以熔融鹽加熱之熱交換器流動沸騰研究

Investigation of Flow Boiling Heat Transfer in Heat Exchanger with Molten Salt Heating

歐益呈

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(2013年)

摘要

本研究針對熔融鹽於太陽熱能發電系統應用設置一個實驗環路，以模擬電廠利用太陽熱能加熱熔融鹽之工作溫度，實驗時應用電熱器加熱熔融鹽到達熔融態後，透過活塞裝置將儲存桶中之高溫熔融鹽推入測試段中，在測試段中高溫熔融態熔融鹽的熱功率以熱交換器作為媒介傳遞給低溫工作流體(水或氦氣)，熱側與冷側流體以逆向流動。研究上採用微流道及迷你流道的兩種熱交換器均以鋁合金6061為基材，其具有良好導熱能力。本研究應用放電、機械加工方式銑出流道分別製作微及迷你流道型熱交換器，再採用不鏽鋼夾合熱交換器並設置流體供應、輸出端的進、出口。微流道熱交換器兩側流道長、寬、深分別為40、0.4、0.4 mm；迷你流道熱交換器兩側流道長60、2、1.5 mm，流道數目均為5條。
研究內容探討氦氣在微流道熱交換器中，改變冷側質量通率(Ghe=16.9、23.7、30.5 kg/m2s)及熱側質量流率下，對於測試段溫度、熱通量、熱傳係數、熱效率影響。水在迷你流道熱交換器中，改變質量通率(Gwa=3.9、4.4、5 kg/m2s)及進口次冷度，對於測試段溫度、熱通量、沸騰曲線、平均熱傳遞係數、熱效率影響。氦氣實驗結果顯示，冷側流量對於熱交換器的熱傳特性影響顯著；在高質量通率氦氣實驗條件下均具有較高的熱通量、總熱傳遞係數及效率，其效率約在24-65%之間隨氦氣流量增加而增加。水的實驗結果顯示，高壁面溫度使水展現"過渡沸騰"的特性，壁溫越高熱通率越低。水的平均熱傳遞係數採用沸騰模式與對數平均溫差計算出結果差異小於20%。以水為工作流體的熱傳效率在75-90%之間。

關鍵字

熔融鹽 ； 熱交換器 ； 流動沸騰

並列摘要

This study explores an experimental investigation on single-phase and boiling heat transfer in micro- and minichannel heat exchangers (counter-current type), respectively, with molten salt heating, which is an application of a solar thermal power plant. For the present study, the heat of the solar thermal on molten salt is simulated by using electrical power supply. The high temperature liquefied molten salt (Hitec) is driven into the test section by the piston device. The working fluid in the hot side is molten salt and in the cold side is water (for the boiling experiment) or helium (for the single-phase flow experiment).
The heat exchangers are prepared from aluminum alloy materials through electric discharge machine and computer numerical control machine processes. Both of micro- and minichannel heat exchangers contain five channels on each side and are covered with stainless steel. The depth and width of the microchannel on both sides are both 0.4 mm, and the length of the microchannel is 40 mm. The depth, width, and length of the minichannel are 2 mm, 1.5 mm, and 60 mm, respectively. During the single-phase flow experiment, i.e., the cold side fluid is helium, the mass flux of helium is varied from 16.9 to 30.5 kg/m2s, and the mass flux of molten salt is varied from 306 to 605 kg/m2s. Moreover, during the boiling experiments, i.e., the cold side fluid is water, the mass flux of water is varied from 3.9 to 5 kg/m2s.
The results of the single-phase flow experiment demonstrate that heat transfer characteristics are significantly influenced by the mass flux of the cold side fluid. The higher mass flux of helium leads to the better heat transfer performance (i.e., heat transfer rate, heat transfer coefficient, and effectiveness). The effectiveness of the single-phase flow experiment is 24%-65%. On the other hand, the results of the boiling experiment show the transition boiling characteristics of water, indicating that heat flux decreases with an increase in the wall temperature. The boiling heat transfer coefficients, calculated by two methods, i.e., boiling model and log-mean-temperature-difference (LMTD), show an insignificant difference, which is within 20%. Moreover, the effectiveness of the boiling experiment is 75%-90%.

並列關鍵字

Molten ； Heat exchanger ； Flow boiling

參考文獻

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[2]IEA(International Energy Agency), Concentrating Solar Power, OCED IEA,2010, Paris.

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