Effect of eccentricity on transitional mixing in vessel equipped with turbine impellers

In this paper, the results of the experimental studies of the mixing time, as well as the power consumption and baffle presence in the stirred tank with dual eccentrically located impellers are presented. The experiments were carried out in an unbaffled flat-bottomed cylindrical vessel. Three types of impellers were used: Rushton turbine, six pitched blade turbine and six flat blade turbine. The obtained data show that eccentricity of dual impeller systems leads to reduction of mixing time. Moreover, the experimental data confirmed the enlargement of power consumption in such systems. In the paper the analysis of relation between eccentricity ratio and mixing time has been performed.     

A jet mixing study in two phase gas-liquid systems

All studies concerned with jet mixing have been focused on liquid phase systems and no studies have been found on jet mixing for gas-liquid two phase systems. In the present study the use of jet fluid as a mixer in gas-liquid systems was proposed. Further by installing an experimental setup, the mixing behavior of liquid phase was studied. Gas flow and jet flow are injected to the mixing vessel countercurrently. In this study, the effect of jet injection, location of the conductivity probe, aeration rate and jet Reynolds number on the mixing time are investigated. The created flow pattern was extracted for each condition and the results often analyzed on the basis of them. It is observed that, for low aeration rates, the injection of jet decreases the mixing time considerably. By increasing the aeration rate, the difference in mixing times between the two cases of jet injection and without jet is reduced. Results also show that the closer the probe is to encounter location of the jet and airflow, the lower the mixing time obtained. Dependence of mixing time on the probe location decreases by increasing the mixing intensity and eliminating dead zones. It is obtained, on the basis of Re_j and the amount of jet travelling in the vessel, increasing the aeration rate has different effects on the performance of mixing. Generally, four different trends for the variation of mixing time with increasing the aeration rate are observed.     

The electrokinetic microfluidic flow in multi-channels with emergent applicability toward micro power generation

In order to elaborate the possible applicability of microfluidic power generation from conceptualization to system validation, we adopt a theoretical model of the electrokinetic streaming potential previously developed for the single channel problem. The ion transport in the microchannel is described on the basis of the Nernst-Planck equation, and a monovalent symmetric electrolyte of LiClO₄ is considered. Simulation results provide that the flow-induced streaming potential increases with increasing the surface potential of the microchannel wall as well as decreasing the surface conductivity. The streaming potential is also changed with variations of the electric double layer thickness normalized by the channel radius. From the electric circuit model with an array of microchannels, it is of interest to evaluate that a higher surface potential leads to increasing the power density as well as the energy density. Both the power density and the conversion efficiency tend to enhance with increasing either external resistance or number of channels. If a single microchannel is assembled in parallel with the order of 10³, the power density of the system employing large external resistance is estimated to be above 1 W/m³ even at low pressure difference less than 1 bar.     

AC Electrokinetic Fast Mixing in Non-Parallel Microchannels

A novel ultra-fast micromixer of a quasi T-channel with electrically conductive sidewalls is presented here and some new phenomena in its mixing process are observed and reported. The mixing is about 10²–10³ times faster than that by purely molecular diffusion, and about 10² times faster than that in existing micromixers, which are based on the electrokinetic instability (EKI). Both parallel and non-parallel channel are investigated and compared by evaluating their mixing. Mixing behaviors in the microchannels are studied in terms of scalar concentration distributions. It is found that with a small angle (about 5° in this case) between the two electrodes sidewalls, mixing can be enhanced rapidly at even low AC voltage. The influence of the applied AC voltage phase shift between the two electrodes on the mixing process is also explored. The result reveals that the mixing is the strongest under a 180° signal phase shift. Fast mixing is also achieved under high AC frequency in this micromixer. Fluorescent micro particles are used to visualize the flow pattern for better understanding of the mixing enhancement mechanism. The design of this micromixer could provide new opportunity for applications where fast mixing is demanded.     

Fluids mixing in devices with connected-groove channels

A novel connected-groove micromixer (CGM) has been designed, fabricated, and investigated thoroughly. Connected grooves in this device, crossing multiple sides of the microchannel induced an intensely transverse field of fluids, and thus generating rapid mixing than patterned grooves on a single side alone. The fabrication of a CGM was facilitated to overcome the complication of fabricating the sidewall and bottom grooves in a channel simultaneously; a CGM hence became highly efficient and compact. We propose here CGM of two types—CGM-1 and CGM-2—and compare their mixing performance with a slanted-groove micromixer (SGM) numerically and experimentally for Re over a wide range (1-100). Numerical analysis demonstrated that a CGM provided intense transverse components in the field and great mixing efficiency; in particular, CGM-2 with co-rotating flows encompassing the mechanisms of cutting and blending of fluids had a mixing performance over 50% better than an SGM for Re=1-100. To systematically analyze the mixing by experiments, mixing of slightly viscous fluids, highly viscous fluids, and bio-fluids were adopted, respectively. The mixing experiments of slightly viscous dye solutions on the basis of the color uniformity of mixture showed that mixing lengths of both CGM were smaller than that of SGM. Based on the mixing results of highly viscous fluids, CGM-1 with sidewall grooves had a shorter pitch of spiral flow and more helical turns than an SGM. With a confocal microscope we explored the mixing sections of fluorescent proteins (B-phycoerythrin, BPE; Allophycocyanin alpha subunit, ApcA) inside a micromixer to confirm the numerical results.     

Evaluation of the mixing performance of three passive micromixers

This work presents a numerical investigation on mixing and flow structures in microchannels with different geometries: zig-zag; square-wave; and curved. To conduct the investigation, geometric parameters, such as the cross-section of the channel, channel height, axial length of the channel, and number of pitches, are kept constant for all three cases. Analyses of mixing and flow fields have been carried out for a wide range – 0.267–267 – of the Reynolds number. Mixing in the channels has been analyzed by using Navier–Stokes equations with two working fluids, water and ethanol. The results show that the square-wave microchannel yields the best mixing performance, and the curved and the zig-zag microchannels show nearly the same performance for most Reynolds number. For all three cases, the pressure drop has been calculated for channels with equal streamwise lengths. The curved channel exhibits the smallest pressure drop among the microchannels, while the pressure drops in the square-wave and zig-zag channels are approximately the same.     

An overlapping crisscross micromixer

We propose a grooved micromixer incorporating an overlapping crisscross inlet port that is located at the intersection of two patterned channels crossing one above another. Both numerical analysis and experimental verification of the flow structure of this design have substantiated the superior mixing features over the existing herringbone mixer. Because of the symmetric feature of this microstructure, fabrication becomes simplified through assembling two identical PDMS-based slabs oppositely. Both experimental results for flow visualization and numerical simulation reveal significant cross flow at the intersection of the two channels and vertical tumbling of the flow. This activated flow feature supplies downstream fluids with vertical momentum to enhance the chaotic advection and enlarges the interfacial area between the two mixing fluids. All these features improve the mixing performance of this novel overlapping crisscross micromixer by 46% compared with the mixing indices of the staggered herringbone mixer for the longitudinal distance . Furthermore, on modulating the ratio of volumetric flow rates between the two inlet streams, an excellent mixing function with a specific prearranged concentration of a mixture and a decreased pressure loss are achieved. The divided ratios Q_t/Q_l, defined as diverted flow rate over forward flow rate, are between 1.86 and 2.88 for fluid A and between 0.52 and 0.67 for fluid B with a variable initial flow rate ratio.     

Rapid Concentration of Nanoparticles with DC Dielectrophoresis in Focused Electric Fields

We report a microfluidic device for rapid and efficient concentration of micro/nanoparticles with direct current dielectrophoresis (DC DEP). The concentrator is composed of a series of microchannels constructed with PDMS-insulating microstructures for efficiently focusing the electric field in the flow direction to provide high field strength and gradient. The location of the trapped and concentrated particles depends on the strength of the electric field applied. Both ‘streaming DEP’ and ‘trapping DEP’ simultaneously take place within the concentrator at different regions. The former occurs upstream and is responsible for continuous transport of the particles, whereas the latter occurs downstream and rapidly traps the particles delivered from upstream. The performance of the device is demonstrated by successfully concentrating fluorescent nanoparticles. The described microfluidic concentrator can be implemented in applications where rapid concentration of targets is needed such as concentrating cells for sample preparation and concentrating molecular biomarkers for detection.     

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.     

A combined active/passive scheme for enhancing the mixing efficiency of microfluidic devices

The straight microchannels used in conventional microfluidic devices yield a poor mixing performance because the fluid flow is restricted to the low Reynolds number regime, and hence mixing takes place primarily as a result of diffusion. In an attempt to improve the mixing efficiency of pressure-driven microfluidic flows, the current study applies periodic velocity perturbations to the species flows at the microchannel inlet and incorporates a wavy-wall section within the mixing channel. Numerical simulations are performed to analyze the respective effects on the mixing efficiency of the geometric amplitude of the wavy surface, the length of the wavy-wall section, and the Strouhal number of the periodic velocity perturbations. Overall, the results reveal that the mixing performance is improved by increasing the geometric wave amplitude or length of the wavy-wall section and by applying a Strouhal number in the range 0.33-0.67.     

Characterization of the continuous-flow mixing of non-Newtonian fluids using the ratio of residence time to batch mixing time

Continuous-flow mixing of pseudoplastic fluids possessing yield stress is a complex phenomenon exhibiting non-ideal flows within the stirred vessels. Electrical resistance tomography (ERT), a non-intrusive technique, was employed to measure the mixing time in the batch mode while dynamic tests were performed to study the mixing system in the continuous mode. This study attempts to explore the effects of the operating conditions and design parameters on the ratio of the residence time (τ) to the mixing time (θ) for the continuous-flow mixing of non-Newtonian fluids. To achieve these objectives, the effects of impeller types (four axial-flow impellers: A310, A315, 3AH, and 3AM; and three radial-flow impellers: RSB, RT, and Scaba), impeller speed (290–754 rpm), fluid rheology (0.5–1.5%, w/v), impeller off-bottom clearance (H/2.7–H/2.1, where H is the fluid height in the vessel), locations of inlet and outlet (configurations: top inlet-bottom outlet and bottom inlet-top outlet), pumping directions of an axial-flow impeller (up-pumping and down-pumping), fluid height in the vessel (T/1.06–T/0.83, where T is the tank diameter), residence time (257–328 s), and jet velocity (0.317–1.66 ms⁻¹) on the ratio of τ to θ were investigated. The results showed that the extent of the non-ideal flows (channeling and dead volume) in the continuous-flow mixing approached zero when the value of τ/θ varied from 8.2 to 24.5 depending on the operating conditions and design parameters. Thus, to design an efficient continuous-flow mixing system for non-Newtonian fluids, the ratio of the residence time to the mixing time should be at least 8.2 or higher.     

Computational study of convective–diffusive mixing in a microchannel mixer

Scalar mixing due to convection and diffusion in a microchannel mixer is studied using CFD. A method is developed to quantitatively measure the effect of false diffusion on scalar decay rate. This method computes an average false diffusivity from a given numerical solution and it is not limited to any particular numerical scheme. It is found that a range of molecular diffusivity exist in which average false diffusion is smaller than molecular diffusion and scalar decay rates can be computed accurately with CFD in the mixer. This range of molecular diffusivity covers most of the liquid solutions encountered in chemical and biochemical engineering. When effective diffusivity is used, this range can be further expanded. The predicted mixing structures agree well with experimental results in literature. The classical lamellar structures of the baker's transformation are strongly affected by diffusion. The striation doubling process is destroyed by diffusion broadening at very early stage in the mixer. The optimal mixing is achieved at low Re when the mixing mechanism in the mixer is the baker's transformation. At higher Re, secondary flow is generated and the mixing mechanism is the competition of the kinematics of the baker's transformation and the dynamics of the cross sectional flow. Results show that the secondary flow hinders mixing and the scalar decays at lower exponential rates than when the mixing is due to the baker's transformation alone.     

Effect of high frequency ultrasound on micromixing efficiency in microchannels

In the present work, an investigation on the effect of high frequency ultrasound wave on micromixing in the studied microchannels was carried out. Three types of microchannels with different shapes are examined. A 1.7 MHz piezoelectric transducer (PZT) was employed to induce the vibration in these microchannels through an indirect contact. A method based on the Villermaux–Dushman reaction was employed to study the micromixing in these microchannels. The segregation intensity was determined for layouts with and in the absence of ultrasound irradiation. Further, the effect of ultrasound waves, in various flow rates and initial concentrations of acid, on the segregation index (X_S) and micromixing time (t_m) was investigated. The experimental results showed that the ultrasound waves have a significant influence on product distribution and segregation index at various flow rate ratios. The data obtained in all cases showed that the segregation index was reduced when the flow rate ratios were increased. Also the results demonstrate that in spite of a low energy consumption of PZT, the relative segregation index improved up to 18–36% at various flow rate ratios.     

螺带式搅拌器在假塑液中的混合及放大的研究

本文综合研究了螺带式搅拌器在假塑性液体中的混合特性及动力特性，测定了双螺带，内外螺带及螺带螺旋３种类型搅拌器的功率常数Ｋｐ和Ｍｅｔａｎｅｒ常数Ｋｓ，并得出混匀操作和传质工艺过程的放大准则。     

Laminar mixing of shear thinning fluids in a SMX static mixer

Flow and mixing of power-law fluids in a standard SMX static mixer were simulated using computational fluid dynamics (CFD). Results showed that shear thinning reduces the ratio of pressure drop in the static mixer to pressure drop in empty tube as compared to Newtonian fluids. The correlations for pressure drop and friction factor were obtained at Re_MR?100. The friction factor is a function of both Reynolds number and power-law index. A proper apparent strain rate, area-weighted average strain rate on the solid surface in mixing section, was proposed to calculate pressure drop for a non-Newtonian fluid. Particle tracking showed that shear thinning fluids exhibit better mixing quality, lower pressure drop and higher mixing efficiency as compared to a Newtonian fluid in the SMX static mixer.     

Numerical simulation of flow and mixing behavior of impinging streams of shear-thinning fluids

Due to their high efficiency, impinging streams, which involve the use of two inlet streams that enter the system through two closely spaced inlets along the same axis in opposite directions, have recently found many applications in the chemical and food industry as alternatives to conventional mixing operations. However, there is no prior work that focuses on the flow and mixing behavior of non-Newtonian impinging streams. Numerical simulation was therefore performed to investigate the flow and mixing behavior of steady-state, two-dimensional laminar confined impinging streams of shear-thinning fluids. The coupled heat, mass and momentum balance equations were solved with the finite element method using commercial software FEMLAB™ 3.0. The effects of various parameters, i.e., inlet jet Reynolds number based on the inlet slot width (Re_j) in the range of 10-200 and the flow behavior index of the fluids in the range of 0.6161-1, on the flow and mixing behavior of impinging streams were then investigated. Temperature of the mixing fluid was used as a passive tracer to monitor mixing in this study. The results were also compared and discussed with those of Newtonian impinging streams.     

Optimization analysis of the staggered herringbone micromixer based on the slip-driven method

In this paper the mixing effect of the staggered herringbone micromixer (SHM) was investigated by using the slip-driven method. This method simplified the 3D flow in the staggered herringbone micromixer into a 2D cavity flow with an axial Poiseuille flow. The solution of the 2D cavity flow was obtained by solving the biharmonic equation. An improved design with a cosine asymmetric factor P(z) was proposed, and its mixing effect was demonstrated by comparing the effect with the original design [Stroock, A.D., Dertinger, S.K.W., Ajdari, A., Mezic, I., Stone, H.A. and Whitesides, G.M., 2002, Chaotic mixer for microchannels, Science, 295: 647–651; Stroock, A.D., Dertinger, S.K.W., Whitesides, G.M. and Ajdari, A., 2002, Patterning flows using grooved surfaces, Anal Chem, 74: 5306–5312]. Four methods evaluating the mixing effect were used: (1) mixing images at different cycles; (2) Poincaré Sections; (3) segregation intensity and (4) stretching computation. Finally, an optimized value of P₀ = 1/6 was obtained, and the mixing effect of the improved design for different P₀ is discussed.     

Mixing efficiency of a multilamination micromixer with consecutive recirculation zones

Adding recirculation zones to a mixer for a microplant is proposed for enhanced mixing efficiency. A multilamination interdigital micromixer has been widely used in microchemical plants for precision or small scale chemical process. The mixing efficiency of this micromixer is relatively low as the mixing of two fluids is executed by the laminar diffusion process. To assist the mixing by fluid action, a series of recirculation zones were added to the mixing chamber. The effectiveness of the recirculation zones on mixing was estimated through a numerical simulation which indicated the dependence on Reynolds number. Mixing efficiency increased at Reynolds number that is relevant to the condition that is prevalent in a microchemical plant. The proposed micromixer was fabricated by the lithography process on the photosensitive glass wafers. The mixing qualities of the fabricated micromixer were measured by two methods; the flow visualization of dilution type experiments and the reactivity measurement. The measurement of color intensity of the mixed fluid followed the predictions by the simulation. For a Reynolds number greater than 400 that was relevant in mixers for microchemical plant, a mixing efficiency higher than 90% was obtained by adding the recirculation zones.     

3种被动式微混合器的性能对比及压损分析

通过数值模拟的方法对3种不同结构的被动式微混合器进行了研究,同时对3种被动式微混合器的混合性能进行了对比,并进一步研究了微混合器的压力损失。结果显示,内肋形微混合器在5种速度条件下的混合性能要优于其他两种微混合器,而且随着流量越来越大,其压力损失也越来越大;内肋形微混合器的压力损失最大,Z形微混合器的次之,Y形微混合器的最小。     

Momentum correction coefficient for two jet flows mixing in a tee junction

Momentum correction coefficient (K) is an important parameter for the mixing process of two jet flows. In this paper, momentum correction coefficient for two jet flows mixing in a tee junction was investigated with systems of air–air, water–water and air–water in a wide region of mass ratio of flow rate. The pressure drop was analyzed, where the total pressure drop was mainly attributed to momentum exchange and friction. Based on the experimental data, pressure drop due to momentum exchange was obtained by extrapolation methods, and then the momentum correction coefficient was fitted. It turned out that the momentum correction coefficient was mainly dependent on the momentum flux ratio (M) of the two jets and almost independent with the physical properties of jet flows. The relationship between K and M was correlated as the following equation:

K=1+0.256M^0.223