甲醇水溶液在單一漸擴微流道之對流沸騰研究

Convective Boiling of Methanol-Water Mixtures in a Single Diverging Micro-channel

林伯翰(Po-Han Lin)

指導教授 ： 潘欽

國立清華大學/原子科學院/工程與系統科學系/碩士(2010年)

摘要

本研究的目的在於發展一種低壓降、高穩定度但仍保有高熱傳能力之微流道蒸發器供雙成分液體，如甲醇水溶液，沸騰與蒸發之用。研究中係利用微機電技術來製作出不同漸擴角度之矽質微流道，藉由改變其通入之流量與加熱功率來進行不同濃度甲醇/水混合溶液在微流道內單相與雙相沸騰熱傳之研究。
研究結果顯示，當工作流體在微流道內為單相流動時，有明顯的流量與濃度效應，且熱通率與熱傳遞係數會隨質量通率增加而增加。在剛開始進入雙相沸騰後，熱通率與熱傳遞係數會有顯著地上升現象。之後，熱通率隨著壁過熱度的增加而逐漸趨緩，熱傳遞係數則隨壁過熱度之增加而減少，最後達臨界熱通率。研究結果顯示，流道漸擴角度對於沸騰熱傳則沒有明顯的影響，甲醇莫耳分率的改變對於微流道內的沸騰熱傳具有顯著的影響。當工作流體進入沸騰雙相對流時，受到表面張力梯度所造成的Marangoni效應之影響，導致氣液介面附近液體牽引對流現象，因而增強其熱傳與質傳機制。在甲醇莫耳分率0 < xm≦0.5時，明顯觀察到氣液介面受Marangoni效應的影響，因表面張力的不平衡造成液膜破碎扭曲現象。尤以甲醇莫耳分率xm=0.3時其流動型態為液膜破碎時所散佈之熱通率與壁過熱度區間較其他莫耳分率為大，進而延遲了液膜乾化的發生，造成了臨界熱通率的提升。
綜觀研究成果發現，在單相流動狀態下，純水具有較高的熱通率與熱傳遞係數。然而，在雙相流動狀態下，甲醇莫耳分率的改變進而增強其熱傳能力，使具有較高的臨界熱通率但亦有較高的壁過熱度與雙相流壓降。

關鍵字

沸騰熱傳 ； 微流道 ； 雙成分 ； 流沸騰

並列摘要

The proposed study aims to develop a low pressure drop, highly stable and efficient microchannel evaporator with binary component mixtures, such as methanol-water mixtures. This study conducts experiments, flow visualization and modeling of convective boiling of methanol-water mixtures, with different concentrations and flow rates, in a diverging microchannel. Flow boiling curve, two-phase flow pattern and two-phase flow pressure drop are explored. All of the microchannels etched on SOI wafer using MEMS technology, such as bulk micro-machining and anodic bonding.
The experimental results reveal that boiling, heat flux increases with increasing superheat, but the slope decreases correspondingly. On the other hand, the results clearly indicate the strong influence of liquid mole fraction on the onset of boiling and the mixture composition results in a pivotal augmentation or reduction in heat transfer. Both critical heat flux and boiling heat transfer are enhanced at xm=0.3 due to the strong Marangoni effect. Flow visualization demonstrates five flow regimes : bubbly flow, slug flow, annular flow, film breakup, and dry out. Nevertheless, the film breakup regime plays an important role in triggering the CHF. At xm=0.3, the span of heat flux and superheat range helps to heighten the given input heat flux without dryout, and the highest CHF is demonstrated one other concentration. The results of present study suggest that by adding suitable amount of methanol in water may result in higher boiling heat transfer capability and higher two-phase flow pressure drop than pure water.

並列關鍵字

boiling heat transfer ； microchannel ； binary mixtures ； flow boiling

參考文獻

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