自再潤濕流體於金屬與高分子毛細結構迴路式熱管熱傳增強研究

Abstract

本文旨在應用自再潤濕流體於迴路式熱管中提升其熱傳性能，並分別對於毛細結構材質為金屬(鎳)與高分子(鐵氟龍)時進行探討。自再潤濕流體(Self-rewetting fluid, 通常為高碳數醇類水溶液)擁有優異的潤濕能力，也具備使表面張力梯度逆轉之特性，當溫度達到特定值，其表面張力將隨著溫度增加而遞增。馬蘭哥尼效應(Marangoni effect)顯示高表面張力之流體會拉動低表面張力流體，形成表面張力之驅動流，有助於二相熱傳裝置內的低溫工作流體推往加熱端潤濕，延後乾涸現象發生並增加熱傳性能。 在表面張力的探討上，本研究量測不同濃度與成分的自再潤濕流體水溶液於25~70 oC時的表面張力值，選擇的工作流體為正丁醇(1-Butanol)，正戊醇(1-Pentanol)，正已醇(1-Hexanol)，二丁醇(2-Butanol)之水溶液，量測結果顯示自再潤濕流體水溶液於飽和濃度時擁有最高的正向表面張力梯度，其中以6 %正丁醇與20 %二丁醇最高，分別為0.30 mN/moC與0.27 mN/ moC。因此本文選用此兩者自再潤濕流體應用於迴路式熱管，探討其熱傳性能。當毛細結構為鎳時，迴路式熱管測試結果顯示當工作流體為6 %正丁醇與20 %二丁醇時，其最大熱通量相較於純水工作流體時的350 W分別提升了40 % (500 W) 與 70 % (600 W)，而最低系統總熱阻也從0.31 oC/W降至0.22 oC/W。 當毛細結構材質為鐵氟龍時，由於鐵氟龍的表面疏水性，純水無法作為其工作流體，因此本文利用自再潤濕流體的潤濕能力，克服純水無法通過鐵氟龍毛細結構之限制。實驗結果顯示當工作流體為20 %二丁醇時，最大熱負載可達250 W，最大熱通量為12.9 W/cm2，最低系統總熱阻為0.54 W/⁰C。最高熱通量與國外文獻平均3.6 W/cm2 比較後，高出了三至五倍。

The objective of this study is the application of self-rewetting fluid as the working fluid on loop heat pipe (LHP), with nickel and teflon(PTFE) as the capillary structure materials respectively. Self-rewetting fluids, generally the dilute aqueous solutions of high carbon alcohols(number of carbon atoms ≥ 4), shows not only better wettability compared with pure water but also a special thermophysical effect that the surface tension increases with increasing temperature. Since the Marangoni effect induces a flow from liquid with lower surface tension to the liquid with higher surface tension, which results a liquid flow to hotter interface, it is advantageous to use self-rewetting fluid as working fluid on heat-transfer devices. In this study, we measure the surface tension of different self-rewetting fluids at different concentration in the temperature range 25~70 oC, and the working fluids of 1-butanol, 1-pentanol, 1-hexanol and 2-butanol(isomer of 1-butanol) are choosed. The result shows that self-rewetting fluids possess higher positive surface tension gradient('σ' _'T' ) at saturation. Furthermore, the 1-butanol (6%) and 2-butanol (20%) show higher 'σ' _'T' than others, which are 0.27 mN/moC and 0.30 mN/moC respectively. These two self-rewetting fluids are then applied to the LHP with nickel capillary structure. The maximum heat load are 500W and 600W relatively when the working fluid is 1-butanol (6%) and 2-butanol (20%) , which was increased by 50 % and 70 % compared with using water as working fluid, and the total thermal resistence decreased from 0.31 oC/W to 0.22 oC/W. Due to the hydrophobicity of PTFE, water can’t be used on PTFE capillary structure. This study applied the self-rewetting fluids, which possess better wettability than water, on LHP with PTFE capillary structure. The results show that when the working fluid is 2-butanol (20%), the LHP system successfully work and the maximun heat load can reach 250W with total thermal resistence 0.54 oC/W, and the maximum heat flux is 12.9 W/cm2. Compared with results in other literatures, the maximum heat flux in this study is higher by 3 to 5 times other reference.