應用自再濕潤流體於開放式熱管模型之冷卻能力與逆Marangoni對流之探討

Abstract

本研究利用自製之開放式平台，以加熱及散熱裝置使兩端產生溫度梯度，模擬熱管之操作情境，並使用不同之自再濕潤流體作為工作流體，觀察並比較其與水在熱傳性能上之差異。此種流體在合適操作環境下顯示了突出的熱傳表現，並展現了獨特的逆Marangoni效應，所獲之結果將提供未來相變化熱傳元件一嶄新的改良與優化之參據。 實驗結果顯示，在低至中熱通量時，自再濕潤流體皆能夠有效的提升實驗系統的熱傳：在低熱通量時 (6.6 W)，元件的燒乾時間皆得到了明顯的提升，相較於水，使用自再濕潤流體時可延長13.4%的燒乾時間；而流體在毛細結構中的流動，其毛細作用所造成的壓力梯度會隨著介面的前進而下降，流體前進速度也會因此而漸漸變小，最終停下。然而，自再濕潤流體所擁有的逆Marangoni效應會產生一額外與行進方向相同的作用力，促使流體往熱端集中，改善元件的散熱；在中熱通量時 (10 W)，自再濕潤流體亦能有效延長燒乾時間，較高的表面溫度也使自再濕潤流體較早達到逆Marangoni效應之作用溫度區間，進而得以提早加速，然而，由於溫度的提高，自再濕潤流體內的醇類蒸發較快，流體會較容易變回純水，縮短逆Marangoni的作用時間；而在高熱通量時 (13.5 W)，各流體皆因過高的表面溫度而快速變回純水，因此所有的熱傳表現皆與純水無異。 本研究亦進行了各流體極限操作功率之探討，實驗結果顯示自再濕潤流體可以將元件之極限操作功率由15 W提升至17 W，增加了13%；極限操作溫度則是由76°C提升至83°C，增加了9.2%，大幅降低了元件在高溫燒乾的可能。 此外，本研究推導出逆Marangoni拉引力與毛細驅動力對於自再濕潤流體在毛細結構中流動的情形。結果顯示，毛細作用主導了流體的流動，而取決於表面張力梯度的方向，Marangoni對流會阻礙或是促進流體的流動，佔比約為毛細作用的5%，然而在實驗的結果中，我們發現Marangoni對流的影響比預期的還要大；而對於0.1%庚醇與0.01%庚醇水溶液而言，濃度越大時會有較強的逆Marangoni對流流速，但是由毛細力造成的速度則較小；濃度越小時則相反。此外，SRWF的種類亦會影響流體的表面張力以及表面張力梯度，因此，如何選取適當的濃度與種類，同時將此兩作用力的優點最大化，將會是優化熱管熱傳表現的重要選擇。

In this study, we evaluated the effectiveness of using self-rewetting fluids (SRWF) in phase-change heat transfer device. By creating a temperature gradient over a copper strip, we simulated the working scenario of a heat pipe, and found the unusual behavior in surface tension of a SRWF was beneficial to fluid replenishment. As the hot zone could be kept wetted, dryout was effectively delayed. The better heat transfer performance was shown within a certain range of heat load. Under low to medium levels of heat load, SRWF performed better compared to that of water. Dryout time was delayed 13.4% under 6.6 W, showing that the inverse-Marangoni convection helped to sustain the fluid velocity and keep the hot zone wetted for a longer duration. However, the long-chain alcohol in the SRWF evaporated faster as heat input increased, and the SRWF immediately became water, deteriorating the cooling performance. We also investigated the maximum operating power of different working fluid and found that the critical heat load of SRWF could be 2 W higher than that of water, proving its potential to improve the heat transfer capability of phase-change heat transfer devices. In addition, we developed a model that incorporated the Marangoni effect on fluid flow in the wicked microstructures. The results indicated that fluid flow was dominated by the capillary force, while the Marangoni convection could act in favor or opposite to the direction of fluid flow. However, the Marangoni effect showed more importance in the experimental results. For the same type of SRWF (0.1wt% Heptanol and 0.01wt% Heptanol for instance), higher content of long-chain alcohol can produce stronger inverse-Marangoni convection due to higher surface tension gradient, but the lower surface tension results in lower capillary force. Hence, the leverage of these two opposite trends would be the key to optimize the SRWF for the better heat transfer performance of the devices in the future.