離子態水溶液次冷態沸騰的探討

Subcooled Pool Boiling of Electrolyte Solutions

蔡昀孜(Yun-Tzu Tsai)

指導教授 ： 潘欽

國立清華大學/原子科學院/核子工程與科學研究所/碩士(2018年)

摘要

本實驗採用線徑為0.3mm，純度為99.95%之白金線作為主要加熱體，利用直流電源供應器提供的電功率轉換為熱功率，受限於窄線徑，針對白金溫度與電阻高度相關的特性，經由事前油浴實驗測量其電阻值及矽油溫度回推白金熱阻絲的壁溫，同時為避免實驗電極和熱阻絲表面與離子態水溶液產生化學變化，在電極外披覆熱縮套管並以兩款電子絕緣膠隔絕其相互接觸。每次實驗以內層不鏽鋼製、外層陶瓷鍍膜的水槽承裝六升之溶液並以下方均溫型PID加熱板調控至所需之次冷度，透過水槽內置的四支T-type熱電偶與MX-100數據擷取裝置記錄池水溫度，且利用水槽前方可視化視窗進行高速攝影機影像拍攝、做圖譜比對。
經由影像觀察、壁溫變化及熱通率計算後，本研究實驗結果可以發現：就氣泡尺寸而言，氯化鈉水溶液較去離子水擁有較小尺寸的氣泡，且受到周圍離子水合力及靜電力影響，氯化鈉水溶液中的氣泡即使相觸易因相斥而不會產生併合現象；而在氣泡生成速率方面，氯化鈉水溶液與去離子水相似，隨著熱通率上升增強其蒸汽產生率，氣泡的生成也隨之加速。去離子水由於並無離子效應發生，在高熱通率時，蒸氣生成速率高於氣泡脫離速率將使鄰近氣泡合併，進一步在熱表面形成氣膜覆蓋，終致白金熱阻絲燒紅，即達臨界熱通率；相反的，氯化鈉水溶液中氣泡不易併合的情況將使熱表面可以與周圍較冷的工作流體接觸，不斷生成、衝出的氣泡也能帶走熱量，達到提高熱傳的效果。

關鍵字

池沸騰 ； 離子態水溶液 ； 次冷態 ； 氣泡併合

並列摘要

In this experiment, a platinum wire with a diameter of 0.3 mm and a purity of 99.95% is used as the test section, and the electric power supplied by the DC power supply is converted into heat flux. Through the oil bath test beforehand, the highly correlation between the platinum temperature and the resistance is used to predict the wall temperature of the platinum wire, which is limited by the narrow wire diameter. In order to avoid the surface changes of the experimental electrodes and the wire in electrolyte solutions, the electrodes are covered with the heat-shrink tubes and two electronic insulating glues are also used to separate the test section from electrolyte solutions preventing electrochemical reaction. The stainless steel tanks with ceramic coating are filled with six liters of solution and the required subcooling are adjusted by PID hot plate. Four T-type thermocouples in the tank are used to measure and MX-100 data acquisition system records the pool temperature and voltage.
Through image comparison, we can find that the bubbles in sodium chloride solution are smaller than in deionized water. With surrounding ion hydration force and Coulomb electrical force, the bubbles in the sodium chloride solution do not combine, even they are touched with each other will get repelled. In terms of bubble generation rate, the sodium chloride solution is similar to deionized water, and vapor generation is enhanced when the heat flux increases. Because there is no ion effect in deionized water, at high heat flux, the bubble generation rate is faster than the bubble detachment rate, which will cause adjacent bubbles to merge, forming a vapor film cover the heating wire, and finally the platinum wire will burn. On the contrary, the bubbles in the sodium chloride solution are not easy to combine, so that the hot surface can contact with the colder working fluid and the continuously generated and rushed away bubbles can also take heat away.

並列關鍵字

pool boiling ； electrolyte solution ； subcooled boiling ； bubble coalescence

參考文獻

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