邊界潤濕性對於矩形密閉容器自然對流熱傳之影響

Effect of boundary wettability on natural convection heat transfer in a rectangular enclosure

黃驛翔(Yi-Shiang Huang)

指導教授 ： 陳炳煇

國立臺灣大學/工學院/機械工程學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.03323

摘要

本研究主要討論不同潤濕性底板表面對於自然對流中的Rayleigh-Bénard Convection (RBC)熱傳效率的影響，傳統RBC實驗熱傳量測邊界皆為未改質底板的情況。而本實驗利用溶膠凝膠法進行銅板表面改質，改變表面的潤濕特性，使銅板具有接觸角10度之親水性與接觸角160度之超疏水性，再與未改質銅板接觸角93度比較，探討不同潤濕邊界下，比較彼此間熱傳差異。實驗測試條件為寬高比為0.5~2，其所對應的Ra數為105~107。
本研究首先驗證未經改質的銅板Nu數，與前人的實驗與分析相符，再與不同潤濕性底板Nu數比較，而具親水性底板，Nu數量測結果，與未改質底板相比，並無明顯增益，而具超疏水性底板，實驗前底板觀察到有氣膜存在，在加熱後底板產生氣泡，因氣泡產生的熱阻，使得Nu數相較於未改質底板明顯降低約16%。本研究亦觀察在無上板的情況下，具超疏水性底板相較於外改質與親水性底板，受熱後易產生氣泡，而影響對流熱傳效果。

關鍵字

自然對流 ； 潤濕性 ； 紐賽數

並列摘要

This study aims to investigate the effect of boundary wettability on natural convection heat transfer in a rectangular enclosure. Different levels of boundary wettability are established by utilizing sol-gel method which is one of the surface modification methods to create the hydrophilic and hydrophobic surface for bottom surface of Rayleigh-Bénard convection. The contact angles of copper plates via the surface modification method are about 10 and 160 degree which can be treated as hydrophilic and hydrophobic surfaces, respectively, and the contact angles of copper plates without surface modification is about 93 degree in this study. To tell the difference in heat transfer, Nusselt number is the dimensionless parameter which is discussed in this study. Aspect ratio of testing condition is 0.5~2, the corresponding Rayleigh number is 10^5~10^7.
The experimental results of plain copper surface in this study are quite match to the experimental results and analysis in previous studies by other researchers. There is no difference between the hydrophilic surfaces and the plain surfaces in Nusselt number. However, compare with the plain plates, the Nusselt number of hydrophobic surfaces is less than 16%. This thesis also observed the relation between temperature and bubble growth in the different levels of boundary wettability.

並列關鍵字

natural convection ； wettability ； Nusselt number

參考文獻

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