微流道內化學反應：雙相流現象與簡單流道設計促進反應之探討

Chemical Reactions in Microchannels: Two-phase Flow Phenomena and Simple Channel Design for Enhancement of Reactions

傅本然(Ben-Ran Fu)

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/博士(2010年)

https://doi.org/10.6843/NTHU.2010.00119

摘要

微流道內單相或雙相流動的應用在很多方面是相當重要，如：微型燃料電池中燃料及產物的輸送、沸騰傳熱應用於微電子元件之冷卻、微流體晶片內的化學反應、微反應器、微混合器等等。流體在微流道內流動通常屬於層流，其混合發生是由於分子的擴散。一般來說，分子的擴散是相當緩慢的。但是，在微流體晶片或微反應器中，不同流體或化學物質的混合是相當重要的，因為有效率的混合會導致較活躍的化學反應。雖然流體混合是由擴散來主導，但是，適當改變流道的微型結構可以顯著地增進混合的效果。
本研究探討混合液體(硫酸和碳酸氫鈉水溶液)在不同微流道(等截面積、漸縮、漸擴)內的單相及雙相流動之現象。硫酸與碳酸氫鈉水溶液可能會產生化學反應，而在微流道內產生二氧化碳氣泡。當化學反應生成二氧化碳氣泡，微流道內流動會由液體的單相流動轉換成為氣液雙相流。反之，若無二氧化碳氣泡生成，微流道內將維持液體的單相流動。實驗結果顯示微流道內的單相混合液體的流動壓降與傳統理論預測及數值模擬計算的結果皆有相當好的吻合，其誤差約在10％。當微流道內有氣泡生成時，本研究利用高速攝影機來觀察微流道內氣泡的成核與成長。本研究首先進行氣泡成長模式的文獻探討，更藉由實驗上的觀察與分析，發展出一個考慮氣泡與液體間對流質傳之氣泡成長模式。在實驗觀察中，本研究歸納出兩種不同氣泡成長的行為。第一種氣泡成長行為如下：氣泡的成核發生在微流道的壁面上，隨後，氣泡會隨著微流道內的流體一起往下游流動並會持續地呈現相當圓形的成長(可視氣泡與液體間沒有相對速度的存在)；其氣泡半徑的成長與時間會呈現平方根的關係，這個結果與文獻上所提出的質傳控制之氣泡成長模式相當吻合。第二種氣泡成長行為則為：氣泡在壁面上成核後，氣泡並不會隨著液體一起流動，而是停留在壁面上持續地成長；其氣泡半徑的成長與時間的三分之一成正比的關係，這個結果與本研究所發展出的氣體生成速率為一定值狀態下的氣泡成長模式相當吻合。
本研究進一步以理論及實驗探討不同微流道對於化學反應及混合效果的影響。藉由流體可視化的觀察，漸擴微流道內呈現較劇烈的化學反應，亦即在漸擴微流道內可以觀察到較多的二氧化碳氣泡生成。在濃度分佈的理論分析方面，本研究以同倫擾動法(Homotopy Perturbation Method)來求解漸擴與漸縮微流道內的濃度分佈。混合效果的實驗及濃度分佈的理論分析皆顯示漸擴微流道內流體的減速度效應會顯著地增加混合的程度(混合的增進會促進化學反應的發生)。並且，微流道內混合效果實驗的擴散特性與理論分析的結果呈現著相當一致的趨勢。實驗的觀察發現，微流道的壁面提供二氧化碳氣泡適當的成核址。並且，當微流道內有氣泡生成後，會增加微流道內液體的相互混合，進而造成更多氣泡的成核。綜合本研究結果，簡單的漸擴微流道設計可以運用在需要增進混合效果的微流道反應器中。

關鍵字

微流道 ； 化學反應 ； 氣液雙相流 ； 混合 ； 同倫擾動法

並列摘要

Fluid flow in microchannels, single-phase or two-phase, with or without bubble generation, is of significant interest for many applications, such as fuel/products delivery in micro fuel cells, boiling heat transfer for microelectronics cooling, chemical reactions in microfluidic devices, microchannel reactors, and micromixers as well as fundamental studies of various adiabatic gas-liquid two-phase flows. Flows in a microchannel are generally laminar, and mixing occurs due to molecular diffusion, which is a slow process. The well mixing of different fluids or reactive chemical species flowing through a microchannel is of importance, especially for chemical reactions in microfluidic devices, as effective mixing leads to highly active chemical reactions. Mixing of solutions as a phenomenon is driven by the diffusion itself, but an appropriate design of the channel cross section may result in the significant improvement in the mixing properties of particular microfluidic structure.
This study investigates the liquid-liquid mixtures flow (using sulfuric acid, H2SO4, and sodium bicarbonate, NaHCO3, as model fluids) in microchannels without a complex microstructure but with different axial cross sections (uniform, converging, and diverging) experimentally. Sulfuric acid may react with sodium bicarbonate, resulting in generation of CO2 bubbles in a microchannel. While chemical reactions take place and produce CO2 bubbles in a microchannel, the flow becomes gas-liquid two-phase flow. On the other hand, the flow remains single-phase (liquid-phase) while little or no chemical reactions occur and no bubble is formed in a microchannel. Firstly, the experimental data of single-phase flow pressure drop agree with the theoretical predictions and CFD simulation results within 10%. Furthermore, bubble nucleation and growth in microchannels under various conditions were observed using a high speed digital camera. The theoretical model for bubble growth with a chemical reaction is reviewed and a new model is developed considering convective effects on mass transfer. The bubble growth behavior for a particular case, without relative motion between the bubble and liquid, shows that the mass diffusion controls the bubble growth; consequently the bubble radius grows as a square root of the time and agrees very well with the model in the literature. On the other hand, for other cases the bubbles stay almost at the nucleation site while growing with a constant gas product generation rate resulting in the instant bubble radius following the one-third power of the time and agree very well with the model developed in the present study.
More importantly, the present study explores an experimental and theoretical investigation into the effect of channel axial cross-section shape on mixing and chemical reactions in microchannels. Flow visualization demonstrated that much more intense chemical reactions occurred in the diverging microchannel, as reflected by much more bubble generation. Results of both qualitative mixing experiments and a theoretical analysis indicated that the flow deceleration effect in the diverging microchannel significantly enhanced diffusive mixing in the lateral direction and, consequently, chemical reactions. Theoretical analysis on concentration distribution in the diverging and converging microchannels are solving by the Homotopy Perturbation Method. The trends in the diffusion characteristics are fairly consistent between qualitative mixing experiments and theoretical analysis. The solid walls provide sites for the nucleation of bubbles from CO2 produced and dissolved in the solution. Once the bubbles nucleate and grow large enough to block the flow, extensive circulation may occur upstream or downstream of the bubbles, which results in an increase in bubble nucleation. Consequently, it is concluded that a microchannel with a simple diverging cross-section design can be recommended to develop microfluidic devices requiring excellent mixing such as microchannel reactors.

並列關鍵字

microchannel ； chemical reactions ； gas-liquid two-phase flow ； mixing ； Homotopy Perturbation Method

參考文獻

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