This dissertation presents the passive and active heat transfer enhancement techniques. Performance of extended surfaces is studied in terms of passive techniques and the effects of electrohydrodynamics (EHD) and piezoelectric fans are discussed in terms of active ones. The present work mainly includes three parts. The first part presents an experimental study on natural convection from pin fin heat sinks subject to the influence of orientation. The second part reports an attempt on the implementation of an EHD integrated thermal module. The third part presents an experimental and theoretical study on the vibration characteristics as well as a test on the thermal performance of piezoelectric fans. In the first part, it is found that the increase in fin surfaces is more effective for downward arrangement and is less effective for sideward arrangement. This argument is supported by showing that the augmentation factor (ξ), defined as the heat transfer of a heat sink relative to that of a flat plate, is around 1.1~2.5 for the upward arrangement, around 0.8~1.8 for the sideward arrangement, and around 1.2~3.2 for the downward arrangement. Turning to the second part, it is observed that heat transfer enhancement is evident when a corona discharge of above 1 μA is established. The enhancement ratio (γ), defined as the average heat transfer coefficient with EHD relative to that without EHD, increases with the electric power input and linearly with the 1/4 power of the corona current. For moderate test condition, the enhancement ratio is around 3~5 with a preferable normalized electrode height of around 0.25~0.35. For a given electrode height, there is an optimal electrode density below which the required area exposed to the corona wind is insufficient, and beyond which the flow interference caused by adjacent electrodes offsets the effective performance. With respect to the third part, a 2-D lumped mass model with a variable air damping factor is proposed to describe the motion of piezoelectric fans. Numerical results show that the vibration amplitude of fan tip linearly increases with the applied voltage without any air loading; however the vibration amplitude decreases with the presence of air loading. In particular, experimental results indicate that the presence of glue layer contributes an additional structure damping that further reduces the vibration amplitude of the system. As for the thermal performance of piezoelectric fans, the enhancement ratio increases with the tip amplitude and with decreasing the separation distance. The effect of fan orientation gradually loses strength as this distance is lengthened. In particular, in similarity with the electric wind, the enhancement ratio decreases with increasing the heat dissipation as a result of augmented buoyancy convection. In the end, it is believed that the present work contributes the understanding of heat sink performances in natural convection and advances the implementation of EHD and the development of piezoelectric fans.

本論文旨在探討被動式與主動式之熱傳增強技術，在被動式增強技術上探討延伸表面積的熱傳特性，在主動式增強技術上研究電液動力學(EHD)與壓電風扇等兩種技術。本文主要的內容可分成三個部份，第一個部份以實驗方式研究針狀散熱座於自然對流下之熱傳特性及角度的影響。第二個部份則嘗試結合EHD之散熱模組。第三個部份先以理論與實驗方式研究壓電風扇的振動特性，並以實驗測試其熱傳性能。第一部分的結果發現增加鰭片面積對熱傳面向下時效率較高，對熱傳面垂直擺放時效率較差，此論點可由熱傳增強係數(augmentation factor)看出，熱傳增強係數定義為散熱座與其底部平板總熱傳的比值。此熱傳增強係數在熱傳面向上時為1.1 ~ 2.5，熱傳面垂直時為0.8 ~ 1.8，熱傳面向下時為1.2 ~ 3.2。第二部分的結果發現熱傳增強效果展現於電暈電流大於1μA時，熱傳增強因子(enhancement ratio)隨輸入電能的增加而增加並且正比於電流的1/4次方，熱傳增強因子定義為電暈風作用下與自然對流下平均熱傳係數的比值。理想的無因次電極距離為0.25~0.35，相應之熱傳增強因子為3~5。當電極距離固定時，存在ㄧ最佳電極密度，少於該電極數時，可影響之鰭片面積不足，過多時則發生鄰近電極的流場干擾，減低熱傳效果。第三部分的理論分析採用二維集中質量的模型(2-D lumped mass model)配合可變的流體阻尼，數值結果顯示當不考慮空氣阻尼時，壓電風扇的尖端振幅隨輸入電壓增加線性上升。當考慮空氣阻尼時，振幅的上升漸趨平緩。特別是比對實驗結果後發現，黏膠層的存在額外增加了系統的結構阻尼，進而減小風扇的振幅。在壓電風扇的熱傳性能方面，實驗結果顯示熱傳增強因子隨扇葉振幅的增加而增加，並且隨風扇與散熱座的距離增加而減小，風扇的角度效應也隨此距離的增加而減小。另外壓電風扇與EHD的效果均隨著散熱瓦數的增加而減少，這種現象乃因自然對流的增強所導致。最終本研究的結果除了對延伸表面的自然對流性能有更深入的了解，相信對EHD技術的應用以及壓電風扇的開發設計都有進一步的助益。

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