微流道內的雙相流與沸騰熱傳對於新能源工程、微電子、汽車、航太、氣體分離等等產業都可能扮演重要角色，也是近年非常受到重視的熱流研究議題。利用微熱管或微冷卻器的高移熱能力則是微電子元件效能進一步提昇的重要關鍵。尤其矽質微流道可以直接蝕刻於矽基，利用微流道內的沸騰熱傳之高熱傳遞係數及微流道組成的高移熱面積，將可移除大量的熱，而提昇微電子元件的效能。然而過去文獻中的研究與本實驗室的探討顯示，微流道內的沸騰與伴隨的雙相流，由於微流道微小的截面積(或水力直徑)產生很大的壓降。如此高的壓降意味著困難的流體推動設計及高幫浦功率。尤其，微流道內的沸騰也可能導致雙相流的不穩定性，特別是在高熱通率及低流量的運轉條件下。熱流不穩定性常導致早發的臨界熱通率而造成流道的乾化與相當高的壁溫。.
本研究的目的在探討漸擴微流道中的沸騰熱傳與雙相流現象，希望發展出具有高移熱能力、低流阻而穩定的微流道設計，進而設計出多平行微流道熱沈的系統，供微冷卻器之用。此外，本研究也將進行等截面積微流道的深寬比對沸騰熱傳與雙相流研究，以為比較之用。新的微流道設計預期可以顯著地的降低微流通沸騰時的流阻並能有效地增加沸騰時雙相流的穩定性。
本研究是應用微機電(MEMS) 技術中的面型微加工及體型微加工來製作矽質微流道，以達製作預定水力直徑的漸擴與深/淺的均勻微流道之設計目標，來進行流動沸騰熱傳的研究，我們將以高速攝影機觀察流道內的氣泡成核現象、氣泡的成長與脫離，及彈狀氣泡的成長與雙相流譜在流道中的演進，並比較漸擴與深/淺的均勻微流道中的異同。此外，我們也將量測沸騰熱傳遞係數、雙相流壓降、與雙相流的穩定性的結果並比較三種流道。
在過去的相關文獻中顯示，在高壁過熱度的微流道中沒有觀測到沸騰起始點但卻有快速且激烈的氣液介面震盪或突然產生氣液介面。文獻中的研究稱這樣微流道內特殊的沸騰(eruptive boiling)現象。本研究利用超高速攝影機，確認這樣微通道內有突沸的現象，為一種超快速的氣泡成核現象。如果沸騰起始點靠近出口(系統壓力較低，氣泡生成頻率較高)時，成核後高速成長的氣泡將與下游的彈狀氣泡合併，而形成有如快速且激烈的氣液介面震盪現象。另一方面，當沸騰起始點靠近入口時，就可觀察到突然產生的氣液介面的現象，亦即單相液體突然變成雙相環狀流。這種突沸現象亦可用傳統預測空穴大小的理論模式來預測，並於實驗量測的結果有良好的比對。
在單一等截面積與小角度(α=0.183°)漸擴矩形矽質微通道內的沸騰現象及雙相流系統的研究結果顯示，在相近的水力直徑與單管微流道於相近的熱通率之流動沸騰條件下，漸擴微通道比等截面積微通道條件下有較高的雙相流穩定度及較高移熱能力且相近的雙相流阻。利用漸擴截面積微型流道特色結構，可以抑制沸騰氣泡逆向迴流進而有效消除或抑制微流道特有的雙相流不穩定性。這使得沸騰氣泡可平穩的流過微流道，因此其散熱能力顯著高於等截面積微通道。此外，以對流沸騰熱傳為主要模式的經驗式亦被提出，並於實驗量測的結果有良好的比對。
深寬比的效應對單一等截面積矩形矽質微通道內的沸騰現象及雙相流系統的研究結果顯示，在相近的水力直徑與微流道於相近的熱通率之流動沸騰條件下，高深寬微通道比低深寬微通道條件下有較高的雙相流穩定度及較高移熱能力且較低的雙相流阻。利用深反應離子蝕刻技術製作的高深寬比微型流道，因其側壁較粗，有利成核沸騰，可以抑制沸騰起始的不穩定(壁過熱度較低)進而壓抑微流道內的雙相流不穩定性。因此其散熱能力顯著高於較淺的微通道。
電雙層的效應對單一等截面積矩形矽質微通道內的沸騰現象及雙相流系統的研究結果顯示，在相近的水力直徑與微流道於相近的熱通率之流動沸騰條件下，低深寬微通道比高深寬微通道條件下有較高移熱能力且較高的單流阻。利用Pyrex玻璃封合的低深寬比微型流道，因其玻璃表面積較高深寬微通道多，有較強的電雙層的效應，使的系統內的磨擦因子較高，反應出較高的系統壓降。由Chilton-Colburn關聯式可知系統內的磨擦因子較高代表有較高移熱能力。因此其單相液體散熱能力顯著高於較深的微通道。
比對研究成果發現，在單相流液體狀態下，漸擴微流道與低深寬比微型流道，兩者都有較高移熱能力但亦有較高的單相流壓降。然而，在雙相狀態下，高深寬比微流道有較高移熱能力(對低深寬比微型流道)且較低的雙相流阻(對漸擴微流道)。如此可知，高深寬比漸擴流道設計不但可用於單相狀態的微流道熱移裝置亦將可以提供較高且穩定的沸騰熱傳與雙相流狀態。

This study conducts the experimental investigation, visualization and modeling of convective boiling of water in the three types of single microchannel with approximately the same hydraulic diameter of 35μm. The aspect ratios, i.e., depth-to-width ratio, of two uniform-cross-section microchannels are 0.20 and 4.43 each with a hydraulic diameter of 33.7 μm and 36.4 μm, respectively. Another one is a diverging microchannel with a diverging angle of 0.183° and a mean hydraulic diameter of 34.8μm. All of the microchannels are made of SOI wafer and prepared using bulk micro-machining and anodic bonding. The surface roughness for both the bottom and the side walls was measured using an atomic force microscope enabling the explanation of convective boiling mechanism in the microchannel.
The evolution of the eruptive boiling of water in the smooth microchannel was clearly examined using an ultra high speed video camera (up to 50,000 frames per second). It is confirmed that eruptive boiling is a form of rapid bubble nucleation after which the bubble merges with a slug bubble downstream in a short distance or evolve to a slug bubble. The bubble frequency in all of the cases studied is provided. Eruptive boiling may be predicted classically with micro-sized cavities that are consistent with the measured surface roughness.
Furthermore, experiments are conducted to study the effect of channel cross-section design on boiling heat transfer in the microchannel. It is found that the slug bubbles tend to grow exponentially in the present microchannels. The results reveal that diverging microchannel presents better performance in boiling heat transfer than that of uniform-cross-section one, primarily due to more stable two-phase flow in the diverging microchannel. Empirical correlations based on convective boiling are developed, respectively, for both types of microchannel. For the same mass flow rate, the diverging microchannel presents higher single phase flow pressure drop, while the two-phase flow in both types of channels shows approximately the same pressure drop for boiling at the same heat flux.
The aspect ratio of channels with approximately the same hydraulic diameter also affects heat transfer and pressure drop data significantly, single- or two-phase. For the same flow rate, the shallow microchannel presents better heat transfer performance in single phase region, while the deep one become better when boiling occurs. Analogically, the shallow microchannel depicts s higher single phase pressure drop and this may be due to EDL effect. While the two-phase flow in the deep one shows the smaller pressure drop for boiling at the same heat flux. Observation of two-phase flow pattern indicates that the flow reversal in the shallow microchannel is more violent than that in the deep one. This is consistent with the higher boiling heat transfer rate for the deep microchannel with a smaller two-phase pressure drop.
The result of present study suggest that diverging cross-section design with a large aspect ratio is the better geometry for microchannels in a heat sink to have stable and high boiling heat transfer capability and yet low pressure drop.

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