多孔性毛細結構表面之池沸騰熱傳增強研究

Enhanced Pool Boiling Heat Transfer by Porous Structure Surface

葉勁廷(Ching-Ting Yeh)

指導教授 ： 陳瑤明

國立臺灣大學/工學院/機械工程學研究所/碩士(2007年)

https://doi.org/10.6342/NTU.2007.00564

摘要

在工業界，藉由發熱面上使用毛細結構來增進沸騰熱傳的技術，可改善熱傳性能，間接縮小蒸發器的體積，降低製造成本、節省空間，並且增進系統的效率。本文旨在使用樹枝狀銅粉和添加孔洞成型劑的方法，燒結具有雙孔徑的毛細結構，藉由增加結構中大孔的體積比例和孔隙度來增強沸騰熱傳。高瓦數時，大孔提供氣體離開的管道，降低液體與氣體之間的相對流阻，同時小孔洞吸入流體補充至相變化發生處，以提升臨界熱通量與熱傳。
本實驗使用絕對壓力5.5 bar 時的飽和冷媒R-134a，於水平測試表面進行池沸騰熱傳增強研究。實驗結果顯示，單孔與雙孔徑毛細結構的熱傳係數最高可達120 與70 kW/m2K。熱傳增強比例最高分別為光滑平面的 11倍與8倍。單孔徑毛細結構性能提升的原因在於樹枝狀粉末擁有較廣大的熱傳面積及通道，能夠加速薄液膜的蒸發速率。雙孔徑毛細結構由於孔隙度較高，有效熱傳導面積減少，以致熱傳性能較差。不過，高熱通量時，具有較高孔隙度的雙孔徑毛細結構，由於擁有許多大孔，所以確實延後了臨界熱通量的發生，達到869 kW/m2，為光滑平面的2.2倍，單粉燒結面的1.3倍。單孔與雙孔徑毛細結構皆有效提升了臨界熱通量與降低壁面過熱度。

關鍵字

熱傳 ； 池沸騰 ； 雙孔徑毛細結構表面 ； 熱傳增強表面

並列摘要

On the tendency of higher and higher power density devices in modern technology, the need for more effective heat exchangers have motivated the development of enhanced surfaces. Therefore, the central purpose in presented research is to enhance boiling heat transfer capacity by utilizing two pore size distributions of a biporous surface structure. This surface is sintered from the mixture of dendritic copper powders and the pore former, Na2CO3, which formed the larger pores in the matrix. By changing the volumetric ratio of pore former, it’s able to alter the porosity and the numbers of larger pores in the porous media. At high heat flux, the larger pores provide venting passages for bubbles generated inside the structure and reduce the liquid-vapor counterflow resistance adjacent to the surface, while the smaller pores continue to function as liquid supply routes.
The experiments were conducted with the 3-cm diameter test surfaces, horizontally oriented, and submerged in saturated R-134a, at an absolute pressure of 5.5 bar. At low heat flux, the results show that the heat transfer coefficients of mono and biporous surfaces are up to 120 and 70 kW/m2K, respectively. The heat transfer enhancement ratios are 9~11 and 4~8 compared to a smooth surface. At high heat flux, due to the larger pores in the porous structure, the biporous surface really prolongs the critical heat flux to 869 kW/m2, which is about 2.2 and 1.3 times over a smooth and monoporous surface, respectively.
Future research is needed to optimize the mixture of the pore former and cooper powders, which can result in substantial further enhancement.

並列關鍵字

Heat transfer ； Pool boiling ； Biporous surface ； Enhanced surface

參考文獻

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