加熱線傾角對池沸騰熱傳之影響

Abstract

核沸騰現象與我們日常生活息息相關，由於沸騰過程中的兩相變化能顯著提升熱傳遞效率，因此在現代化社會中被廣泛應用；然而，核沸騰過程中的熱通量存在明顯的限制，稱為「臨界熱通量(Critical Heat Flux, CHF)」，當熱通量超過CHF時，系統的溫度會迅速上升，進而引發材料損壞甚至系統失效。因此掌握CHF的大小能有效地提高系統運行的安全性。核沸騰的熱傳遞機制包括汽泡的生成、成長、分離以及液體再濕潤。這些機制受到多種因素的影響，包括液體的物理特性(黏度、表面張力)、加熱表面特性(粗糙度、潤濕性、傾角)。故本研究的主旨為利用鋁線以及鎳線及0.1 mm、0.2 mm、0.3 mm三種不同線徑作為加熱表面，並使用去離子水(Deionized water, DI water)、及電子氟化液(HFE-7100)為工作流體進行0°、30°、60°及90° 四種不同傾角之池沸騰實驗，並觀察加熱表面的差異及不同工作流體對沸騰熱傳性能的影響。 研究結果顯示，高熱傳導率的鋁線在線徑由0.3 mm減至0.1 mm時，CHF顯著提升53%，歸因於較小韋伯數促進汽泡脫離與液體回流；相反地，低熱傳導率的鎳線呈現相反趨勢，0.2 mm與0.1 mm線徑分別使CHF下降23.3%與27.5%，同時核沸騰起始點(Onset of Nucleate Boiling, ONB)過熱度延後4°C與4.6°C。 傾角對沸騰熱傳之影響結果顯示，DI water在30°傾角達最佳CHF值，而後隨角度增加而下降，90°時最高下降32.5%；而HFE-7100因較低表面張力特性， 0.1 mm鋁線在30°與60°時CHF分別提升9.2%與13%，然90°下降13.8%。

The two-phase change that occurs during boiling significantly enhances heat transfer efficiency, making it widely applicable in modern systems. However, nucleate boiling is limited by the Critical Heat Flux (CHF); exceeding this threshold can lead to rapid temperature increases and potential material damage. Therefore, enhancing CHF is crucial for improving safety. Heat transfer in nucleate boiling involves several processes: bubble nucleation, growth, departure, and liquid rewetting. These processes are influenced by the properties of the liquid properties (viscosity, surface tension) and the characteristics of the heating surface characteristics (roughness, wettability, and inclination). This study utilized aluminum and nickel wires of three different diameters (0.1 mm, 0.2 mm, and 0.3 mm) as heating surfaces. Pool boiling experiments were conducted using deionized water and HFE-7100 at four inclination angles (0°, 30°, 60°, and 90°) to assess the effects of material, diameter, fluid, and angle on boiling performance. For high-conductivity aluminum wires, reducing the diameter from 0.3 mm to 0.1 mm increased CHF by 53%, attributed to improved bubble departure and rewetting resulting from a lower Weber number. In contrast, low-conductivity nickel wires exhibited opposite trends; diameters of 0.2 mm and 0.1 mm decreased the CHF by 23.3% and 27.5%, respectively, and delayed ONB by 4 to 4.6°C. Regarding inclination, DI water exhibited peak CHF at an angle of 30°, which decreased by 32.5% at 90°. In contrast, HFE-7100, with its lower surface tension, resulted in CHF increases of 9.2% and 13% at 30° and 60°, respectively, while experiencing only a 13.8% decline at 90°.