Performance Analysis and Numerical Evaluation of Mixing in 3-D T-Shape Passive Micromixers

Design and Mixing Analysis of a Passive Micromixer with Circulation Promoters

A novel passive micromixer equipped with circulation promoters is proposed, and its mixing performance is simulated over a broad range of Reynolds numbers (0.1≤Re≤100). To evaluate the effectiveness of the circulation promoters, three different configurations are analyzed in terms of the degree of mixing (DOM) at the outlet and the associated pressure drop. Compared to other typical passive micromixers, the circulation promoter is shown to significantly enhance mixing performance. Among the three configurations of circulation promoters, Case 3 demonstrates the best performance, with a DOM exceeding 0.96 across the entire range of Reynolds numbers. At Re = 1, the DOM of Case 3 is 3.7 times larger than that of a modified Tesla micromixer, while maintaining a comparable pressure drop. The mixing enhancement of the present micromixer is particularly significant in the low and intermediate ranges of Reynolds numbers (Re<40). In the low range of Reynolds numbers (Re≤1), the mixing enhancement is primarily due to circulation promoters directing fluid flow from a concave wall to the opposite convex wall. In the intermediate range of Reynolds numbers (2≤Re<40), the mixing enhancement results from fluid flowing from one concave wall to another concave wall on the opposite side.

Mixing Performance of a Passive Micromixer Based on Split-to-Circulate (STC) Flow Characteristics

We propose a novel passive micromixer leveraging STC (split-to-circulate) flow characteristics and analyze its mixing performance comprehensively. Three distinct designs incorporating submerged circular walls were explored to achieve STC flow characteristics, facilitating flow along a convex surface and flow impingement on a concave surface. Across a broad Reynolds number range (0.1 to 80), the present micromixer substantially enhances mixing, with a degree of mixing (DOM) consistently exceeding 0.84. Particularly, the mixing enhancement is prominent within the low and intermediate range of Reynolds numbers (0.1 Collapse

Key Words

• concave wall
• convex wall
• degree of mixing (DOM)
• flow impingement
• saddle point
• split-to-circulate
• submerged circular wall

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Affiliation(s)

Makhsuda Juraeva
Dong-Jin Kang School of Mechanical Engineering, Yeungnam University, 280 Daehak-ro, Gyoungsan 38541, Republic of Korea;

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Design and Mixing Analysis of a Passive Micromixer Based on Curly Baffles

A novel passive micromixer based on curly baffles is proposed and optimized through the signal-to-noise analysis of various design parameters. The mixing performance of the proposed design was evaluated across a wide Reynolds number range, from 0.1 to 80. Through the analysis, the most influential parameter was identified, and its value was found to be constant regardless of the mixing mechanism. The optimized design, refined using the signal-to-noise analysis, demonstrated a significant enhancement of mixing performance, particularly in the low Reynolds number range (Re< 10). The design set obtained at the diffusion dominance range shows the highest degree of mixing (DOM) in the low Reynolds number range of Re< 10, while the design set optimized for the convection dominance range exhibited the least pressure drop across the entire Reynolds number spectrum (Re< 80). The present design approach proved to be a practical tool for identifying the most influential design parameter and achieving excellent mixing and pressure drop characteristics. The enhancement is mainly due to the curvature of the most influential design parameter.

Mixing Performance of a Passive Micromixer Based on Multiple Baffles and Submergence Scheme

A novel passive micromixer based on multiple baffles and a submergence scheme was designed, and its mixing performance was simulated over a wide range of Reynolds numbers ranging from 0.1 to 80. The degree of mixing (DOM) at the outlet and the pressure drop between the inlets and outlet were used to assess the mixing performance of the present micromixer. The mixing performance of the present micromixer showed a significant enhancement over a wide range of Reynolds numbers (0.1 ≤ Re ≤ 80). The DOM was further enhanced by using a specific submergence scheme. At low Reynolds numbers (Re < 5), submergence scheme Sub24 produced the highest DOM, approximately 0.57, which was 1.38 times higher than the case with no submergence. This enhancement was due to the fluid flowing from or toward the submerged space, creating strong upward or downward flow at the cross-section. At high Reynolds numbers (Re > 10), the DOM of Sub1234 became the highest, reaching approximately 0.93 for Re = 20, which was 2.75 times higher than the case with no submergence. This enhancement was caused by a large vortex formed across the whole cross-section, causing vigorous mixing between the two fluids. The large vortex dragged the interface between the two fluids along the vortex perimeter, elongating the interface. The amount of submergence was optimized in terms of DOM, and it was independent of the number of mixing units. The optimum submergence values were 90 μm for Sub24 and Re = 1, 100 μm for Sub234 and Re = 5, and 70 μm for Sub1234 and Re = 20.

Mixing Performance of the Modified Tesla Micromixer with Tip Clearance

A passive micromixer based on the modified Tesla mixing unit was designed by embedding tip clearance above the wedge-shape divider, and its mixing performance was simulated over a wider range of the Reynolds numbers from 0.1 to 80. The mixing performance was evaluated in terms of the degree of mixing (DOM) at the outlet and the required pressure load between inlet and outlet. The height of tip clearance was varied from 40 μm to 80 μm, corresponding to 25% to 33% of the micromixer depth. The numerical results show that the mixing enhancement by the tip clearance is noticeable over a wide range of the Reynolds numbers Re < 50. The height of tip clearance is optimized in terms of the DOM, and the optimum value is roughly h = 60 μm. It corresponds to 33% of the present micromixer depth. The mixing enhancement in the molecular diffusion regime of mixing, Re ≤ 1, is obtained by drag and connection of the interface in the two sub-streams of each Tesla mixing unit. It appears as a wider interface in the tip clearance zone. In the intermediate range of the Reynolds number, 1 < Re ≤ 50, the mixing enhancement is attributed to the interaction of the flow through the tip clearance and the secondary flow in the vortex zone of each Tesla mixing unit. When the Reynolds number is larger than about 50, vortices are formed at various locations and drive the mixing in the modified Tesla micromixer. For the Reynolds number of Re = 80, a pair of vortices is formed around the inlet and outlet of each Tesla mixing unit, and it plays a role as a governing mechanism in the convection-dominant regime of mixing. This vortex pattern is little affected as long as the tip clearance remains smaller than about h = 70 μm. The DOM at the outlet is little enhanced by the presence of tip clearance for the Reynolds numbers Re ≥ 50. The tip clearance contributes to reducing the required pressure load for the same value of the DOM.

Optimal Design of Water Treatment Contact Tanks

In water treatment facilities, the last step of the treatment process includes disinfectant application to improve the water quality appropriate for a specific end-use purpose. At this step, contact tanks are used to mix water with the disinfectant. Mixing in contact tanks mainly relies on mechanical mixing processes to mix water with the disinfectant to activate the removal process. Thus, mixing efficiency of the contact tank design is critical for the reduction in the amount of disinfectant used to treat a fixed volume of water, to reduce the energy requirements to derive the treated volume of water through the system and to improve other design considerations of the contact tanks. There are numerous design alternatives reported in the literature that do achieve some of these purposes to a certain extent. Among the recent and more successful designs, one can cite the slot-baffle, the perforated-baffle, and the porous-baffle designs. Although these designs provide important improvements to the mixing process, the studies in which these concepts are reported did not provide an optimal design for the baffle geometry used in the design that would include other important considerations beyond the baffle geometry. In this paper, a new optimal design concept is introduced where important design considerations that are not considered in earlier studies are included in the analysis. The results show that new baffle geometries are possible for the optimal design of contact tanks when these innovative design criteria are included in the analysis.

Mixing Performance of a Passive Micro-Mixer with Mixing Units Stacked in Cross Flow Direction

A new passive micro-mixer with mixing units stacked in the cross flow direction was proposed, and its performance was evaluated numerically. The present micro-mixer consisted of eight mixing units. Each mixing unit had four baffles, and they were arranged alternatively in the cross flow and transverse direction. The mixing units were stacked in four different ways: one step, two step, four step, and eight step stacking. A numerical study was carried out for the Reynolds numbers from 0.5 to 50. The corresponding volume flow rate ranged from 6.33 μL/min to 633 μL/min. The mixing performance was analyzed in terms of the degree of mixing (DOM) and relative mixing energy cost (MEC). The numerical results showed a noticeable enhancement of the mixing performance compared with other micromixers. The mixing enhancement was achieved by two flow characteristics: baffle wall impingement by a stream of high concentration and swirl motion within the mixing unit. The baffle wall impingement by a stream of high concentration was observed throughout all Reynolds numbers. The swirl motion inside the mixing unit was observed in the cross flow direction, and became significant as the Reynolds number increased to larger than about five. The eight step stacking showed the best performance for Reynolds numbers larger than about two, while the two step stacking was better for Reynolds numbers less than about two.

Mixing Enhancement of Non-Newtonian Shear-Thinning Fluid for a Kenics Micromixer

In this work, a numerical investigation was analyzed to exhibit the mixing behaviors of non-Newtonian shear-thinning fluids in Kenics micromixers. The numerical analysis was performed using the computational fluid dynamic (CFD) tool to solve 3D Navier-Stokes equations with the species transport equations. The efficiency of mixing is estimated by the calculation of the mixing index for different cases of Reynolds number. The geometry of micro Kenics collected with a series of six helical elements twisted 180° and arranged alternately to achieve the higher level of chaotic mixing, inside a pipe with a Y-inlet. Under a wide range of Reynolds numbers between 0.1 to 500 and the carboxymethyl cellulose (CMC) solutions with power-law indices among 1 to 0.49, the micro-Kenics proves high mixing Performances at low and high Reynolds number. Moreover the pressure losses of the shear-thinning fluids for different Reynolds numbers was validated and represented.

Fused Filament Fabrication (FFF) for Manufacturing of Microfluidic Micromixers: An Experimental Study on the Effect of Process Variables in Printed Microfluidic Micromixers

The need for accessible and inexpensive microfluidic devices requires new manufacturing methods and materials as a replacement for traditional soft lithography and polydimethylsiloxane (PDMS). Recently, with the advent of modern additive manufacturing (AM) techniques, 3D printing has attracted attention for its use in the fabrication of microfluidic devices and due to its automated, assembly-free 3D fabrication, rapidly decreasing cost, and fast-improving resolution and throughput. Here, fused filament fabrication (FFF) 3D printing was used to create microfluidic micromixers and enhance the mixing process, which has been identified as a challenge in microfluidic devices. A design of experiment (DoE) was performed on the effects of studied parameters in devices that were printed by FFF. The results of the colorimetric approach showed the effects of different parameters on the mixing process and on the enhancement of the mixing performance in printed devices. The presence of the geometrical features on the microchannels can act as ridges due to the nature of the FFF process. In comparison to passive and active methods, no complexity was added in the fabrication process, and the ridges are an inherent property of the FFF process.

Toward the Next Generation of Passive Micromixers: A Novel 3-D Design Approach

Passive micromixers are miniaturized instruments that are used to mix fluids in microfluidic systems. In microchannels, combination of laminar flows and small diffusion constants of mixing liquids produce a difficult mixing environment. In particular, in very low Reynolds number flows, e.g., Re < 10, diffusive mixing cannot be promoted unless a large interfacial area is formed between the fluids to be mixed. Therefore, the mixing distance increases substantially due to a slow diffusion process that governs fluid mixing. In this article, a novel 3-D passive micromixer design is developed to improve fluid mixing over a short distance. Computational Fluid Dynamics (CFD) simulations are used to investigate the performance of the micromixer numerically. The circular-shaped fluid overlapping (CSFO) micromixer design proposed is examined in several fluid flow, diffusivity, and injection conditions. The outcomes show that the CSFO geometry develops a large interfacial area between the fluid bodies. Thus, fluid mixing is accelerated in vertical and/or horizontal directions depending on the injection type applied. For the smallest molecular diffusion constant tested, the CSFO micromixer design provides more than 90% mixing efficiency in a distance between 260 and 470 µm. The maximum pressure drop in the micromixer is found to be less than 1.4 kPa in the highest flow conditioned examined.

Technical Model of Micro Electrical Discharge Machining (EDM) Milling Suitable for Bottom Grooved Micromixer Design Optimization

In this paper, development of a technical model of micro Electrical Discharge Machining in milling configuration (EDM milling) is presented. The input to the model is a parametrically presented feature geometry and the output is a feature machining time. To model key factors influencing feature machining time, an experimental campaign by machining various microgrooves into corrosive resistant steel was executed. The following parameters were investigated: electrode dressing time, material removal rate, electrode wear, electrode wear control time and machining strategy. The technology data and knowledge base were constructed using data obtained experimentally. The model is applicable for groove-like features, commonly applied in bottom grooved micromixers (BGMs), with widths from 40 to 120 µm and depths up to 100 µm. The optimization of a BGM geometry is presented as a case study of the model usage. The mixing performances of various micromixer designs, compliant with micro EDM milling technology, were evaluated using computational fluid dynamics modelling. The results show that slanted groove micromixer is a favourable design to be implemented when micro EDM milling technology is applied. The presented technical model provides an efficient design optimization tool and, thus, aims to be used by a microfluidic design engineer.

Single and Multi-Objective Optimization of a Three-Dimensional Unbalanced Split-and-Recombine Micromixer

The three-dimensional geometry of a micromixer with an asymmetrical split-and-recombine mechanism was optimized to enhance the fluid-mixing capability at a Reynolds number of 20. Single and multi-objective optimizations were carried out by using particle swarm optimization and a genetic algorithm on a modeled surrogate surface. Surrogate modeling was performed using the computational results for the mixing. Mixing and flow analyses were carried out by solving the convection–diffusion equation in combination with the three-dimensional continuity and momentum equations. The optimization was carried out with two design variables related to dimensionless geometric parameters. The mixing effectiveness was chosen as the objective function for the single-objective optimization, and the pressure drop and mixing index at the outlet were chosen for the multi-objective optimization. The sampling points in the design space were determined using a design of experiment technique called Latin hypercube sampling. The surrogates for the objective functions were developed using a Kriging model. The single-objective optimization resulted in 58.9% enhancement of the mixing effectiveness compared to the reference design. The multi-objective optimization provided Pareto-optimal solutions that showed a maximum increase of 48.5% in the mixing index and a maximum decrease of 55.0% in the pressure drop in comparison to the reference design.

Novel 3-D T-Shaped Passive Micromixer Design with Helicoidal Flows

Laminar fluid flow and advection-dominant transport produce ineffective mixing conditions in micromixers. In these systems, a desirable fluid mixing over a short distance may be achieved using special geometries in which complex flow paths are generated. In this paper, a novel design, utilizing semi-circular ridges, is proposed to improve mixing in micro channels. Fluid flow and scalar transport are investigated employing Computational Fluid Dynamics (CFD) tool. Mixing dynamics are investigated in detail for alternative designs, injection, and diffusivity conditions. Results indicate that the convex alignment of semi-circular elements yields a specific, helicoidal-shaped fluid flow along the mixing channel which in turn enhances fluid mixing. In all cases examined, homogenous concentration distributions with mixing index values over 80% are obtained. When it is compared to the classical T-shaped micromixer, the novel design increases mixing index and mixing performance values by the factors of 8.7 and 3.3, respectively. It is also shown that different orientations of ridges adversely affect the mixing efficiency by disturbing the formation of helicoidal-shaped flow profile.

Computational Evaluation of Mixing Performance in 3-D Swirl-Generating Passive Micromixers

Computational Fluid Dynamics (CFD) tools are used to investigate fluid flow and scalar mixing in micromixers where low molecular diffusivities yield advection dominant transport. In these applications, achieving a numerical solution is challenging. Numerical procedures used to overcome these difficulties may cause misevaluation of the mixing process. Evaluation of the mixing performance of these devices without appropriate analysis of the contribution of numerical diffusion yields over estimation of mixing performance. In this study, two- and four-inlet swirl-generating micromixers are examined for different mesh density, flow and molecular diffusivity scenarios. It is shown that mesh densities need to be high enough to reveal numerical diffusion errors in scalar transport simulations. Two-inlet micromixer design was found to produce higher numerical diffusion. In both micromixer configurations, when cell Peclet numbers were around 50 and 100 for Reynolds numbers 240 and 120, the numerical diffusion effects were tolerable. However, when large cell Peclet number scenarios were tested, it was found that the molecular diffusivity of the fluid is completely masked by false diffusion errors.

Laser-Inscribed Glass Microfluidic Device for Non-Mixing Flow of Miscible Solvents

In recent years, there has been significant research on integrated microfluidic devices. Microfluidics offer an advantageous platform for the parallel laminar flow of adjacent solvents of potential use in modern chemistry and biology. To reach that aim, we worked towards the realization of a buried microfluidic Lab-on-a-Chip which enables the separation of the two components by exploiting the non-mixing properties of laminar flow. To fabricate the aforementioned chip, we employed a femtosecond laser irradiation technique followed by chemical etching. To optimize the configuration of the chip, several geometrical and structural parameters were taken into account. The diffusive mass transfer between the two fluids was estimated and the optimal chip configuration for low diffusion rate of the components was defined.

Editorial for the Special Issue on Passive Micromixers

Micromixers are important components of microfluidic systems [...].