請用此 Handle URI 來引用此文件:
http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/65541
標題: | 潤濕性對微流道冷凝器熱傳熱傳增強之研究 Wettability Effect on Microchannel Condenser Heat Transfer Enhancement |
作者: | Kuei-Yen Chen 陳奎延 |
指導教授: | 陳瑤明 |
關鍵字: | 微流道,冷凝熱傳增強,親水或疏水表面, microchannel,condensation heat transfer enhancement,hydrophobic and hydrophilic surface, |
出版年 : | 2012 |
學位: | 碩士 |
摘要: | 近年來,利用兩相熱傳的微流道蒸發器由於具有高熱通量、低工質需求的特性被視為極有潛力的散熱元件之一。當在有限面積下的傳統單相熱交換器無法有效冷卻時,搭配具有相變化的微流道冷凝器被學者認為是有發展潛力的冷凝元件。由於科技發展日新月異,各項產品發熱量日益增高,熱傳增強的微流道冷凝器更有其應用價值。疏水性表面由於接觸角大,潤濕性較差,能在冷凝過程形成液珠冷凝。在大尺度下具疏水性的冷凝器熱傳性能已有大幅提升,預期在微流道冷凝器也能有同樣顯著效果。本研究設計並製作具親水及疏水性微流道冷凝器並比較改質前後之熱傳係數、壓降的影響。
測試段是由無氧銅表面製作的30條寬、深各為500μm×155μm流道。利用奈米粒子疊層薄膜組裝技術以製作親水及疏水性結構套用在相同尺寸的微流道冷凝器表面上,並以水為工質進行熱性能測試。 在未改質微流道的實驗中,熱傳係數以及壓降都有隨著質量通率上升而上升的趨勢。當質量通率增加,流速加快,使得壁面剪應力上升,造成液膜厚度變薄,熱阻減小,熱傳係數因而增加。實驗結果與傳統流道的熱傳經驗公式比較後,發現明顯低估,顯示已不適用於微流道中。與不同工質的微流道熱傳經驗式比較後,結果顯示誤差仍太大,目前的微流道熱傳經驗式仍有改進空間。壓降方面比較近年發展的微流道壓降經驗公式,結果相當吻合,顯示具有一定可靠度。 表面改質部分,本實驗利用奈米粒子靜電薄膜組裝的技術改變水在銅表面上的接觸角形成親水及疏水性結構,對於親水性結構接觸角可由87°變作43°,疏水性結構則可使接觸角增加為134°。 與未改質微流道熱傳係數相較之下,疏水性微流道在相同質量通率範圍下平均約可提升100%,具有顯著的改變。疏水性表面會產生液珠冷凝,其機制與液膜冷凝有所不同。因此有更高的熱傳係數。 液珠冷凝能提升熱傳係數的機制在本實驗較小質量通率下沒有明顯的提升。可能原因為當質量通率較小時,流速較慢,易形成液膜。因此,當質量流率增加,流速加快不易形成液膜,使得疏水性結構所產生的液珠冷凝過程則越趨明顯,造成熱傳係數顯著增加。 在壓降方面,因親水性結構接觸角較小,能延展液體,使得流動較為順暢。本實驗結果壓降平均約降低40%,顯示親水改質對微流道冷凝器的流動壓降有明顯的改善。 In recent years, the microchannel evaporator with two phase heat transfer, due to its highly heat flux and little requirement for coolant flow rate, is considered as one of potential cooling techniques. When the traditional single phase heat exchanger cannot efficiently cool in a limited area, collocating the microchannel condenser with two phase heat transfer is regarded as a potential cooling component. Because of the highly developed technology, the heat dissipation rate raise in many products. Therefore the microchannel condenser with enhanced heat transfer is more applicale. The hydrophobic surface has wide contact angle, and worse wettbility. In the process of condensation it will form dropwise condensation. The heat transfer coefficient is increased dramatically in the large scale condenser within hydrophobic surface. We assume that it will show the same result in the microchannel condenser. Thus, this research design and manufacture the hydrophobic and hydrophilic microchannel condenser, compare to the heat transfer coefficient and pressure drop with uncoated microchannels condenser. The test section has 30 channels 500μm in width and 155μm in depth using water as working fluid. Using layer-by-layer (LbL) assembly method manufacture the hydrophobic and hydrophilic structure the same geometric dimensions microchannel condenser. In the experiment of the uncoated microchannel, the heat transfer coefficient and pressure drop is positive correlative with the increasing mass flux. Because increasing the mass flux, the velocity becomes faster, along with increasing the wall shear stress. This will make the thickness of the liquid film become much thinner, and reduce the heat resistant, further increase the heat transfer coefficient. Compared with the heat transfer correlation of the conventional channel, the result shows the MAE is still large. Currently, there is still much room to make progress on the heat transfer correlation of the microchannels. With regard to the pressure drop, compare with the correlation of micrchannel developed recently, it correlated with our result and shows that our result is reliable. In coated surface, we use layer-by-layer (LbL) assembly method to change the contact angle between water and copper, to manufacture the hydrophobic and hydrophilic structure. The contact angle of the hydrophilic structure change from 87°to 43°. Also the contact angle of the hydrophobic structure rise up to 135°. Compared with the heat transfer coefficient of the uncoated surface, the hydrophobic microchannels can increase roughly 100% on average, with remarkable difference. The droplet cannot adhere on the hydrophobic surface to cause the dropwise condensation. The mechanism is different from the filmwise condensation. Therefore, the heat transfer coefficient is much higher. When the mass flux is small, the heat transfer coefficient doesn’t increase remarkably. Because the velocity is slower, the liquid film is easy to form. Therefore, the mechanism that dropwise condensation could increase the heat transfer coefficient cannot be observed within the small mass flux in this experiment. As increasing the mass flux, the velocity is so fast that is not easy to form the liquid film. This will make the heat transfer coefficient increase remarkably. For the pressure drop, the contact angle of the hydrophilic structure is smaller, and it can extend liquid, making the flow easier. In this experiment the pressure drop decrease about 40% on average. It shows that the hydrophilic structure can greatly improve the pressure drop for the microchannel condenser. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/65541 |
全文授權: | 有償授權 |
顯示於系所單位: | 機械工程學系 |
文件中的檔案:
檔案 | 大小 | 格式 | |
---|---|---|---|
ntu-101-1.pdf 目前未授權公開取用 | 2.16 MB | Adobe PDF |
系統中的文件,除了特別指名其著作權條款之外,均受到著作權保護,並且保留所有的權利。