雙孔徑毛細結構表面於池沸騰之熱傳增強研究

Abstract

工業界常使用燒結型毛細結構於發熱面上來增進沸騰熱傳性能，其優異之表現已經獲得普遍的肯定。但近年來隨著能源短缺與散熱需求不斷增加，如何再提升在毛細結構整體性能將為重要的議題。以往文獻探討皆以單孔徑毛細結構為主，雙孔徑毛細結構的報導仍付之闕如，且熱傳性能多針對低瓦數的條件下進行實驗，鮮少有完整的報導。本文旨在利用添加孔洞成型劑於樹枝狀銅粉的方法，燒結製作具雙孔徑的毛細結構並有效控制孔徑參數。在絕對壓力5.5 bar 時的飽和冷媒R-134a於水平測試表面進行池沸騰研究，探討不同孔洞變化對熱傳性能之影響。研究方法藉由改變不同厚度、孔洞成型劑的粒徑與含量，搭配二階因子的統計方法，分析各參數對熱傳性能的影響程度與趨勢，並提供性能改進的空間與方向。最後透過和單孔徑毛細結構與商用增強管比較，以了解雙孔徑毛細結構的熱傳性能與沸騰熱傳特性。 驗結果經統計分析後，顯示孔洞成性劑含量為影響熱傳性能的最主要關鍵，貢獻百分比為56%，其次是厚度銅粉粒徑比與厚度孔洞成型劑比，分別為20%與14%。在低孔洞成型劑含量、較高厚度銅粉粒徑比和厚度孔洞成型劑比，可獲得性能較佳之毛細結構。實際測試結果，性能較佳之雙孔徑毛細結構，在熱通量約150 KW/m2以下，和單孔徑毛細結構性能差異不大，熱傳係數為光滑表面的6~7倍，當輸入熱通量超過150 KW/m2時，雙孔徑毛細結構與單粉燒結面熱傳係數分別為光滑表面的5.1~6.3倍和2.4~4.2倍；臨界熱通量分別為671 kW/m2和631 kW/m2。高熱通量下熱傳性能差異的主因為大孔提供足夠氣體脫離的通道與增加蒸發面積，降低液氣間之流阻，同時小孔洞吸入流體補充至相變化發生處，有效的提升了熱傳性能。

The purpose of this research is to enhance boiling heat transfer capacity by utilizing two pores distribution structure (biporous structure) on the saturated pool boiling heat transfer of R134a. This surface is fabricated with sintered dendritic copper powders and the pore former, Na2CO3, which forms the larger pores in the matrix. By changing the volumetric ratio of pore former, we are able to alter the porosity and the numbers of larger pores in the porous media. The study was conducted based on a statistical method, with a two-level factorial plan involving three variables (coating thickness/particle size of copper: 6 and 10, coating thickness/particle size of pore former content: 16 and 5, and pore former content: 15% by volume and 25% by volume). Finally, the performance of biporous surface, monoporous surface and commercially enhanced surfaces were compared. he preceding statistical analysis of experiment data show that the boiling performance and characteristics are strongly dependent on the pore former contents (56% of percent contribution) and the better performance tend to have less pore former contents, higher thickness/particle size of copper, and higher thickness/particle size of pore formers. This information provides a direction of the potential improvement. The heat transfer coefficients of biporous and monoporous surface are not much different at low heat flux(less than about 150 kW/m2). The heat transfer enhancement ratios are 6~7 times compared to a smooth surface. For high heat flux removal (higher than 150kW/m2), the heat transfer enhancement ratios of biporous surfaces are 5.1~6.3 and 2.4~4.2 times over a smooth and monoporous surfaces, respectively. The critical heat fluxes for each kind are 671 kW/m2 and 631 kW/m2. At high heat flux, the biporous surface can continuously remove heat at high heat transfer coefficient. The larger pores provide more vapor pathways 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. Therefore, biporous surface is very attractive for high heat flux application.