A Pulsating Heat Pipe (PHP) with alternating hydrophilic/superhydrophobic channels was tested at vertical position, bottom heat mode and different heat power inputs. The device consists in a copper tube (internal/external diameters of 3.18/4.76 mm), bent into a planar serpentine of ten channels and five U-turns. The tube is partially functionalized with a superhydrophobic coating, in such a way to create an alternation of hydrophilic and superhydrophobic surfaces on the straight tubes along the loop, in the condenser zone. The aim is to investigate how the alternated wettability affects the startup, the fluid motion along the tubes and the overall thermal performance of the device, which is compared to another PHP, having the same geometry and under the same working conditions, but completely hydrophilic. Distilled water, at 50% filling ratio is the working fluid. Power inputs varying from 20 to up to 350W, in a stepwise increasing and decreasing fashion, are applied to the PHP. The condenser temperature is kept constant, at 20°C. The device is monitored by sixteen thermocouples and one pressure transducer, mounted in contact to the fluid in the condenser region. Data analysis shows that the alternating wettability of tube sections strongly affects the flow motion, the start-up and the overall performance. In general, the alternating PHP presents a worse overall thermal performance: the thermal resistance is always higher and the start-up is achieved at higher heating power levels. Temperature at superhydrophobic surfaces exhibit a flat trend, suggesting that the flow was blocked in the functionalized area surface, while, for the hydrophilic inserts, more pronounced temperature fluctuations are observed. It is believed that the superhydrophobic coating hinders the liquid film formation, decreasing locally the flow motion. On the other hand, the enhancement of the inner wettability improves the flow motion, as the liquid film that covers the inner surface acts as lubricant.
This work aims to discuss the application of diffusion bonding process in the fabrication of flat plate pulsating heat pipes and to analyze the thermal performance effect of porous media structures applied to alternating channels in the evaporator section and compare it with a smooth conventional flat plate pulsating heat pipe. For that, two different copper flat plate pulsating heat pipes with five turns and 1.5 mm of inner diameter channels were fabricated, using diffusion bonding. Samples were made with two different diffusion process to evaluate the ability of the resulting device to keep the vacuum, avoiding the use of filler material during the welding process, which could result in contamination and, therefore, undesirable corrosion on both internal and external heat pipe surfaces. The results allowed to assess the quality of the bonded interface of two different configurations of the diffusion process. Microscopic results obtained on controllable pressure furnace and non-pressure control were analyzed, where the diffusion process with controlled pressure presented best results of continuity, alignment, and tightness. A comparison of the heat transfer characteristics between the two devices was performed under the same experimental conditions; for both devices, the thermal resistance in horizontal position showed to be like heat transfer of pure conductive copper plate. However, pulsating heat pipe containing smooth channels showed the lower thermal resistance in vertical position and filling ratio of 50%.
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