探討帶有熱源之圓棒在去離子水和海水中的淬冷

The quenching of cylinders with heating power in deionized water and sea water

何元宏(Ho, Yuan Hong)

指導教授 ： 潘欽 ； 蘇育全

國立清華大學/原子科學院/工程與系統科學系/碩士(2015年)

摘要

當一高溫熱體與低溫冷體接觸時，膜沸騰現象極可能被誘發。此種沸騰現象會抑制熱傳效果而阻礙冷卻的過程。在設計緊急爐心冷卻系統時，膜沸騰所造成的影響值得納入考慮，特別是當核電廠發生冷卻水流失事故時，爐心溫度極可能超過最低膜沸騰溫度而引發膜沸騰。再者，緊急事故發生後，雖然爐心立刻停機，但由於運轉時所產生的不穩定產物，仍會持續地衰變並釋出所謂的衰變熱。除此之外，冷卻水再淹沒的過程中，冷卻水的次冷度可能劇烈的改變。因此，了解衰變熱及次冷度對於淬冷過程的影響，即成為一重要且有趣的課題。由於核電廠大多建造於鄰近海岸的地方，因此海水成為一豐富的冷卻水來源。正因為如此，海水的淬冷行為也值得探討，以了解其在緊要時，對於核能安全可以扮演的角色。
本研究展示了無加熱與加熱的垂直黃銅圓棒，於不同次冷度之去離子水及海水中的淬冷。圓棒直徑與長度分別為24mm及112mm。六個K-type的熱電偶按照不同軸向深度擺放在距離表面2mm的位置。在經過每一次的淬冷實驗後，圓棒都會藉由相同的拋光處理程序使表面性質大約維持一致。首先，圓棒先在高溫加熱爐中預熱至約550°C，接續啟動圓棒內嵌的彈筒式加熱棒，當熱電偶1號達600°C 時，啟動氣壓缸將圓棒快速沒入淬冷池中。同一時間，啟動溫度數據擷取系統與高速攝影機，同步地記錄溫度的變化與淬冷圖譜的改變。淬冷池為一長195mm，寬195mm，高150mm的鋁框水缸。實驗進行前在其中注入去離子水或是海水，並藉由四個角落的T-type熱電偶監控池水溫度。實驗結果指出，在相同的池水溫度下，特別是在高池水溫度，例如池水溫度為95°C，去離子水實驗中，圓棒加熱時的膜沸騰時間為圓棒無加熱時的3.5倍。此外，隨著池水溫度的增加，膜沸騰時間也會增長。淬冷峰移動的速度除了隨著池水溫度增加而下降，圓棒有加熱時的淬冷峰移動速度也比圓棒無加熱時慢。萊登佛洛斯特溫度隨著池水溫度增加而下降，然而，熱源似乎對於萊登佛洛斯特溫度沒有顯著的影響。本研究結果指出，海水擁有相較於去離子水更加優異的熱移除能力，臨界熱通率約是去離子水的1.5倍。值得注意的是，不同於傳統的認知，高溫圓棒與淬冷液接觸後，並非立即形成蒸氣膜，而是藉由高溫核沸騰所產生的氣泡彼此間合併所致。

關鍵字

淬冷 ； 膜沸騰 ； 萊登佛洛斯特溫度 ； 衰變熱 ； 離子態溶液

並列摘要

Film boiling is usually induced while a very hot object contacts with a coolant. Such phenomena will deteriorate the heat transfer and degrade the cooling process. Film boiling is of significant concern for the design of an emergency core cooling system after a hypothetical loss of coolant accident happens in a nuclear power plant. Furthermore, after a nuclear power plant is shut down, the fuel rods will continue to release the heat due to decay of fission products. Moreover, the subcooling of coolant might be changed dramatically during the reflood process. Therefore, it is of significant importance and interest to understand the effect of decay heat and subcooling of coolant on the quenching process of a hot object. Besides, because the nuclear power plants are usually located in the place near the coast, the sea water is considered as an abundant source of coolant. Thus, it is also important to study the quenching behavior of sea water.
This study demonstrates the quenching of a vertical brass cylinder without and with heating power in deionized water or sea water with different subcoolings. The diameter and length of the cylinder is 24 mm and 112 mm, respectively. Six K-Type thermocouples are embedded 2mm below the cylinder surface at different axial locations. The cylinder is welled polished with the same process after every single test to maintain the surface condition approximately the same. The cylinder is first heated up to an initial temperature of about 550 °C in a radiant furnace, then the heater inside the cylinder is turned on and subsequently immersed into the quench pool by a pneumatic cylinder when the thermocouple (based on TC1, the lowest one ) reaches 600°C. The dimension of quench pool is 195 mm x 195 mm x 150 mm (depth), which is partially filled with deionized water or sea water. The temperatures of the quench pool are measured with T-Type thermocouples at four corner. The quenching behavior is visualized by a high-speed video camera simultaneously with the temperature measurements. The experimental results reveal that, with heating power of 105W, corresponding to the mean flux of 13.5kW/m2, which is to simulate the heat flux due to decay heat at about 1hour after reactor shut down, the duration of film boiling becomes much larger than the case without heating power under the same subcooling condition, especially for the low subcooling condition. For instance, the duration of film boiling in the case with heating power is 3.5 times longer than that in the case without heating power in deionized water with subcooling of 5˚C. Besides, the duration of film boiling increases with decreasing subcooling. The heating power and decreasing subcooling slow down the quench speed. The Leidenfrost temperature decreases significantly with decreasing subcooling. However, the heating power has no significant effect on the Leidenfrost temperature. This study also reveals that the sea water has better cooling capability than that of deionized water. The critical heat flux of sea water is 1.5 times larger than that of deionized water. Significantly, the formation of vapor film around the cylinder was not immediate as traditionally thought, but formed through the bubble coalescence of nucleate boiling at high temperature.

並列關鍵字

Quenching ； Film boiling ； Leidenfrost temperature ； Decay heat ； dissolved salt

參考文獻

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