多孔性燒結結構蒸發器於低過熱度之熱傳研究

Heat Transfer in Porous Sintered Structural Evaporator at Low Superheat Levels

李卓翰(Cho-Han Lee)

指導教授 ： 蔡曜陽

國立臺灣大學/工學院/機械工程學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.10803

摘要

隨著半導體製程與高功率發光二極體於高性能、高功率及微型化的發展，高熱密度的熱交換需求應用漸被重視。高熱密度下，由於熱傳面積很小，為了降低熱交換時的熱阻，熱傳必須在很小的溫差內進行。而於高熱傳量與低溫差的熱傳條件下，無論是熱擴散或熱傳遞，相變化熱傳元件皆為一個適合的解決方案。在這些元件內部，發生於蒸發端的薄膜蒸發相變化機制，扮演著元件能運作於低過熱度下一個重要的角色。比較各種毛細結構，薄膜蒸發機制在粉末燒結的多孔性結構內，能夠最明顯且有效率地以潛熱的形式進行熱傳。本研究在各粉末燒結結構的結構參數間，包含45 μm、75 μm、150 μm的粉末尺寸，球狀、樹枝狀的粉末形狀及三個結構厚度等級，比較過熱度需求與熱通量之間的關係，並探討其影響薄膜蒸發的機制。
本研究包含了兩個部分的實驗，分別為多孔性結構的固體等效熱傳導實驗與多孔性結構的相變化熱傳實驗。其中，相變化熱傳實驗又可分為相變化機制轉換實驗及薄膜蒸發實驗。由於本研究著重於低過熱度的應用，為了準確量測相變化熱傳實驗中，微小的溫差與熱通量，本研究研發製作了一套設備，以期能夠達到準確量測的目的。此設備的特色包含加熱式低溫差型的絕熱腔體、直接燒結結構於熱通量量測柱以避免接觸熱阻的設計、穩定的環境真空循環系統、高解析度與取樣頻率的資料擷取系統。實驗結果顯示，無論是球狀或樹枝狀粉末，較小尺寸粉末的燒結結構表現出較高的等效熱傳導係數。而相同粉末尺寸時，球狀粉末的燒結結構表現出較樹枝狀高的等效熱傳導係數，其差異達到兩倍。而在薄膜蒸發實驗中，過熱度範圍從2 K至5 K，熱通量隨著過熱度的提高成正相關地增加。相同過熱程度時，較薄結構厚度、較小粉末尺寸及樹枝狀粉末的燒結結構，表現出較高的熱通量及較低的熱阻。
輔以影像處理流程，對燒結結構進行影像辨識，偵測出單一截面下，固體結構的輪廓與其長度，這個輪廓為固體燒結結構與液體工作流體間，可能形成交界的位置，亦代表工作流體形成薄膜的可能區域，藉此量化形成薄膜量的趨勢。總體來說，結合影像辨識的評估與兩個部分的實驗結果，即可組合出多孔性燒結結構蒸發器進行薄膜蒸發相變化熱傳時的總熱阻。並探討低過熱度時，薄膜量為一個影響薄膜蒸發熱傳的重要因素。

關鍵字

過熱度 ； 粉末燒結結構 ； 薄膜蒸發 ； 熱阻 ； 蒸發器

並列摘要

With the developement of semiconductor manufacturings and high power light emited diodes, the heat exchange abilitiy of high heat density has been noticed recently. In the small heat transfer area, the requirement of a smaller temperature difference is necessary to reduce the thermal resistance. Two-phase heat transfer devices are a proper thermal solution for this high heat density application. At low superheat levels, thin-film evaporation at the evaporator of a two-phase heat transfer device plays an important role in its overall heat transfer performance. Among various wick structures, the mechanism of thin-film evaporation transfers latent heat obviously and efficiently in sintered powder structures. With various structural parameters, such as the powder sizes of 45 μm, 75 μm, 150 μm, the powder shapes of spherical, dendritic, and three levels of structural thickness, this study investigates the correlations between superheat levels and heat fluxes.
A two-part experiment in this study consists of effective thermal conductivity and phase-changing heat transfer. The evaporative heat transfer experiment includes transformation of phase-changing mechanisms and thin-film evaporation heat transfer. To measure the small temperature differences and heat fluxes at low superheat levels, this study developed an apparatus composed of a thermal guard test chamber, a direct sintering design, a pressure control loop, and a data acquisition system. The experimental results show that a smaller powder size achieved a higher effective thermal conductivity in both powder shapes. Spherical powder structures achieved twice the effective thermal conductivity of dendritic powder structures for each powder size. The thin-film evaporation heat transfer measurement showed that the heat flux increases proportionally with the superheat between 2 to 5 K. At the same superheat level, structures with thinner structural thickness and smaller powder size have a higher heat flux and lower thermal resistance, and dendritic powder structures perform better than spherical powder structures.
Assisted by image recognition process, the edge length of solid sintered structures cross-sectional could be detected. The length of the contour is the contact interface between the solid structure and the liquid working fluid, and may represent the tendency of the total amount of thin film. However, this evaluation and the experimental results produce the total thermal resistance of heat transfer in the evaporator. In these structural parameters, the amount of thin film may be the primary factor affecting thin-film evaporation heat transfer.

並列關鍵字

superheat level ； sintered powder structure ； thin-film evaporation ； thermal resistance ； evaporator

參考文獻

[1] S. S. Murthy, Y. K. Joshi, W. Nakayama, Single chamber compact thermosyphons with micro-fabricated components, Proceedings of the 7th Intersociety Conference on Thermal and Thermomechanical Phenomenon in Electronic Systems, Vol. 2, (2000), pp. 321-327.

[2] R. S. Gaugler, Heat transfer devices, U.S. Patent 2350348, 1944.

[3] G. M. Grover, Evaporation-condensation heat transfer devices, U.S. Patent 3229759, 1966.

[4] T. Nguyen, M. Mochizuki, K. Mashiko, Y. Saito, I. Sauciuc, R. Boggs, Advanced cooling system using miniature heat pipes in mobile PC, IEEE Transactions on Components and Packing Technology, Vol. 23, (2000), pp. 86-90.

[5] J. Legierski, B. Wiecek, Steady state analysis of cooling electronic circuits using heat pipes, IEEE Transactions on Components and Packing Technology, Vol. 24, (2001), pp. 549-553.