μPHP containing nonuniform channels.

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