Computational Fluid Dynamics of Mixing Performance in Microchannel (original) (raw)
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A two-dimensional domain of multiphase flow analyses in this study using the Volume of Fluid (VOF) model was carried out in order to simulate and predict the fluid flows and mixing performance of two miscible liquids in various microchannel configurations. The various microchannels configurations were designed accordingly and the simulation was carried out based on the justified conditions, assumptions and considerations by using the commercial computational fluid dynamics (CFD) software, FLUENT. The grid type and size of the computational domain were verified in terms of stability by performing the grid independence analysis. The result showed that static mixing would be possible to achieve in various configurations of microchannels, however, the simulation results predicted that it appeared to be more efficient in complex and retrofitted microchannels. It showed the potential to promote and enhance chaotic advection, compositions distribution, and diffusivity as compared to basic microchannels that are mostly dependent only on the injection focus.
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Micromixers have received much interest as essential part of microfluidic devices. Therefore, enhancement of mixing quality has gained a lot of attention in recent years. In the present study, improvement of mixing quality for two different miscible liquids is considered in passive micromixers. Numerical approach is based on a second order finite volume Jameson scheme in order to solve two dimensional incompressible Navier-Stocks and mass transport equations by implementing artificial compressibility. Mixing quality is influenced by Reynolds and Schmitt numbers as well as size and location of the ribs. Diffusion mechanism has the main role for mixing in micro scale fluid flows; therefore, increasing Peclet number leads to extend mixing time. In order to enhance mixing quality, ribs are used in different locations through the microchannel which cause more instability in the fluid flow and leads to a better mixing. The Reynolds number is constant while the Schmitt number is in the range of 10 to 100. However, in order to laminar fluid flow, ribs just have an influence near itself and faraway, mixing mechanism return to earlier state. Therefore, in low Reynolds numbers they have no effective influence. When Reynolds number increase, flow instability that is created by different ribs leads to a better mixing.
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One-dimensional and two-dimensional models for microchannel flow with noncontinuum (slip-flow) boundary conditions have been presented here. This study presents an efficient numerical procedure using pressure-correction-based iterative SIMPLE algorithm with QUICK scheme in convective terms to simulate a steady incompressible two-dimensional flow through a microchannel. In the present work, the slip flow of liquid through a microchannel has been modeled using a slip length assumption instead of using conventional Maxwell's slip flow model, which essentially utilizes the molecular mean free path concept. The models developed; following this approach lend an insight into the physics of liquid flow through microchannels.
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Effective mixing is an important task in microfluidics for chemical and biochemical applications. Due to the small size and consequently the low Reynolds number, mixing in microchannels relies on diffusive transport. This paper discusses an analytical model of diffusive mixing in microchannels. The dimensionless analysis generalizes the solution for different channel sizes and different diffusion coefficients. The Peclet number is the only parameter of the model. Furthermore, the paper presents the result of a nonlinear model of diffusive mixing in microchannels. The nonlinear model considers the dependence of the diffusion coefficient on the concentration. A simple micromixer was fabricated using a lamination technique. Measurement results with the micromixer verify the analytical results.
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A finite element model has been used in order to study the mixing process of species in a 100-µm-wide zigzag microchannel integrating a "Y" inlet junction. The distribution of the concentration was obtained by solving successively the Navier-Stokes equation and the diffusion-convection equation in the steady state form. Because of the large range of Reynolds numbers studied (1 < Re < 800), the 2D diffusion-convection simulations are carried out with high diffusion coefficients. The results illustrated the effects of both flow rate and channel geometry on hydrodynamics and mixing efficiency. Below a critical Reynolds number of ∼80, the mixing is entirely ensured by molecular diffusion. For higher Reynolds numbers, simulations revealed the mixing contribution of laminar flow recirculations. This effect increases for lower values of diffusion coefficients. Experimental studies on the mixing of species at different flow rates are reported showing the same hydrodynamic tendency.
Computational study of convective–diffusive mixing in a microchannel mixer
Chemical Engineering Science, 2011
Scalar mixing due to convection and diffusion in a microchannel mixer is studied using CFD. A method is developed to quantitatively measure the effect of false diffusion on scalar decay rate. This method computes an average false diffusivity from a given numerical solution and it is not limited to any particular numerical scheme. It is found that a range of molecular diffusivity exist in which average false diffusion is smaller than molecular diffusion and scalar decay rates can be computed accurately with CFD in the mixer. This range of molecular diffusivity covers most of the liquid solutions encountered in chemical and biochemical engineering. When effective diffusivity is used, this range can be further expanded. The predicted mixing structures agree well with experimental results in literature. The classical lamellar structures of the baker's transformation are strongly affected by diffusion. The striation doubling process is destroyed by diffusion broadening at very early stage in the mixer. The optimal mixing is achieved at low Re when the mixing mechanism in the mixer is the baker's transformation. At higher Re, secondary flow is generated and the mixing mechanism is the competition of the kinematics of the baker's transformation and the dynamics of the cross sectional flow. Results show that the secondary flow hinders mixing and the scalar decays at lower exponential rates than when the mixing is due to the baker's transformation alone.
Numerical Investigation of Flow and Mixing in a Microstirrer
Journal of Fluid Science and Technology, 2006
Low Reynolds number (Re ~ 0.001) flow and mixing in a Y-shaped micromixer with a four-paddle rotor placed in its junction were investigated using computational fluid dynamics (CFD). The rotation speed of the rotor ω ranges from 100 to 2000 RPM (revolutions per minute). The flow visualization shows the enhance mixing by the rotating paddle that are stretching the fluid particles, up and down flows in the junction, transverse velocity in a section of the mixing channel. Further, at a high value of ω, a circulatory flow formed in the mixing channel greatly enhances the mixing rate. The mixing potential of the mixer is measured by the strain rate. The dispersive mixing efficiency coefficient is considered as a criterion for evaluating the mixer. The other criterion is the distributive mixing efficiency, which is based on the homogeneity of the concentration distribution. Both the criteria indicate that the mixing significantly improves with increasing rotation speed. With regard to obtaining similar mixing efficiency, a mixer with a rotor reduces the required length of the mixing channel as compared with that of a mixer without a rotor. The reduced length is proportional to ω.