结构参数对布置窄缝和挡板的微混合器内流体流动和混合的影响

基于混沌对流原理设计了一种布置窄缝和挡板结构的被动式微混合器,并采用三维数值模拟和可视化实验对该微混合器内流体流动与混合特性进行了研究。窄缝和挡板的共同作用使微混合器水平面内形成了扩展涡和分离涡,垂直流动方向的截面内形成了对称的反向旋涡,多维度涡系显著提高了混合效率。窄缝和挡板的结构尺寸对流体流动和混合有重要影响。综合考虑混合强度和压降,利用场协同原理分析窄缝宽度、窄缝长度、挡板高度对微混合器综合性能的影响并得到了不同Reynolds数条件下的最优结构参数。     

Numerical simulation of geometry effect on mixing performance in L-shaped micromixers

Abstract

This study accomplishes a numerical analysis of mixing in a microchannel with repeating L-shaped units in order to research the effect of the extension of L-shaped units in a three-dimensional (3D) space and the angle of repeating units on the process of mixing. In the first part, Geometry 2 and Geometry 3 are designed by extending the units of Geometry 1 in a 3D space. In the second part, an L-shaped micromixer, a 90° V-shaped micromixer, and a 60° V-shaped micromixer are analyzed. It is observed that Geometry 1 and Geometry 2 perform better than Geometry 3 in terms of mixing due to the spiral path with 360° rotation of the flow. The L-shaped micromixer is more efficient than the 90° and 60° V-shaped micromixers. A maximum mixing index of about 88% is achieved in all serpentine microchannels at the following Reynolds numbers: Re = 150 and Re = 200.     

垂直内挡板对流态化流体动力学及谷物干燥特性之影响

The effect of vertical internal baffles on the particle mixing and grain drying characteristics in a batch fluidized bed column is investigated. Experimental work was carried out in a 3 m high rectangular fluidized bed dryer of cross sectional area of 0.15 mx0.61 m at different operating conditions using paddy, a group D particle, as the fluidizing material. The results of the study showed that the fluidized bed dryer system with vertical internal baffles gave better particle mixing effect in the bed of particles than that without vertical internal baffles. This is due to the fact that the vertical internal baffle act as gas bubble breakers by breaking up the large gas bubbles into smaller ones. The smaller bubbles cause a more vigorous mixing in the bed of particles before finally erupting at the bed surface. This improves the contacting efficiency and enhanced the heat and mass transfer of the fluidized bed system. Thus a higher drying rate was obtained in the falling rate period because the higher contactin efficiency increases the evaporation rate at the particle surface. However, the drying rate in the diffusion regiol shows little improvement because the moisture diffusivity does not depend on the contacting efficiency. The fluidized bed dryer with vertical internal baffles could therefore be used in the initial rapid drying stage in a two stage drying strategy for paddy. The insertion of vertical internal baffles into a fluidized bed system improves the processing of Group D particles in a fluidized bed system especially if the system is large in scale.     

机械搅拌反应器中挡板的结构设计

研究了内径为0．786m的搅拌釜中挡板尺寸及结构对圆盘透平桨RT和翼型桨k5及其组合在气液两相中的气体分散与混合特性的影响。对不同形式的挡板的搅拌功率、气含率及气液混合特性进行了对比分析。研究结果表明：挡板尺寸结构应根据搅拌特性需要进行优化设计;挡板系数为0．12时,组合浆的功率输入已与同一转速下的全挡板系数时的功率输入相近,它同时可改善微观混合、提高混合效率：采用开槽挡板可提高复杂快反应的选择性,混合效率提高20％～25％。     

声场驱动气泡增强微流体混合的数值模拟

微流体研究中，由于雷诺数较低，流体呈层流流动，流体混合主要依靠分子扩散，混合时间长，效率低，故流体混合成为亟待解决的问题。声场激振气泡可以有效促进流体混合，已经引起了广泛关注。本文模拟研究了声场作用下气泡振动对流体混合的影响，探索了微尺度流体在声场激振下的流动特性，分析了微通道高度、入口速度、气泡间距及布置方式对流体混合的影响。结果发现，微通道高度较低时，气泡振动可以更好地促进流体混合；入口速度较小时，流体在气泡附近滞留时间较长，混合较为均匀；气泡半径较大时，旋涡扰动增强，混合效率提高；两个气泡的混合效果优于单个气泡，而气泡间距对混合效率基本无影响；微通道高度较低时，气泡同侧布置和异侧布置对流体的混合效果相接近，随着微通道高度的升高，两种布置方式对混合效果的差异逐渐显现，异侧布置具有更好的混合效果。     

当量螺旋角对三分螺旋折流板换热器性能的影响

提出了非连续螺旋折流板换热器的当量螺旋角定义,并建议以组合数ho/Δp1o/3来反映换热器的综合性能。对可拆卸芯体管束折流板的倾斜角为10°扇形、15°扇形、15°椭圆、20°扇形、20°扇形搭接的三分螺旋折流板换热器和弓形折流板换热器进行了传热和压降性能测试实验。实验结果表明:其中当量螺旋角最大的20°倾斜角扇形折流板方案的壳侧换热系数最高且压降较低,其综合性能指标ho/Δp1o/3比弓形折流板换热器的数值高26%左右;20°倾斜角扇形搭接方案与当量螺旋角相仿的15°倾斜角扇形方案的性能大致相当;15°扇形方案比当量螺旋角较小的15°椭圆方案的性能更高。可见对换热器的性能起决定作用的是当量螺旋角而不是倾斜角;实验预示在当量螺旋角更大的范围存在最佳值。     

Combination of PDMS microfilters and micromixers based on flexible thermoplastic films for size sorting and mixing of microparticles

We present a study for the development of flexible microfilters based on sealing microstructured poly(dimethylsiloxane) (PDMS) to different functionalized thermoplastic films [polyimide (PI), polyethylene naphthalate (PEN), and polyethylene terephthalate (PET)]. The microfilter was manufactured by soft‐lithography and replica molding and then combined with plasma activation and chemical treatment using 3‐(aminopropyl)triethoxysilane (APTES). To demonstrate the functionality of the PDMS microfilters, poly(lactic‐co‐glycolic acid) (PLGA) microparticles (MPS) were filtered through the microfluidic device based on the three thermoplastic films. Subsequently, the mixing capabilities of a passive PDMS micromixer was observed with the injection of polymeric MPS (fluorescent and nonfluorescent) as fluidic mixers are not generally effective at mixing particles. On mixing nonfluorescent MPS (～<10–30 µm in diameter) a mixing performance of 13.3% at 5 mm was observed. Therefore, a PDMS microfiltering device was integrated with a PDMS micromixer using a simple and cost effective home‐made polymeric connector for filtration at a size sorting of 11 µm. The results exhibit that the combination of the two microfluidic devices can be achieved with size sorting and mixing of MPS with an improved mixing performance of 62.5% at 3 mm. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42088.     

基于循环灰热载体煤热解的固-固颗粒冷态混合特性

以开发循环流化床（CFB）燃烧/煤热解分级转化工艺为应用背景,为在移动床混合段内利用有限高度空间达到理想的灰/煤混合效果,建立了内置锥面形挡板结构的固-固冷态混合实验装置,并对颗粒的混合特性进行了研究。分别以石英砂和硅胶模拟CFB循环灰和煤,采用挡板重力混合的方法,探究了挡板角度、层数、放置方式（对向和旋转放置）以及物料混合比等因素对混合效果的影响,并与机械混合实验进行了对比。结果表明,在混合过程中颗粒的混合和分散是并存的,旋转放置30°的挡板层数越多,硅胶和石英砂颗粒混合越均匀,混合物料中石英砂比例越高,混合效果越好。通过优化挡板结构及设置方式强化对流混合和剪切混合可明显改善固-固混合效果。虽然与机械混合相比,挡板混合效果略差,但在一定操作范围内仍可满足灰/煤混合热解工艺的要求。     

交叉导流式微型混沌混合器

针对微尺度混合的特点,提出了一种以玻璃湿法刻蚀加工技术为基础,基于混沌对流混合机理的三维静态微混合器.微通道内,周期排列的导流块在轴向的压力梯度作用下产生了横向的速度分量,可以诱发混沌对流,同时实现微通道内流体的快速混合.运用流场可视化实验和计算流体动力学（CFD）方法研究了微混合器的混沌混合行为.混合试验表明,SOR微混合器在低Reynolds数和高Reynolds数条件下都可以获得好的混合效果.     

Effect of louver baffles on hydrodynamics and gas mixing in a fluidized bed of FCC particles

The effect of louver baffles on the particle concentration profiles, pressure fluctuations, bed expansion, and gas mixing of a fluidized bed was investigated in a transparent 2-D column of cross-section 500×30 mm and height 6 m over a broad range of operating conditions covering both the bubbling and turbulent flow regimes. Visual observations, pressure fluctuations and steady gas tracer experiments showed that louver baffles can break bubbles, as indicted by the lower amplitudes and higher mean frequencies of differential pressure fluctuations, but they were only effective for superficial gas velocities <∼0.7 m/s for the FCC particles considered in this study. The ability of louver baffles to break bubbles reached a maximum near the onset of the turbulent flow regime. A gas cushion of low particle concentration appeared below the louver baffle, and its height increased with increasing superficial gas velocity, indicating increasing suppression of solids backmixing. Internal emulsion circulation was promoted above the louver baffle, causing an uneven distribution of gas flow. The addition of louver baffles reduced the upstream tracer gas concentrations by 80-90%, indicating a significant decrease in the backmixing fluxes of both gas and solids across the baffle layer. The tracer gas concentrations above the louver baffles increased resulting from the promoted emulsion circulation by louver baffles.     

Improvement of mass and heat transfer efficiency in a scale-up microfluidic mixer designed by CFD simulation

Due to scale effects, directly enlarging the size of the micromixer is an easy way to reduce the efficiency of mass and heat transfer in the continuous flow chemical process. It is urgently needed to solve the problem of mass and heat transfer efficiency of the scale-up mixer. A scale-up microfluidic mixer with a porous structure was designed to improve the mass and heat transfer efficiency using computational fluid dynamics (CFD) simulations. The effects of rotation angle, porosity, and baffle spacing were studied to optimize the mixer structure. Compared with the 1 mm mixer without structure, the scale-up mixer has a higher mixing efficiency and an 80% reduction in energy consumption at Re ≥ 700. A Nusselt number was used to evaluate the heat transfer efficiency of the mixer during fluid heating. The results show that the porous baffle promotes the generation of secondary flow and enhances the heat transfer effect, making its Nu increase by three times compared with the unstructured mixer. The scale-up microfluidic mixer with a porous structure can effectively improve the mass and heat transfer performance. This study can provide a reference for the design or development of a novel scale-up mixer.     

Experimental and Numerical Investigation of a Novel Spiral Micromixer with Sinusoidal Channel Walls

A novel spiral micromixer with sinusoidal channel walls was designed to enhance the mixing index in the low to intermediate Reynolds number range (1 < Re < 100). To analyze the fluid flow, a set of numerical simulations were performed using the finite-difference method. The microchip was fabricated from polydimethylsiloxane, employing the soft-lithography technique. The degree of mixing was increased by 99.11 % when using the proposed micromixer, compared to 59.44 % for a simple spiral micromixer. The introduced microchannel drastically reduced the mixing length, increasing the mixing index of a 0.5-loop spiral-sinusoidal microchannel compared to that of the simple spiral microchannel with 1.5 loops. The mixing index of the 3-loop mixer was higher than that of the microchannel with 1.5 loops, and its pressure drop was increased.     

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.     

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.     

Shape Optimization of a Three-Dimensional Serpentine Split-and-Recombine Micromixer

Optimization of a three-dimensional split-and-recombine micromixer with serpentine structure was performed using Navier–Stokes analysis and optimization techniques. The optimization study was performed at a fixed Reynolds number of 15, with four dimensionless design variables, viz. the ratio of the subchannel width to the main channel width, the ratio of the subchannel length to the pitch length, the ratio of the subchannel depth to the main channel depth, and the ratio of the recombination channel width to the main channel width. The design space was investigated by a parametric study, and the design points within the design space were selected by the Latin hypercube sampling method. Two different objective functions, viz. the mixing index at the micromixer exit and mixing effectiveness, were used alternately for the single-objective optimizations. Mixing effectiveness was defined as the ratio of the mixing index to the pressure drop. A surrogate modeling technique based on a radial basis neural network was used to approximate the objective functions. Optimum configurations of the micromixer were found through the mixing-index and mixing-effectiveness optimizations. The optimum design of the micromixer obtained by the mixing-index optimization confirmed 33.0% relative increase in the mixing index compared with the reference micromixer. The mixing index, 0.86, which was achieved by the optimization of the micromixer, is much higher than those of the other split-and-recombine micromixers at the same mixing length and Reynolds number, and the optimized micromixer could be integrated with microfluidic systems including lab on a chip and micro total analysis system.     

Centrifugal Micromixery

We present a modular centrifugal micromixer comprising a mixing unit hosting a planar network of low‐aspect‐ratio microfluidic channels (“disk”), a fixed rotating drive (“player”) and contact‐free dispensers for the continuous feed of educts. The modular setup allows a simple fabrication and a child's play exchange of the mixing unit. High‐speed micromixing is powered by the Coriolis force at volume throughputs of up to milliliters per minute and microchannel! These outstanding characteristics are demonstrated by experiments and accompanying CFD‐simulations.     

Design and Simulation of a Microfluidic Blood-Plasma Separation Chip Using Microchannel Structures

Current clinical methods for the separation of whole blood into blood cells and cell-free plasma are currently based on large facility equipment, such as centrifuges. The disadvantage of this process is that the patients must have assays performed at the hospital or laboratory where the separation facility is located. The present study presents a design for microfluidic chips with different microchannel structures, which utilizes backward facing step geometry and centrifugal force to extract the cell-free plasma from whole blood samples at the branch of the microchannel for further assay, avoiding the influence of blood cells. Numerical simulation was performed on a personal computer to analyze the effects of inlet velocity and the structures of the microchannel on the flow field and back–flow in the microchannel, as well as the efficiency of separation and the volumetric fraction of the flowrate of plasma extraction. The minimum radius of particles (R) that can be excluded from the side channel, and fraction of the volumetric flowrate were obtained to evaluate the efficiency of plasma extraction. Based on the numerical simulations, the design with both converging and bending channels was the best design among the four layouts proposed. In this design, the value of R could be set to less than the critical value (set as 1 µm because of the radius of platelets), and the volumetric fraction of the extraction flowrate was approximately 8.4% when Re was about 20. The preliminary experiments indicated the fluorescent particles with 2.5 µm in radius were successfully excluded from side (plasma outlet) channel of the microfluidic chip with converging a inlet channel and the bent microchannel, when the Reynolds number of the inlet flowrate equals 50.     

Cavitation behavior and mixing performance of antisolvent precipitation process in an ultrasonic micromixer

A facile and robust ultrasonic micromixer was developed to intensify antisolvent precipitation via ultrasonic cavitation. The gas supersaturation created from solvent–antisolvent mixing was found to be a novel driving force which facilitated the generation of cavitation bubbles (CBs). Instead of being attached on the channel wall, numerous CBs translated across the microchannel at a speed up to 1.7 m/s, inducing intense transverse flow over the cross-section. The unique cavitation behavior enabled rapid mixing (mixing time 15–45 ms at 30 W) of solvent–antisolvent over wide Reynolds number range (70–500) and flow rate ratio (5:1–2:3), providing better operability for antisolvent precipitation. The effects of ultrasonic power, total flow rate, flow rate ratio, and solvent on cavitation behavior and mixing performance were quantitatively studied. Finally, the potential of the ultrasonic micromixer as a new tool for antisolvent precipitation was demonstrated by synthesizing size-controllable and monodisperse polymeric nanoparticles in a high-throughput and reproducible manner.     

Taylor-vortex membrane reactor for continuous gas-liquid reactions

A unique Taylor-vortex membrane reactor (TVMR) design for continuous gas-liquid reactions is presented in this work. The reactor consists of a cylindrical rotor inside a stationary concentric cylindrical vessel, and a flexible system of equispaced baffle rings surrounding the rotor. This restricts the annular cross section to a small gap between the baffles and the rotor, and divides the annulus into 18 mixing zones. The baffles support a 6 m long PFA tubular membrane that is woven around the rotor. At 4 mL/min inlet flow rate, the TVMR showed a plug-flow behavior and outperformed the unbaffled reactor, having 5–12 times lower axial dispersion. The continuous aerobic oxidation of benzyl alcohol was performed for 7 h using the Pd(OAc)₂/pyridine catalyst in toluene at 100 °C and 1.1 MPa oxygen pressure. A stable conversion of 30% was achieved with 85% benzaldehyde selectivity, and no pervaporation of organics into the gas phase.     

Mixing performance of a planar micromixer with circular obstructions in a curved microchannel

A numerical investigation of the mixing and fluid flow in a new design of passive micromixer employing several cylindrical obstructions within a curved microchannel is presented in this work. Mixing in the channels is analyzed using Navier–Stokes equations and the diffusion equation between two working fluids (water and ethanol) for Reynolds numbers from 0.1 to 60. The proposed micromixer shows far better mixing performance than a T-micromixer with circular obstructions and a simple curved micromixer. The effects of cross-sectional shape, height, and placement of the obstructions on mixing performance and the pressure drop of the proposed micromixer are evaluated.