由於近來微流體系統的快速發展，微尺度下的氣泡誘導聲流開始被廣泛應用，且擾動流場的特質已成功應用於流體混合上，可幫助生醫微晶片之流體操控。因此本研究旨在探討氣泡誘導聲流的流場現象，以及應用在熱傳增益之可能性。本研究應用前人常用的流場可視化技術，並首次搭配微粒子影像測速法與溫度螢光感測塗料之量測技術，針對氣泡誘導聲流效應進行定性與定量的流場量測以及熱傳分析。實驗設置選用PDMS矩形直管微流道並在流道兩側安置凹槽結構，再以因表面張力被困在凹槽內的氣泡為氣泡誘導聲流中的主要激擾裝置，當此氣泡受到外界振動激發而產生共振時，會形成一對互相反向轉動的渦旋，而氣泡共振頻率能透過理論方程式估算與實驗量測而得，本研究中氣泡的共振頻率為89 kHz。
本研究整合PDMS微流道、壓電片、微型加熱器等裝置，並結合微粒子影像測速法與流場可視化技術應用於觀測氣泡誘導聲流之流場，以渦度大小進行氣泡誘導聲流影響的定量分析與討論，其中在長寬比5、2、1之凹槽中，長寬比2的凹槽氣泡誘導聲流強度最強。實驗中觀察到氣泡誘導聲流產生的一對渦旋並不容易完全對稱，而是會隨著製程導致凹槽導角的不同、以及實驗中氣泡形成時的差異有所不同，在同一個凹槽的實驗中，氣泡誘導聲流呈現對稱分佈的機率僅有13 %。另外，實驗也觀察到氣泡高度亦會影響氣泡誘導聲流的強度，氣泡越高時則氣泡誘導聲流會越弱。在微流道裝置加入主流的環境下，原先氣泡誘導聲流的一對渦旋將變為上、下游吸引點，其中位於下游的吸引點有機率會形成小型渦旋。在雷諾數2~8時，氣泡誘導聲流造成的橫向動量變化量大致相同，然而擾動造成的流場影響則在雷諾數2時有著最大變化以及影響範圍。
最後利用單一對凹槽流場分析的結果設計多凹槽流道，並運用溫度螢光感測塗料技術量測雷諾數2以及熱通量為0.12 W/mm2時的液體溫度及壁面溫度，並計算對應的紐索數變化與流場進行比對分析。多凹槽流道在氣泡誘導聲流作用後，流經視野範圍內的焓值差會有12.4 %的上升；觀察實驗量測的紐索數分佈圖，發現在靠近凹槽的區域增益最大可達15~20 %，驅動壓電片的能耗僅需18 mW。
 

Since the rapid development of microfluidic systems, the bubble-induced acoustic streaming has been extensively applied in many fields of research, such as the flow manipulation in micro-biochip because of its great capability on the fluid mixing. As a result, this study aims to investigate the flow field in bubble-induced acoustic streaming and the feasibility of heat transfer enhancement. For the first time, not only flow visualization (FV) technique but also Micro Particle Image Velocimetry (µ-PIV) and Temperature-Sensitive Paint (TSP) techniques are utilized for the measurement of temperature and velocity in bubble-induced acoustic streaming. A PDMS rectangular microchannel has been fabricated and cavities were positioned at the side walls of the microchannel. Bubble-induced acoustic streaming is mainly constructed by trapping air bubbles in cavities, which is caused by surface tension, and exciting the air bubbles by a piezo-actuator. The trapped air bubble will start to resonate during the excitation driven by a piezo-actuator and a pair of vortex will appear which rotates clockwise and counterclockwise. The resonance frequency of the trapped air bubble can be calculated by theoretical equations and verified by the experiments. The resonance frequency of 89 kHz has been identified as the resonance frequency of the bubble-induced streaming in current study.
A PDMS microchannel, a piezo-actuator and a micro-heater are integrated in a microfluidic device for the experimental investigation of temperature profiles and flow field. µ-PIV technique and flow visualization are utilized to measure the velocity profiles of bubble-induced acoustic streaming. The vorticity distributions have been analyzed from the velocity profile as quantified information in the flow field. The bubble-induced acoustic streaming has been investigated with different aspect ratios of cavities (L/W) which equals to 5, 2 and 1. The strongest bubble-induced acoustic streaming has been identified while the aspect ratio of cavity equals to 2. However, it is also observed that the vortex pattern of bubble-induced acoustic streaming is highly asymmetrical. Only 13% possibility of symmetrical vortex pattern would be observed even in the flow field using identical cavity structure. In addition, the height of the bubble and fluid interface is also a factor that changes the magnitude of vortex pattern in bubble-induced acoustic streaming. The magnitude of bubble-induced acoustic streaming reduces if the bubble/fluid interface becomes higher. With the main stream flowing in the main channel, the vortex pair formed by bubble-induced acoustic streaming will evolve to two attraction points located at upstream and downstream of the cavity. The attraction point at downstream of the cavity would become a small vortex with recirculation flow. The disturbance as lateral momentum difference in the flow field induced by bubble-induced acoustic streaming are almost the same at Reynolds number varying from 2 to 8. The greatest disturbance and the large affected region from bubble-induced acoustic streaming is identified as the Re=2 of the main flow.
At last, a microchannel with multi-cavity structure is designed which is based on the information acquired from the experiment with single pair cavity. Fluid and surface temperature profiles at Reynolds number of 2 and bottom side heated at constant heat flux of 0.12 W/mm2. Up to 12.4% heat transfer enhancement has been observed from the enthalpy change after bubble-induced acoustic streaming applied. From the Nusselt number map around the cavity region in the microchannel, up to 15~20 % increase of Nusselt number have been identified in the region near the cavity structure with the excitation power of 18 mW applied to the piezo-actuator.

摘要
Abstract
誌謝
目錄
圖目錄
表目錄
第一章 緒論
1.1 研究動機
1.2 文獻回顧
1.3 研究目的
1.4 論文架構
第二章 實驗原理
2.1 螢光影像擷取
2.2 微粒子影像測速法量測原理
2.3 溫度螢光感測塗料
2.4 Intensity base TSP
2.5 熱傳特性
2.6 氣泡理論共振頻率
第三章 實驗架設
3.1 微流道製作
3.2 微型加熱器製作
3.3 速度、流場可視化及溫度量測系統
3.4 CCD相機及訊號產生器時序設定
3.5 調配工作流體及溫度螢光感測塗料
3.6 TSP校正曲線
3.7 壓電片振動頻率及振幅
3.8 氣泡膨脹現象及解決辦法
3.9 熱損計算
3.10 影像處理方法
3.11 誤差分析
第四章 無主流氣泡誘導聲流效應流場
4.1 凹槽幾何對氣泡誘導聲流的影響
4.2 氣泡誘導聲流偏向趨勢
4.3 氣泡高度對於氣泡誘導聲流的影響
第五章 有主流氣泡誘導聲流效應流場
5.1 有主流氣泡誘導聲流之PIV及FV結果
5.2 有主流氣泡誘導聲流速度分量流場分析
5.3 有主流氣泡誘導聲流流動方向分析
5.4 多凹槽結構之PIV及FV結果
第六章 氣泡誘導聲流效應熱傳分析
6.1 單凹槽結構分析
6.2 多凹槽結構分析
6.3 多凹槽結構之流場與熱傳比較
第七章 結論與未來工作
7.1 結論
7.2 未來建議工作
參考資料
 

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