氣體加熱叉流式微流道熱交換器之流動沸騰熱傳研究

An Investigation of Flow Boiling Heat Transfer on Cross-flow Microchannel Heat Exchanger with Gas Heating

黃文利

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(2012年)

https://doi.org/10.6843/NTHU.2012.00299

摘要

本研究針對重組式甲醇燃料電池 (RMFC) ，設計一矽質叉流式微流道熱交換器，以有效地利用重組器 (reformer) 產出的廢熱，一方面降低重組器產生出來的氫氣溫度，另一方面將此熱量傳給液態甲醇，使其蒸發以利重組反應進行。
根據上述構想，實驗使用的叉流式微流道熱交換器採用微機電 (MEMS) 體型加工技術製作，材料以單晶矽晶圓為基材，其具有良好的導熱能力。其中微流道熱交換器晶片大小為2×2 cm2，且在其上下面蝕刻深度為200μm冷熱端流場板，再與pyrex 7740玻璃做陽極接合 (anodic bonding)。根據實驗室多年來的研究結果顯示，漸擴截面積的微流道設計可以降低液體在沸騰過程中出現的不穩定現象，因此本研究透過漸擴矩形微流道的設計來製作冷側(甲醇端)的流道，而熱側(氦氣端)則是採用均勻截面積矩形微流道。
根據甲醇重組器的操作條件，系統需要達到約200℃以上的高溫才能由甲醇轉化為氫氣，且重組反應為放熱反應，所以產生的氫氣於出口處可能會達到250℃以上的高溫。因此，本實驗設定進入微熱交換器之氦氣(替代氫氣)高溫至250-300℃。藉由改變熱測的溫度，來探討不同流動方向配置在微流道熱交換器所產生的熱交換效率差異及單相與雙相沸騰熱傳。實驗並利用高速攝影機，觀察流體沸騰時流道的雙相流譜。
綜觀研究成果發現，叉流式微流道熱交換器之效率在甲醇沸騰時隨熱側熱功率的增加而逐漸上升。此外，效率也隨甲醇質量通率的增加而增加，且此流量效應在單相時較為顯著。本研究之微流道熱交換器的效率最高約可達0.91且移熱能力約為50 kWm-2 。

關鍵字

微流道 ； 熱交換器 ； 氣體加熱 ； 沸騰熱傳

並列摘要

To utilize the exhausted heat produced from a methanol reformer (a part of a reformed methanol fuel cell) effectively, the development of a highly effective microchannel heat exchanger (MCHE) is of critical importance. This design could not only reduce the temperature of the hydrogen released from a reformer but also transfer the heat to the liquid methanol. Following our previous researches on the co- and counter-flow MCHEs, the two-phase flow boiling heat transfer characteristics in a cross-flow MCHE is investigated in the present work. The MCHE is made from 4-inches silicon wafer prepared through microfabrication processes with a dimension of 20 mm × 20 mm. Eighteen microchannels are etched on each side, and covered with Pyrex 7740 glass by anodic bonding. The channel depth on both cold- and hot-side is 200 μm, and the thickness of the wall between is the same of 200μm. To make the boiling flow more stable, the microchannels in the cold side employ a diverging design, as suggested in our previous studies. Liquid methanol is used as the boiling fluid, while hot helium gas is employed to simulate hydrogen.
From the experiment results, it indicates that in the efficiency with methanol boiling in the cold side gradually increases with an increase in hot-side thermal power. Moreover, the efficiency increases significantly with an increase in the mass flux. The highest efficiency could achieve about 0.91 and the transferred cooling power is around 50 kWm-2.

參考文獻

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