Residence-time distribution as a measure of mixing in T-junction and multilaminated/elongational flow micromixers

The ineffective mixing in microchannel mixers or reactors, primarily due to the laminar flow behavior in such microfluidic devices, has become an issue of significant interest to many researchers working in the field of microreaction engineering and related disciplines. The present study describes the numerical and experimental investigation of mixing performance in a proposed multilaminated/elongational flow micromixer (herein referred to as MEFM-4) and a standard T-junction micromixer (TjM). These two micromixers that employ different mixing enhancement strategies were fabricated from silicon using micro-electromechanical systems (MEMS) technology. Computational fluid dynamics (CFD) approach was first used to establish the experimental platform for the mixing study. Tracer experiment utilizing UV–vis absorption spectroscopy detection technique was used to obtain the required concentration data for residence-time distribution (RTD) analysis. The RTD and its coefficient of variation (CoV) were used for indirect characterization of flow and mixing behavior in the micromixers. Using this measure, the proposed MEFM-4, as expected, exhibits a better mixing performance (with its narrower RTD and lower CoV values) than the standard TjM. The comparison of results from the CFD simulation and the experiment shows very good agreement, especially in the low Reynolds number flow regime (Re<29). In combination with matching experiment and advanced microfabrication techniques, CFD simulation is a powerful tool for effective design and evaluation of simple to complex microfluidic devices for useful applications in chemical analysis and synthesis.     

Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel

The rapid development of microfabrication techniques creates new opportunities for applications of microchannel reactor technology in chemical reaction engineering. The extremely large surface-to-volume ratio and the short transport path in microchannels enhance heat and mass transfer dramatically, and hence provide many potential opportunities in chemical process development and intensification. Multiphase reactions involving gas/liquid reactants with a solid as a catalyst are ubiquitous in chemical and pharmaceutical industries. The hydrodynamics of the flow affects the reactor performance significantly; therefore it plays a prominent role in reactor design. For gas/liquid two-phase flow in a microchannel, the Taylor slug flow regime is the most commonly encountered flow pattern. The present study deals with the numerical simulation of the Taylor flow in a microchannel, particularly on gas and liquid slugs. A T-junction empty microchannel with varying cross-sectional width (0.25, 0.5, 0.75, 1, 2 and 3 mm) served as the model micro-reactor, and a finite volume based commercial computational fluid dynamics (CFD) package, FLUENT, was adopted for the numerical simulation. The gas and liquid slug lengths at various operating and fluid conditions were obtained and found to be in good agreement with the literature data. Several correlations in the T-junction microchannel were developed based on the simulation results. The slug flows for other geometries and inlet conditions were also studied.     

T型微通道内流体流动的FLUENT模拟

在流体力学研究中,微流控技术是当今研究的一个热点。利用微尺度下流体流动的一些特性可以提高流体间传质、传热以及反应速率。T型微通道是一个比较常见的微流控装置,利用ICEM CFD软件建立T型微通道的几何模型;然后通过FLUENT软件对微通道内流体流动进行模拟仿真。模拟实验结果表明,通道内两相流体的速度和粘度对流型有显著影响。     

Computational Fluid Dynamic Simulation of Liquid-Liquid Mixing in a Static Double-T-shaped Micromixer

The laminar flow structure and mixing performance of T-shaped and double-T-shaped micromixers with rectangular cross-section have been investigated using computational fluid dynamic (CFD) simulation. FLUENT software is used to evaluate the mixing efficiency. The numerical simulation results show that the presented double-T-micromixer is highly efficient over T-shaped micromixer. The performance of double-T-micromixer with and without static mixing elements (SME) is also investigated. The enhancement in mixing performance is thought to be caused by the generation of eddies and lateral velocity component when the mixture flows through these elements. Mixing efficiency as higher as 97% is reached within a mixing length of 320 mm downstream from the first T-junction with the enhancement of three SMEs.     

On the CFD modelling of Taylor flow in microchannels

With the increasing interest in multiphase flow in microchannels and advancement in interface capturing techniques, there have recently been a number of attempts to apply computational fluid dynamics (CFD) to model Taylor flow in microchannels. The liquid film around the Taylor bubble is very thin at low Capillary number (Ca) and requires careful modelling to capture it. In this work, a methodology has been developed to model Taylor flow in microchannel using the ANSYS Fluent software package and a criterion for having a sufficiently fine mesh to capture the film is suggested. The results are shown to be in good agreement with existing correlations and previous valid modelling studies. The role played by the wall contact angle in Taylor bubble simulations is clarified.     

Laminar Drag Reduction in Hydrophobic Microchannels

The apparent slip effects of laminar water flow in smooth hydrophobic microchannels and patterned hydrophobic microchannels were investigated. A series of experiments were performed to demonstrate the drag reductions for laminar water flow in hydrophobic microchannels. These microchannels were fabricated from silicon wafers using photolithography and were coated with hydrophobic octadecyltrichlorosilane (OTS). To generate a larger drag reduction, the patterned hydrophobic microchannels were fabricated to allow the liquid to flow over a region of trapped air in the cavity between the microridges. With the geometrical dimensions used, pressure drop reductions ranging from 10 to 30 % were found in the smooth microchannels and patterned microchannels. The pressure drop reduction was shown to increase with increasing microridge spacing and decreasing microchannel width. Using micro‐particle image velocimetry (PIV), we measured an apparent slip velocity at the wall of approximately 8 % of the centerline velocity, yielding a slip length of approximately 2 μm in the smooth hydrophobic microchannel. Theoretically, the analytical solution derived for three‐dimensional flow in a rectangular duct is presented to predict the slip velocity and slip length at the wall based on the pressure drop measurement. These results are in agreement with the experimental data obtained using micro‐PIV.     

INVESTIGATION OF A NOVEL MICROREACTOR FOR ENHANCING MIXING AND CONVERSION

A novel microreactor with a network of omega-shaped microchannels has been designed, fabricated, and tested for enhanced chemical species mixing and reaction conversion. Fluidic and mixing properties of the omega channel reactor have been investigated by means of computational fluid dynamic (CFD) simulation. Also, a stochastic model describing particle transport in the axial direction was applied to characterize the residence time distribution or the cumulative probability of a particle exiting the microreactor over time. Both fluidic simulation and stochastic model approaches revealed the advantage of the omega-shaped microchannels as compared to straight or zigzag-shaped microchannels. Fischer-Tropsch reactions were carried out using sol-gel encapsulated iron and cobalt catalysts in the omega-shaped microchannels. The experimental results showed that the conversion rate for the omega-shaped microchannels was considerably higher than that for the conventional straight microchannel or for the zigzag-shaped microchannels. These results were consistent with the fluidic simulation and the stochastic modeling results.     

INVESTIGATION OF A NOVEL MICROREACTOR FOR ENHANCING MIXING AND CONVERSION

A novel microreactor with a network of omega-shaped microchannels has been designed, fabricated, and tested for enhanced chemical species mixing and reaction conversion. Fluidic and mixing properties of the omega channel reactor have been investigated by means of computational fluid dynamic (CFD) simulation. Also, a stochastic model describing particle transport in the axial direction was applied to characterize the residence time distribution or the cumulative probability of a particle exiting the microreactor over time. Both fluidic simulation and stochastic model approaches revealed the advantage of the omega-shaped microchannels as compared to straight or zigzag-shaped microchannels. Fischer-Tropsch reactions were carried out using sol-gel encapsulated iron and cobalt catalysts in the omega-shaped microchannels. The experimental results showed that the conversion rate for the omega-shaped microchannels was considerably higher than that for the conventional straight microchannel or for the zigzag-shaped microchannels. These results were consistent with the fluidic simulation and the stochastic modeling results.     

捕捉微通道内Taylor流特性的一种渐变网格划分方法

提出了一种适用于捕捉通道内Taylor流液膜厚度的渐变网格划分方法。为了平衡模拟精度和计算效率,提出了针对此方法的具体规则来获得一种高效的网格划分。利用CFD软件Fluent进行了一系列的二维数值模拟,采用VOF模型来捕捉微通道内气液两相界面,所用微通道横截面宽度分别为0.1,0.2,0.3,0.4,0.5,0.8和1.0mm。采用提出的网格划分方法获得的模拟结果与关联式计算值和实验数据很接近。基于模拟结果,提出了新的气液弹长度的关联式,并且利用实验数据对此关联式进行了验证。     

气体在任意截面形状微尺度槽道中的滑移流动

利用正交函数法对气体在具有任意截面形状的微尺度槽道内的充分发展层流滑移流动特性进行了理论分析,获得了任意截面形状微槽道内的速度分布和流动阻力特性的分析解,并以矩形微槽为例分析了微槽截面上的速度分布和阻力特性.结果表明：随Kn数的增加,由于壁面处滑移流动的影响,气体流经微槽的流动阻力常数小于大尺度理论预测值;理论分析解的结果与实验结果吻合较好,表明在一定的Kn数范围内Navier-Stokes方程在考虑了速度滑移后可以描述微通道内的气体流动过程;正交函数法在微槽内滑移流动的分析中是可行的.     

不同三维微通道中被动混合特性的研究

Based on the transport phenomena theory, the passive mixing of water and ethanol in different three-dimensional microchannels is simulated numerically. The average variance of water volume fraction is used to index the mixing efficiency in the cases with different Reynolds number and different fabricated mixers. The results show that the efficiency of liquid mixing is progressively dependent on the convective transport as the Reynolds number increases. The efficiency of serpentine microchannel decreases with the increasing Reynolds number in the laminar regime. Altering the aspect ratio of channel inlet section has no significant effect on the mixing efficiency. Increasing the area of channel inlet section will cause the decrease of the mixing efficiency. The mixing in serpentine channels is the most efficient among three different mixers because of the existence of second flow introduced by its special structure.     

Simulation of the hydrodynamics and mass transfer in a falling film wavy microchannel

The flow in a liquid falling film is predominantly laminar, and the liquid-side mass transfer is limited by molecular diffusion. The effective way to enhance the mass transfer is to improve the liquid film flow behavior. The falling film behaviors of water, ethanol and ethylene glycol in nine different wavy microchannels were simulated by Computational Fluid Dynamics. The simulation results show that the falling film thickness exhibits a waveform distribution resulting in a resonance phenomenon along the wavy microchannel. The fluctuation of liquid film surface increases the gas–liquid interface area, and the internal eddy flow inside the liquid film also improves the turbulence of liquid film, the gas–liquid mass transfer in falling film microchannels is intensified. Compared with flat microchannel, the CO_2 absorption efficiency in water in the wavy microchannel is improved over 41%. Prediction models of liquid film amplitude and average liquid film thickness were established respectively.     

Using CFD and Ultrasonic velocimetry to Study the Mixing of Pseudoplastic Fluids with a Helical Ribbon Impeller

A commercial CFD package was used to simulate the 3D flow field generated in a cylindrical tank by a helical ribbon impeller. The study was carried out using a pseudoplastic fluid with yield stress in the laminar mixing region. Ultrasonic Doppler velocimetry (UDV), a noninvasive fluid flow measurement technique for opaque systems, was used to measure xanthan gum velocity. From flow field calculations and tracer homogenization simulations, power consumption and mixing time results were obtained. The torque and power characteristics remain the same for upward and downward pumping of the impeller, but the mixing times are considerably longer for the downward pumping mode. Overall, the numerical results showed good agreement with experimental results and correlations developed by other researchers. From the power and mixing time results, two efficiency criteria were utilized to determine the best pumping mode of the impeller.     

Open-Source CFD Simulations of Liquid–Liquid Flow in the Annular Centrifugal Contactor

Simulation of the fluid dynamics of solvent extraction in centrifugal contactors requires advanced models to account for complex physical phenomena including turbulent free-surface flow and liquid-liquid dispersion physics. The use of an open-source computational fluid dynamics (CFD) framework allows for implementation of advanced models not feasible in commercial CFD applications. The open-source CFD package OpenFOAM has been used to simulate turbulent, multiphase flow in the annular centrifugal contactor, including simulations of the mixing zone (annular region), and of the coupled operation of the mixing and separation (rotor interior) zones. These simulations are based on the Volume of Fluid (VOF) methodology along with Large Eddy Simulation (LES) for turbulence. The results from these simulations compare favorably with previous simulations using a commercial CFD tool and with available experimental data. They also give insight into the requirements for more advanced multiphase models needed to accurately capture flows in these devices.     

A simple mechanism of bubble and slug formation in Taylor flow in microchannels

A simple mechanism is proposed to explain and predict the bubble and slug lengths in Taylor (slug) flow in microchannels. The results obtained using the proposed approach are in good agreement with a correlation based on numerical experiments [Qian, D. and Lawal, A., 2006, Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci, 80: 7609–7625] and available experimental data.     

仿蜂巢分形微管道网络中的流动与换热

受自然界中蜂巢结构分形特征的启发,设计和加工了仿蜂巢分形微管道网络,并进行了参数优化.在微管道截面参数、对流传热系数、传热温差均相同的条件下,对流动与换热特性的理论分析表明：加热底面积相同时,仿蜂巢分形微管道网络所能带走的热量可达平行阵列微管道网络的5倍以上;不计分流、合流效应,总换热量一定时,仿蜂巢分形微管道网络所需的泵送功率约为传统平行阵列微管道网络的1/10.恒定热流条件下的去离子水层流对流换热实验也证明：仿蜂巢分形微管道网络比传统的平行阵列微管道网络具有更高的Nusselt数和更低的流动压降.这种分形微管道网络除用于电子器件冷却,还可用于微燃料电池极板、微混合器、微生化反应器等微化工系统结构设计.     

Simulation of on-Board Fuel Conversion in Catalytic Microchannel Reactor-Heat Exchanger Systems

Catalytic conversions of methanol, ethanol, and propane in two parallel microchannels, repeating unit of a multichannel reactor involving successive exothermic combustion and endothermic reforming channels, are simulated to investigate the effect of design parameters on hydrogen yield. Simulations conducted using computational fluid dynamics (CFD) at steady-state show that parameters such as distance between and dimensions of the microchannels and use of micro-baffles within the channels affect heat transfer to the reforming zone and hydrogen yield. Dynamics of the microchannel conversion are also investigated by transient CFD simulations.     

LASP and Villermaux/Dushman protocols for mixing performance in microchannels: Effect of geometry on micromixing characterization and size reduction

The paper reports an experimental investigation on the effect of geometrical design of double jet impingement microchannels on mixing efficiency. Three arrangements of microchannel reactors (MCRs) were designed with 800 μm in diameter by 30 mm in length in various confluence angles of 45°, 90°, and 135°. Mixing performance of the microchannels was first evaluated via competitive parallel reactions of Villermaux/Dushman. The mixing quality was then described under various total liquid flow rates and initial acid concentration using segregation index (X_S). In the second protocol, mixing performance was further investigated via complicated liquid anti-solvent precipitation (LASP) process for nanodrug production. Curcumin was utilized as a model of an insoluble drug in water and particle size and SEM imaging were then employed to characterize the produced nanosuspentions. The whole results show that despite considerable differences in nature of these two processes, the microchannel with confluence angle of 135° works more efficiently in both protocols, due to its higher mixing quality.     

Micromixing using induced-charge electrokinetic flow

The mixing effect of induced-charge electrokinetic flow (ICEKF) in a rectangular microchannel with embedded conducting hurdles is investigated in this paper. A correction method is suggested to estimate numerically the induced zeta potential on the conducting surface. Two-dimensional pressure-linked Navier-Stokes equation is used to model the flow field in the channel. The numerical results show flow circulations generated from the induced non-uniform zeta potential distribution along the conducting hurdle surfaces. It is demonstrated that the local flow circulations provide effective means to enhance the flow mixing between different solutions. The mixing enhancement effect is experimentally validated using PDMS based microchannels with embedded platinum hurdles. The dependence of the degree of mixing enhancement on the hurdle geometries and hurdle numbers is also predicted. The mixing using ICEKF described in this paper can be used in various microfluidics and lab-on-a-chip applications.     

Liquid–liquid two-phase flow and mass transfer characteristics in packed microchannels

In this work, the flow hydrodynamic characteristics and the mass transfer performance of immiscible fluids in the packed microchannels are investigated experimentally. Water–kerosene system is used for visually identifying the flow hydrodynamic characteristics in PMMA microchannels, and water–succinic acid–n-butanol is chosen for investigating mass transfer performance in stainless steel microchannels. Quartz sand micro-particles are used as packing particles. In packed microchannels, high liquid–liquid dispersions can be obtained, and the diameter of droplets produced in the packed microchannel can be even less than 10 μm. It ensures better mixing performance and larger effective interfacial area of two immiscible fluids, and improves the mass transfer performance obviously. Compared to the extraction efficiency (46–61%) in the non-packed microchannel, it can reach 81–96% in the packed microchannel. The effects of packing length, micro-particle size on liquid–liquid dispersions and extraction efficiency are investigated. The pressure drop and the specific energy dissipation in the packed microchannels are also discussed.