Flow Pulsation and Geometry Effects on Mixing of Two Miscible Fluids in Microchannels

2014 ◽  
Vol 136 (12) ◽  
Author(s):  
Houssein Ammar ◽  
Ahmed Ould el Moctar ◽  
Bertrand Garnier ◽  
Hassan Peerhossaini
Keyword(s):  
Reynolds Number ◽  
Residence Time ◽  
Pure Water ◽  
Mixing Efficiency ◽  
Mixing Enhancement ◽  
Flow Pulsation ◽  
Mean Flow ◽  
Mixing Index ◽  
Two Fluids ◽  
Passive Mixing

Many microfluidic applications involve chemical reactions. Most often, the flow is predominantly laminar, and without active or passive mixing enhancement the reaction time can be extremely long compared to the residence time. In this work we demonstrate the merits of the combination of flow pulsation and geometrical characteristics in enhancing mixing efficiency in microchannels. Mixing was studied by introducing a mixing index based on the gray level observed in a heterogeneous flow of pure water and water colored by rhodamine B. The effects of the injection geometry at the microchannel inlet and the use of pulsed flows with average Reynolds numbers between 0.8 and 2 were studied experimentally and numerically. It appeared that the mixing index increases with the nondimensional residence time (τ), which is inversely proportional to the Reynolds number. In addition, we show that the mixing efficiency depends strongly on the geometry of the intersection between the two fluids. Better mixing was achieved with sharp corners (arrowhead and T intersections) in all cases investigated. In pulsed flow, the mixing efficiency is shown to depend strongly on the ratio (β) between the peak amplitude and the mean flow rate. Optimal conditions for mixing in the microchannels are summarized as a function of Reynolds number Re, the ratio β, and the geometries.

Mixing Enhancement by a Micro-Rotor in Step Microchannel

2008 ◽  
Author(s):  
Thien Xuan Dinh ◽  
Yoshifumi Ogami
Keyword(s):  
Strouhal Number ◽  
Passive Scalar ◽  
Mixing Efficiency ◽  
Mixing Enhancement ◽  
Exit Channel ◽  
Mean Flow ◽  
Two Fluids ◽  
Convective Mixing ◽  
Active Mixer ◽  
And Performance

This paper describes numerically the design and performance of an active mixer which aims to exploit the chaos generated by a micro-rotor in continuous flow. The mixer consists of a step contraction-expansion microchannel and a micro-rotor placed at the step. The micro-rotor can be set into motion by laser power or magnetic field, inducing 3D motion in the surrounding fluid. By tracking streaklines from the inlet, we observed that fluids from the inlet can penetrate into the space between the paddles of the rotor, and then are mixed here. The streaklines also show that two fluids are twisted 90 degrees after passing the rotor region. It implies that mixing in the exit channel occurs in the height instead in the width of the exit channel. It makes the mixer applicable for channels with high aspect ratio of the cross section. The effectiveness mixing of the mixer is measured by the homogeneity distribution of a passive scalar on the outlet of the mixer. The results show that effectiveness of convective mixing induced by the rotor depends on Strouhal number, which is defined as the ratio of tip paddle velocity to mean flow in the channel. Mixing efficiency increases with increasing Strouhal number.

A Planar Labyrinth Micromixer

2010 ◽  
Author(s):  
Jeremy T. Cogswell ◽  
Peng Li ◽  
Mohammad Faghri
Keyword(s):  
Soft Lithography ◽  
Lab On A Chip ◽  
Fluorescein Isothiocyanate ◽  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Mixing Length ◽  
Two Fluids ◽  
Curved Channels ◽  
Passive Mixing ◽  
Important Challenge

Rapid mixing of two fluids in microchannels has posed an important challenge to the development of many integrated lab-on-a-chip systems. In this paper, we present a planar labyrinth micromixer (PLM) to achieve rapid and passive mixing by taking advantage of a synergistic combination of the Dean vortices in curved channels, a series of perturbation to the fluids from the sharp turns, and an expansion and contraction of the flow field via a circular chamber. The PLM is constructed in a single soft lithography step and the labyrinth has a footprint of 7.32 mm × 7.32 mm. Experiments using fluorescein isothiocyanate solutions and deionized water demonstrate that the design achieves fast and uniform mixing within 9.8 s to 32 ms for Reynolds numbers between 2.5 and 30. Compared to the mixing in the prevalent serpentine design, our design results in 38% and 79% improvements on the mixing efficiency at Re = 5 and Re = 30 respectively. An inverse relationship between mixing length and mass transfer Pe´clet number (Pe) is observed, which is superior to the logarithmic dependence of mixing length on Pe in chaotic mixers. Having a simple planar structure, the PLM can be easily integrated into lab-on-a-chip devices where passive mixing is needed.

scholarly journals A Computational Study on Mixing of Two-Phase Flow in Microchannels

2003 ◽  
Author(s):  
Farshid Bondar ◽  
Francine Battaglia
Keyword(s):  
Computational Study ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Mixing Index ◽  
Wave Type ◽  
Two Phase ◽  
Chaotic Flows ◽  
Two Fluids ◽  
Passive Mixing ◽  
Better Than

The passive mixing of water and alcohol, as two fluids with different densities, is carried out computationally in three-dimensional microchannels. Four designs of microchannels are considered to investigate the efficiency of mixing for Reynolds numbers ranging between 6 and 96. In a straight-type microchannel, mixing is very poor. In a square-wave-type microchannel, mixing is marginally better than the straight one. Mixing in the serpentine-type and twisted-type microchannels develops considerable better than the first two microchannels, especially at higher Reynolds numbers. However, in the twisted microchannel, the mixing index is substantially larger compared to the serpentine microchannel for the Reynolds number of 35. The higher mixing index implies the occurrence of spatially chaotic flows with a higher degree of chaos compared to the case of the serpentine microchannel. The results are compared quantitatively and qualitatively in Eulerian and Lagrangian frameworks and a correlation between Lagrangian chaos and Eulerian chaos is concluded.

scholarly journals A LES Study on Passive Mixing in Supersonic Shear Layer Flows Considering Effects of Baffle Configuration

2014 ◽  
Vol 6 ◽  
pp. 836146 ◽  
Author(s):  
Ren Zhao-Xin ◽  
Wang Bing
Keyword(s):  
Pressure Loss ◽  
Total Pressure ◽  
Large Scale ◽  
Mixing Layer ◽  
Total Pressure Loss ◽  
Mixing Efficiency ◽  
Mixing Enhancement ◽  
Supersonic Mixing ◽  
Passive Mixing ◽  
Supersonic Mixing Layer

Under the background of dual combustor ramjet (DCR), a numerical investigation of supersonic mixing layer was launched, focused on the mixing enhancement method of applying baffles with different geometric configurations. Large eddy simulation with high order schemes, containing a fifth-order hybrid WENO compact scheme for the convective flux and sixth-order compact one for the viscous flux, was utilized to numerically study the development of the supersonic mixing layer. The supersonic cavity flow was simulated and the cavity configuration could influence the mixing characteristics, since the impingement process of large scale structures formed inside the cavity could raise the vorticity and promote the mixing. The effect of baffle's configurations on the mixing process was analyzed by comparing the flow properties, mixing efficiency, and total pressure loss. The baffle could induce large scale vortexes, promote the mixing layer to lose its stability easily, and then lead to the mixing efficiency enhancement. However, the baffle could increase the total pressure loss. The present investigation could provide guidance for applying new passive mixing enhancement methods for the supersonic mixing.

Mixing by Time-Dependent Orbits in Spatiotemporal Chaotic Advection

2014 ◽  
Vol 137 (1) ◽  
Author(s):  
Mohammad Karami ◽  
Ebrahim Shirani ◽  
Mojtaba Jarrahi ◽  
Hassan Peerhossaini
Keyword(s):  
Secondary Flow ◽  
Lyapunov Exponents ◽  
Amplitude Ratio ◽  
Flow Patterns ◽  
Chaotic Advection ◽  
Mixing Enhancement ◽  
Flow Pulsation ◽  
Mean Flow ◽  
Homoclinic Connections ◽  
Vorticity Vector

The simultaneous effects of flow pulsation and geometrical perturbation on laminar mixing in curved ducts have been numerically studied by three different metrics: analysis of the secondary flow patterns, Lyapunov exponents and vorticity vector analysis. The mixer that creates the flow pulsation and geometrical perturbations in these simulations is a twisted duct consisting of three bends; the angle between the curvature planes of successive bends is 90 deg. Both steady and pulsating flows are considered. In the steady case, analysis of secondary flow patterns showed that homoclinic connections appear and become prominent at large Reynolds numbers. In the pulsatile flow, homoclinic and heteroclinic connections appear by increasing β, the ratio of the peak oscillatory velocity component of the mean flow velocity. Moreover, sharp variations in the secondary flow structure are observed over an oscillation cycle for high values of β. These variations are reduced and the homoclinic connections disappear at high Womersley numbers. We show that small and moderate values of the Womersley number (6 ≤ α ≤ 10) and high values of velocity amplitude ratio (β ≥ 2) provide a better mixing than that in the steady flow. These results correlate closely with those obtained using two other metrics, analysis of the Lyapunov exponents and vorticity vector. It is shown that the increase in the Lyapunov exponents, and thus mixing enhancement, is due to the formation of homoclinic and heteroclinic connections.

Mixing Enhancement in a Chaotic Micromixer Using Pulsating Flow

2014 ◽  
Author(s):  
Mohammad Karami ◽  
Mojtaba Jarrahi ◽  
Ebrahim Shirani ◽  
Hassan Peerhossaini
Keyword(s):  
Reynolds Number ◽  
Lyapunov Exponent ◽  
Steady Flow ◽  
Concentration Distribution ◽  
Pulsating Flow ◽  
Chaotic Advection ◽  
Mixing Enhancement ◽  
Mean Flow ◽  
Number Range ◽  
Velocity Amplitude

This study determines the simultaneous effects of spatial disturbance and flow pulsation on micromixing by using three different metrics: concentration distribution, Lyapunov exponent and axial vorticity. Numerical simulations are performed for both steady and pulsating flows through a microchannel made up of C-curved repeating units. Moreover, a straight microchannel is analyzed to compare the effects of chaotic advection and molecular diffusion, the main mechanisms of transverse mixing in the chaotic and straight mixer respectively. Simulations are carried out in the steady flow for the Reynolds number range 1≤Re≤50 and in the pulsating flow for velocity amplitude ratios 1≤β≤2.5, and the ratio of the peak oscillatory velocity component to the mean flow velocity, Strouhal numbers 0.1≤St≤0.5. It was found that chaotic advection improves mixing without significant increase in pressure drop. The analysis of concentration distribution implied that full mixing occurs after Reynolds number 50 in the steady flow. When the flow is pulsatile, small and moderate values of the Strouhal number (0.1≤St≤0.3) and high values of velocity amplitude ratio (β ≥ 2) are favorable conditions for mixing enhancement. Moreover, mixing has an oscillating trend along the microchannel due to the coexistence of regular and chaotic zones in the fluid. These results correlate closely with those obtained using two other metrics, analysis of the Lyapunov exponent and axial vorticity.

scholarly journals Effects of Channel Wall Twisting on the Mixing in a T-Shaped Micro-Channel

2019 ◽  
Author(s):  
Dong Jin Kang
Keyword(s):  
Reynolds Number ◽  
Cross Section ◽  
Channel Wall ◽  
Numerical Study ◽  
Twist Angle ◽  
Mixing Enhancement ◽  
Micro Channel ◽  
Number Range ◽  
Mixing Performance ◽  
Two Fluids

A new design scheme is proposed for twisting the walls of a microchannel, and its performance is demonstrated numerically. The numerical study was carried out for a T-shaped microchannel with twist angles in the range of 0 to 34π. The Reynolds number range was 0.15 to 6. The T-shaped microchannel consists of two inlet branches and an outlet branch. The mixing performance was analyzed in terms of the degree of mixing and relative mixing cost. All numerical results show that the twisting scheme is an effective way to enhance the mixing in a T-shaped microchannel. The mixing enhancement is realized by the swirling of two fluids in the cross section and is more prominent as the Reynolds number decreases. The twist angle was optimized to maximize the DOM, which increases with the length of the outlet branch. The twist angle was also optimized in terms of the relative mixing. The two optimum twisting angles are generally not coincident. The optimum twist angle shows a dependence on the length of the outlet branch but it is not affected much by the Reynolds number.

scholarly journals Effects of Channel Wall Twisting on the Mixing in a T-Shaped Micro-Channel

Micromachines ◽  
2019 ◽  
Vol 11 (1) ◽  
pp. 26 ◽  
Author(s):  
Dong Jin Kang
Keyword(s):  
Reynolds Number ◽  
Cross Section ◽  
Channel Wall ◽  
Numerical Study ◽  
Twist Angle ◽  
Mixing Enhancement ◽  
Micro Channel ◽  
Number Range ◽  
Mixing Performance ◽  
Two Fluids

A new design scheme is proposed for twisting the walls of a microchannel, and its performance is demonstrated numerically. The numerical study was carried out for a T-shaped microchannel with twist angles in the range of 0 to 34π. The Reynolds number range was 0.15 to 6. The T-shaped microchannel consists of two inlet branches and an outlet branch. The mixing performance was analyzed in terms of the degree of mixing and relative mixing cost. All numerical results show that the twisting scheme is an effective way to enhance the mixing in a T-shaped microchannel. The mixing enhancement is realized by the swirling of two fluids in the cross section and is more prominent as the Reynolds number decreases. The twist angle was optimized to maximize the degree of mixing (DOM), which increases with the length of the outlet branch. The twist angle was also optimized in terms of the relative mixing cost (MC). The two optimum twisting angles are generally not coincident. The optimum twist angle shows a dependence on the length of the outlet branch but it is not affected much by the Reynolds number.

Optimal parametric mixing analysis of active and passive micromixers using Taguchi method

2019 ◽  
Vol 233 (6) ◽  
pp. 1292-1303 ◽  
Author(s):  
Imran Shah ◽  
Han Su Jeon ◽  
Muhsin Ali ◽  
Doh Hoi Yang ◽  
Kyung-Hyun Choi
Keyword(s):  
Taguchi Method ◽  
Passive Mixer ◽  
External Energy ◽  
Mixing Index ◽  
Two Fluids ◽  
Active Mixer ◽  
Passive Mixing ◽  
Mixing Ability ◽  
Active Mixing ◽  
Micro Mixer

Mixing of fluids flowing through channels and chambers is a crucial step in chemical and biochemical reactions inside microfluidic devices due to laminar flow because of small size channel and chamber dimensions. Mixing can be enhanced by passive or active mechanism which makes convection dominant over diffusion. To address this challenge, the study proposes three novel mixing designs: passive mixer, active mixer and a combination of active and passive mixing. These designs mixing performance has been studied by numerical simulation using COMSOL 5.3. According to the preliminary results of the study, pure active micromixer design has superior mixing ability. The mixing ability was proved by concentration line plots, concentration contours and videos. In order to further optimize the mixing index of the pure active micromixer, Taguchi method is applied against various input parametric values for micromixer such as frequency, voltage and velocity. The velocity is required for two fluids to flow, while frequency and voltages are for providing an external energy for active mixing. A total of nine cases were analyzed; the two best cases out of nine were selected for comparing mixing index line plots. The result of the study conclude that pure active micro-mixer at an optimal set of parameters, frequency of 10 Hz, velocity of 0.05 mm s–1 and voltage of 0.5 V achieved 99.6% mixing index at t = 0.2 s.

2D Numerical Study of a Micromixer Based on Blowing and Vortex Shedding Mechanisms

2019 ◽  
pp. 79-113
Author(s):  
Maria Sanchez-Claros ◽  
Joaquin Ortega-Casanova ◽  
Francisco Jose Galindo-Rosales
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Numerical Study ◽  
Fluid Flows ◽  
Input Power ◽  
Mixing Efficiency ◽  
Injection Angle ◽  
Two Fluids ◽  
Optimum Configuration ◽  
Intensity Ratios

In this chapter, a numerical study and assessment of the mixing efficiency of a novel microfluidic device for mixing two fluids are presented. The device under study consists of a two-dimensional straight microchannel with a square pillar centered across the channel. The main fluid flows through the microchannel from the main inlet to the outlet, while the second fluid is injected through the pillar as two small jets at its upstream corners. For different values of the Reynolds number, intensity ratio between the jets and the main channel stream and jets injection angle, the authors have conducted several numerical simulations to characterize both the mixing efficiency and the required input power to make the fluids flow. The optimum configuration has been revealed for high values of the Reynolds number, low intensity ratios, and high injection angles. Thanks to vortex shedding and the corresponding downstream oscillations, a mixing efficiency of around 90% can be reached. The worst mixing efficiency is obtained for a configuration without vortex shedding, having a mixing efficiency of only around 2%.

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