scholarly journals A stretchable micromixer with enhanced performance for intermediate Reynolds numbers

2021 ◽  
Author(s):  
Hedieh Fallahi ◽  
Jun Zhang ◽  
Jordan Nicholls ◽  
Pradip Singha ◽  
Nhat-Khuong Nguyen ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Molecular Diffusion ◽  
Inertial Force ◽  
Reynolds Numbers ◽  
Chaotic Advection ◽  
Low Flow ◽  
Viscous Force ◽  
Mixing Efficiency ◽  
Potential Candidate ◽  
Low Reynolds

Abstract Chemical reactions in microscale require good mixing at a relatively low flowrate. However, mixing in microscale faces the major challenge of stable laminar flow associated with the low Reynolds number, the relative ratio between inertial force and viscous force. For low Reynolds numbers of less than unity, mixing occurs due to molecular diffusion. For high Reynolds number of more than several tens, chaotic advection enhances mixing. However, in the intermediate regime, mixing is not efficient. This paper reports a stretchable micromixer with dynamically tuneable channel dimensions. Periodically stretching the device changes the channel geometry and the curvature induced secondary Dean flows. The dynamically evolving secondary and main flows in the mixing channel result in chaotic advection and enhance mixing. The concept was demonstrated in a stretchable micromixer with a serpentine channel. We evaluated the performance of this stretchable micromixer both experimentally and numerically. At the intermediate range of Reynolds numbers from 4 to 17, the periodically stretched micromixer showed a better mixing efficiency than the non-stretched counterpart. Therefore, our stretchable micromixer is a potential candidate for applications where precious reagents need to be mixed at relatively low flow rate conditions.

scholarly journals A stretchable micrometer with enhanced performance for intermediate Reynolds numbers

2021 ◽  
Author(s):  
Hedieh Fallahi ◽  
Jun Zhang ◽  
Jordan Nicholls ◽  
Pradip Singha ◽  
Nhat-Khuong Nguyen ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Molecular Diffusion ◽  
Inertial Force ◽  
Reynolds Numbers ◽  
Chaotic Advection ◽  
Low Flow ◽  
Viscous Force ◽  
Mixing Efficiency ◽  
Potential Candidate ◽  
Low Reynolds

Abstract Chemical reactions in microscale require good mixing at a relatively low flowrate. However, mixing in microscale faces the major challenge of stable laminar flow associated with the low Reynolds number, the relative ratio between inertial force and viscous force. For low Reynolds numbers of less than unity, mixing occurs due to molecular diffusion. For high Reynolds number of more than several tens, chaotic advection enhances mixing. However, in the intermediate regime, mixing is not efficient. This paper reports a stretchable micromixer with dynamically tuneable channel dimensions. Periodically stretching the device changes the channel geometry and the curvature induced secondary Dean flows. The dynamically evolving secondary and main flows in the mixing channel result in chaotic advection and enhance mixing. The concept was demonstrated in a stretchable micromixer with a serpentine channel. We evaluated the performance of this stretchable micromixer both experimentally and numerically. At the intermediate range of Reynolds numbers from 4 to 17, the periodically stretched micromixer showed a better mixing efficiency than the non-stretched counterpart. Therefore, our stretchable micromixer is a potential candidate for applications where precious reagents need to be mixed at relatively low flow rate conditions.

scholarly journals Numerical Analysis of the Heterogeneity Effect on Electroosmotic Micromixers Based on the Standard Deviation of Concentration and Mixing Entropy Index

Micromachines ◽  
2021 ◽  
Vol 12 (9) ◽  
pp. 1055
Author(s):  
Alireza Farahinia ◽  
Jafar Jamaati ◽  
Hamid Niazmand ◽  
Wenjun Zhang
Keyword(s):  
Reynolds Number ◽  
Zeta Potential ◽  
Electroosmotic Flow ◽  
Molecular Diffusion ◽  
Slip Surface ◽  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Slip Coefficient ◽  
Mixing Rate ◽  
Heterogeneous Distribution

One approach to achieve a homogeneous mixture in microfluidic systems in the quickest time and shortest possible length is to employ electroosmotic flow characteristics with heterogeneous surface properties. Mixing using electroosmotic flow inside microchannels with homogeneous walls is done primarily under the influence of molecular diffusion, which is not strong enough to mix the fluids thoroughly. However, surface chemistry technology can help create desired patterns on microchannel walls to generate significant rotational currents and improve mixing efficiency remarkably. This study analyzes the function of a heterogeneous zeta-potential patch located on a microchannel wall in creating mixing inside a microchannel affected by electroosmotic flow and determines the optimal length to achieve the desired mixing rate. The approximate Helmholtz–Smoluchowski model is suggested to reduce computational costs and simplify the solving process. The results show that the heterogeneity length and location of the zeta-potential patch affect the final mixing proficiency. It was also observed that the slip coefficient on the wall has a more significant effect than the Reynolds number change on improving the mixing efficiency of electroosmotic micromixers, benefiting the heterogeneous distribution of zeta-potential. In addition, using a channel with a heterogeneous zeta-potential patch covered by a slip surface did not lead to an adequate mixing in low Reynolds numbers. Therefore, a homogeneous channel without any heterogeneity would be a priority in such a range of Reynolds numbers. However, increasing the Reynolds number and the presence of a slip coefficient on the heterogeneous channel wall enhances the mixing efficiency relative to the homogeneous one. It should be noted, though, that increasing the slip coefficient will make the mixing efficiency decrease sharply in any situation, especially in high Reynolds numbers.

Low Reynolds-Number Experiments in an Axial-Flow Turbomachine

1964 ◽  
Vol 86 (3) ◽  
pp. 257-295 ◽  
Author(s):  
J. Neustein
Keyword(s):  
Reynolds Number ◽  
Guide Vane ◽  
Axial Flow ◽  
Reynolds Numbers ◽  
Low Flow ◽  
Working Fluid ◽  
Low Reynolds Numbers ◽  
Blade Row ◽  
Overall Performance ◽  
Low Reynolds

The performance of a single-stage, axial-flow turbomachine was studied experimentally at low Reynolds numbers. The study was made with a turbomachine modeled from a large jet-engine type of axial-flow compressor. Low Reynolds numbers were obtained by using a mixture of glycerine and water as the working fluid. The overall performance was determined over a range of Reynolds numbers RT (based on rotor-tip speed and rotor chord) from 2000 to 150,000. The flow rate at each Reynolds number was varied from near shutoff to the maximum permitted by the turbomachine-tunnel systems. Blade-row characteristics were studied by means of quantitative flow surveys before and after each blade row, and by means of extensive flow-visualization experiments within each blade row. The investigation established that sudden or critical changes in performance do not occur in the type of machine tested, between RT of 150,000 and 20,000. Below 20,000 the performance deteriorated more rapidly. A relatively sharp change in performance occurred between RT of 20,000 and 10,000. The results clarified many of the viscous flow details in each blade row which are associated with the deterioration of performance. These effects were very pronounced at RT of 4000 and below. Consequently, a considerable part of the paper is concerned with results obtained at these lower Reynolds numbers. From the point of view of a designer, information is presented in regard to overall performance, guide-vane turning, and guide-vane and stator total-pressure losses, all as functions of Reynolds number. These results are expected to be indicative of performance in turbomachines similar to the one tested here. Other details are concerned with problems such as wall boundary layers, flow reversal at low flow coefficients, lip-clearance flow, flow patterns near shutoff, and flow comparisons in stators with rotating and stationary hubs.

Passive Microextractor with Internal Fluid Recirculation for Two Immiscible Liquids

2014 ◽  
Vol 12 (1) ◽  
pp. 285-293 ◽  
Author(s):  
Cong Xu ◽  
Jiao Wang
Keyword(s):  
Mass Transfer ◽  
Reynolds Number ◽  
Molecular Diffusion ◽  
Reynolds Numbers ◽  
Immiscible Liquid ◽  
Chaotic Advection ◽  
Inlet Channel ◽  
External Energy ◽  
Outlet Channel ◽  
Internal Fluid

Abstract A microextractor comprising an inlet channel, a mixing chamber, two feedback channels, and an outlet channel and having no moving parts was designed for immiscible liquid–liquid extraction. Two liquids were mixed passively without any external energy input, and the extraction was completed in the microextractor. The extractor performance with or without a splitter was investigated by visualization and mass transfer experiments. Two mixing mechanisms were observed: (i) molecular diffusion at lower Reynolds number and (ii) chaotic advection at higher Reynolds number. The transition point between the two mechanisms was at Reynolds numbers 375.2 and 179.9 for the aqueous phase (3 mol/L HNO3 solution) and the organic phase (30% tributyl phosphate (TBP)–kerosene solution), respectively. In the chaotic advection mode, two vortexes rotating in opposite directions were formed on both sides of the main flow, which enhanced the mass transfer between the two liquids. Mass transfer between the 3 mol/L HNO3 and 30% TBP–kerosene solutions was achieved with an efficiency of 92.8% at the extractor exit when the extractor operated in the chaotic advection mode.

Correlation of Mixing Efficiency and Entropy Generation Rate in a Square Cross Section Tee Junction Micromixer

2017 ◽  
Author(s):  
Aric M. Gillispie ◽  
Evan C. Lemley
Keyword(s):  
Reynolds Number ◽  
Entropy Generation ◽  
Molecular Diffusion ◽  
Chaotic Advection ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Analytical Relationship ◽  
Mixing Efficiency ◽  
Mixed Solution ◽  
Tee Junction

The potential applications of micromixers continues to expand in the bio-sciences area. In particular, passive micromixers that may be used as part of point-of-care (POC) diagnostic testing devices are becoming commonplace and have application in developed, developing, and relatively undeveloped locales. Characterizing and improving mixing efficiency in these devices is an ongoing research effort. Micromixers are used in some lab-on-chip (LOC) devices where it is often necessary to combine two or more fluids into a mixed solution for testing or delivery. The simplest micromixer incorporates a tee junction to combine two fluid species in anti-parallel branches, with the mixed fluid leaving in a branch perpendicular to the incoming branches. Micromixers rely on two modes of mixing: chaotic advection and molecular diffusion. In micro-mixers flow is typically laminar, making chaotic advection occur only via induced secondary flows. Hence, micromixers, unless carefully designed, rely almost exclusively on molecular diffusion of fluid species. A well designed micromixer should exhibit significant chaotic advection; which is also a sign of large strain rates and large entropy generation rates. This paper describes the development of an analytical relationship for the entropy generation rate and the mixing efficiency as function of the outgoing branch Reynolds number. Though there has been extensive research on tee junctions, entropy generation, and the mixing efficiencies of a wide variety of micromixers, a functional relationship for the mixing efficiency and the entropy generation rate has not been established. We hypothesize a positive correlation between the mixing index and the entropy generation rate. The worked described here establishes a method and provides the results for such a relationship. A basic tee junction with square cross sections has been analyzed using computational fluid dynamics to determine the entropy generation rate and outgoing mixing efficiencies for Reynolds numbers ranging from 25–75. The mixing efficiency is determined at a location in the outgoing branch where the effects of molecular diffusive mixing is minimized and chaotic advective mixing is the focus. The entropy generation rate has been determined for the indicated range of Reynolds number and decomposed into its viscous and diffusive entropy terms. The functional relationships that have been developed are applicable for micromixer design and serve as a reference for more complex passive micromixer designs.

The Numerical Investigation of an Interdigital Micromixer With the Circular-Sector Obstacles

2009 ◽  
Author(s):  
Yan Feng Fan ◽  
Ibrahim Hassan
Keyword(s):  
Surface Area ◽  
Molecular Diffusion ◽  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Mixing Enhancement ◽  
Mixing Length ◽  
Circular Sector ◽  
Lateral Direction ◽  
Low Reynolds Numbers ◽  
Low Reynolds

In this paper, a passive interdigital micromixer with the circular-sector obstacles is proposed and the mixing performance is estimated by numerical simulation. The tested Reynolds numbers range from 0.01 to 10. Flow recirculation or vortices seems impossible to generate to enhance the mixing at such low Reynolds numbers. Hence, molecular diffusion is the dominant mixing mechanism. Based on the diffusion principle, enlarging the mixing length, reducing the diffusion length and increasing the surface area between species are major methods to obtain mixing enhancement. In order to achieve rapid mixing, shortening the mixing length is necessary. However, the reduced mixing length induces the decreased mixing time which the species take to mix. The circular-section obstacles are placed in the straight microchannels to enlarge the contact surface area between species. The flow path is distorted after passing the obstacles so that the real mixing length increases compared with traditional T-shape micromixers. Furthermore, flow advection takes a part role in mixing since the velocity direction is no longer perpendicular to diffusion direction. Different geometries and layouts of obstacles are analyzed for optimization. The results of optimal design show the worst mixing efficiency, around 50%, occurs at Re = 1. In order to improve the lower limitation of mixing efficiency, the duplicated layouts of obstacles in lateral direction with interdigital inlet are applied to reduce the diffusion path and increase the interface area so that the mixing efficiency could be enhanced. The results show that the mixing efficiency could achieve 85% at Re ≤ 1 with a low pressure drop of 100 Pa. It has the potential to be used in applications with low Reynolds numbers.

Parametric Study of Obstacle Geometry Effect on Mixing Performance in a Convergent-Divergent Micromixer with Sinusoidal Walls

2017 ◽  
Vol 12 (1) ◽  
Author(s):  
Fazlollah Heshmatnezhad ◽  
Halimeh Aghaei ◽  
Ali Reza Solaimany Nazar
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Aspect Ratio ◽  
Molecular Diffusion ◽  
Reynolds Numbers ◽  
Secondary Flows ◽  
Mixing Efficiency ◽  
Main Concept ◽  
Mixing Index ◽  
Operational Parameter

Abstract This study presents a numerical simulation through computational fluid dynamics on mixing and flow structures in convergent-divergent micromixer with a triangular obstacle. The main concept in this design is to enhance the interfacial area between the two fluids by creating a transverse flow and split, and recombination of fluids flow due to the presence of obstacles. The effect of triangular obstacle size, the number of units, changing the position of an obstacle in the mixing channel and operational parameter like the Reynolds number on the mixing efficiency and pressure drop are assessed. The results indicate that at inlet Reynolds numbers below 5, the molecular diffusion is the most important mechanism of mixing, and the mixing index is almost the same for all cases. By increasing the inlet Reynolds number above 5, mixing index increases dramatically, due to the secondary flows. Based on the simulation results, due to increasing the effect of dean and separation vortices as well as more recirculation of flow in the side branches and behind the triangular obstacle, the highest mixing index is obtained at the aspect ratio of 2 for the triangular obstacle and its position at the front of the mixing unit. Also the highest value of mixing index is obtained by six unit of mixing chamber. The effect of changing the position of the obstacle in the channel and changing the aspect ratio of the obstacle is evident in high Reynolds numbers. An increase in the Reynolds number in both cases (changing the aspect ratio and position of the obstacle) leads to pressure drop increases.

Analysis of a Vibrating-Beam-Based Micromixer

2007 ◽  
Author(s):  
Hongwei Sun ◽  
Pengtao Wang
Keyword(s):  
Molecular Diffusion ◽  
Reynolds Numbers ◽  
Laminar Flows ◽  
Chaotic Advection ◽  
Mixing Efficiency ◽  
Microfluidic Channels ◽  
Low Reynolds Numbers ◽  
Practical Applications ◽  
Mixer Design ◽  
Beam Movement

The mixing of two or more streams in microscale devices is a slowly molecular diffusion process due to the unique laminar flows, and some ‘turbulence’ based mixing technologies which are effective in macroscales become hard to implement in such small dimensions. The chaotic advection based mixing, depending on the stretching and folding of interface, has been proved to be effective for low Reynolds numbers (Re) and is a very promising technology for micro mixing. We propose a new mixing concept based on a vibrating micro-beam in microfluidic channels to generate chaotic advection to achieve an efficient mixing. The simplicity of the proposed mixer design makes microfabrication process easy for practical applications. The feasibility of the concept is evaluated computationally and moving mesh technique (ALE) is utilized to trace the beam movement. The simulation shows that the mixing quality is determined by parameters such as flow velocities, amplitudes and frequencies of vibrating beam. The Reynolds number (Re) is less than 2.0, Pelect number (Pe) ranges from 5 to 1000, and Strohal number (St) 0.3 to 3.0. It was found that vortex type of flows were generated in microchannel due to the interaction between beam and channel wall. The mixing efficiency with this design is well improved comparing with the flows without beam vibration.

Passive Micro Mixers for Applications in the Micro Reactor and µTAS Field

2004 ◽  
Author(s):  
Steffen Hardt ◽  
Klaus Drese ◽  
Volker Hessel ◽  
Friedhelm Scho¨nfeld
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Dynamical Behavior ◽  
Reynolds Numbers ◽  
Chaotic Advection ◽  
Superior Performance ◽  
Hydrodynamic Focusing ◽  
Process Technology ◽  
Two Fluids ◽  
Low Reynolds

An overview is given of current developments in micro mixing technology, where the emphasis is on liquid mixing in passive micro mixers. The mixers presented are differentiated by the hydrodynamic principle employed, and four important principles are discussed in some detail: hydrodynamic focusing, flow separation, chaotic advection and split-and-recombine flows. It is shown that these principles offer an excellent mixing performance in various dynamical regimes. Hydrodynamic focusing is a concept working very much independently of the Reynolds number of the flow. Flow separation offers a rich dynamical behavior over a Reynolds number scale of several hundred, with superior performance compared to purely diffusive mixing already found at low Reynolds numbers. For chaotic advection different implementations tailor-made for low and comparatively high Reynolds numbers exist, both leading to an exponential increase of the interface between two fluids. Split-and-recombine flows can only be realized in a close-to-ideal form in the low Reynolds number regime. Corresponding mixers can be equipped with comparatively wide channels, enabling a favorable ratio of throughput to pressure drop. The overview given in this article should enable a potential user of micro mixing technology to select the most favorable concept for the application envisaged, especially in the field of chemical process technology.

Low Reynolds number flow around and heat transfer from a heated circular cylinder

2010 ◽  
Vol 1 (1-2) ◽  
pp. 15-20 ◽  
Author(s):  
B. Bolló
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Lift Coefficient ◽  
Reynolds Numbers ◽  
Low Reynolds Numbers ◽  
Reynolds Number Flow ◽  
Two Dimensional Flow ◽  
Number Flow ◽  
Low Reynolds ◽  
Increasing Temperature

Abstract The two-dimensional flow around a stationary heated circular cylinder at low Reynolds numbers of 50 < Re < 210 is investigated numerically using the FLUENT commercial software package. The dimensionless vortex shedding frequency (St) reduces with increasing temperature at a given Reynolds number. The effective temperature concept was used and St-Re data were successfully transformed to the St-Reeff curve. Comparisons include root-mean-square values of the lift coefficient and Nusselt number. The results agree well with available data in the literature.

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