| Summary: | 碩士 === 國立高雄應用科技大學 === 機械與精密工程研究所 === 100 === In recent years, various portable electronic products have been developed with good performance as well as small size. Among them, the piezoelectric products apparently demonstrate many advantages such as like weight, small size, high power-efficiency, and low electromagnetic interference, well suited for the portable applications. On the other hand, to avoid large power consumption for traditional air conditioning systems, much attention is paid to the emerging development of a novel cooling system based on droplet evaporation; the problems of traditional cooling systems can be solved including the high noise of motors, non-uniformity of droplets (size, speed, and directionality) as well as high energy consumption.
Therefore, here we propose such a novel cooling system primarily consisting a ring of piezoelectric material and a nickel nozzle plate. By driving the piezoelectric ring, spray-like droplets can be generated out of the nozzle plate, where the evaporative cooling effect can be further evaluated through the infrared thermograph. In this study, many geometrical parameters of nozzles such as their sizes, shapes, arrangements, numbers and structural outlines are deeply explored to find the most suitable design of nozzle plates.
To implement the nozzle design, a photolithography process of LIGA electroforming is required in order to achieve the thick plate with high aspect ratio of nozzle thickness to nozzle diameter. In the study, many parameters of electroforming process including the electrical current, voltage, spatial and temporal duration are further tuned in experiments to find the most suitable processing of the product. Upon completion of the nozzle plate, bon ding it with a piezoelectric ring, which is electrically connected to a voltage amplifier and a waveform generator, fluidic droplets can be jetted from the nozzles to air in environment. Thus, the evaporative cooling effect of the present system is able to be verified by using a thermal imaging equipment (thermograph). Finally, to optimize the cooling system, the influences of velocity and size of the drop lets as well as the environmental moisture on the cooling effect of evaporation are explored in experiments. In addition, using high-speed CCD shot of spray droplets, the calculated droplet volume in average was found with about 37 picoliters. In the experiment, the natural cooling rate of -0.66°C/min was found, while the unstructured sprayer yielded a cooling rate of -1.242 °C/min, demonstrating the spray cooling system with good cooling performance. In addition, the upward-spray exhibited a cooling rate of -0.77℃/min, compared to the upward-spray with a cooling rate of -1.242°C/min. To explore the differences in ring-structured and unstructured nozzle plates, it was found in the cooling experiments that the ring-structured sprayer showed more hydrophilic property on the nozzle-plate surface than that of unstructured one with accumulation of water at the bottom of cooling space. This indicates the fine effect of the atomization on cooling performance by using the ring structure.
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