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

• Main Authors: Fu, Ben-Ran, 傅本然
• Other Authors: Pan, Chin
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/70008806196495823677

博士 === 國立清華大學 === 工程與系統科學系 === 99 === 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.