DEM simulation for optimal design of powder mixing in a ribbon mixer

A ribbon mixer is often employed in powder mixing in a wide range of engineering fields. The structure of the ribbon mixer is extremely complicated. This structure makes it difficult to understand the mixing mechanism by experimental approaches due to problems related to accurate sampling. At present, the mixing mechanism in the ribbon mixer is empirically identified as convection, despite a lack of precise assessment. Additionally, experimental investigations to find the optimal design of the ribbon mixer have not been sufficiently conducted because of its prohibitive cost. As such, there is a lack of sufficient discussion concerning the design for better mixing in the ribbon mixer. Numerical technologies represent a promising approach for solving the aforementioned problems. Significant improvements in computer hardware have enabled numerical models such as the discrete element method (DEM) to be positively employed in powder mixing. In the current study, an identification approach is developed for convective mixing, and besides, the study explores an effective parameter for better mixing in the ribbon mixer using the DEM. A swept volume measurement approach due to paddle movement is newly developed to identify the main mixing mechanism as convection. Sensitivity analyses are performed to find an effective parameter for better mixing. Through the sensitive analyses, the blade width is indicated as an important factor for achieving better mixing. Moreover, this study shows that the relationship between the swept volume and mixing index remains, even if the paddle width changes. Thus, the swept volume measurement method is revealed as useful for identifying the mechanism as convection in the ribbon mixer. Thus, not only novel finding regarding the blade width for better mixing but also the development of an approach for identifying convective mixing in the ribbon mixer is presented herein. Incidentally, convection being the dominant mechanism is consistent with the novel finding regarding blade width achieving better mixing.     

Optimization of a microfluidic mixer for studying protein folding kinetics

We have applied an optimization method in conjunction with numerical simulations to minimize the mixing time of a microfluidic mixer developed for protein folding studies. The optimization method uses a semideterministic algorithm to find the global minimum of the mixing time by varying the mixer geometry and flow conditions. We describe the minimization problem and constraints and give a brief overview of the optimization algorithm. We present results of the optimization, including the optimized geometry and parameter sensitivities, and we demonstrate the improvement in mixing performance with experiments using microfabricated mixers. The dye-quenching experiments of the original and optimized mixer designs show respective mixing times of 7 and 4 mus, a 40% reduction. The new design also provides more uniform mixing across streamlines that enter the mixer. The optimized mixer is the fastest reported continuous flow mixer for protein folding.     

A multi-dimensional population balance model approach to continuous powder mixing processes

This study is concerned with the development of a novel population balance model (PBM) framework that can qualitatively capture the dynamics of a continuous powder mixing process. For the first time, a PBM has been developed to model powder mixing and it accounts for key design and process parameters such as mixer RPM, processing angle in terms of powder fluxes, along with the effect of number of axial and radial compartments. Via this approach, results clearly show the qualitative validity of the PBM as a tool to capture the dynamics of the process that affect API composition, RSD and RTD. The model also demonstrates the use of the PBM as an overall multi-scale modeling tool to combine micro-level models such as DEM in a hybrid framework. Due to the relative computational simplicity of solving the PBM (as compared to DEM), the developed model can be used effectively in control and optimization of the mixing process.     

Hydrodynamics and mass transfer of the coaxial jet mixer in flow injection analysis

A coaxial jet mixer that was previously proposed for rapid and efficient mixing under laminar flow conditions has been studied both theoretically and experimentally. A mathematical model that consists of a set of Navie-Stokes equations that determine the flow velocities and three diffusion-convection reaction equations that determine the reactant and product concentrations has been developed. Equations are solved with the help of finite difference techniques for different flow conditions. The quality of sample and reagent mixing is characterized by the mean product concentration and the amount of product produced. Theoretical results are compared with experimental ones for the mixing of bromothymol blue (a pH indicator) in the outer capillary with NaOH in the inner capillary of the jet mixer.     

通道式微混合器的设计及性能

利用计算流体力学和数字图像处理技术,研究"Y"型通道式微混合器的结构及混合性能,分析了混合器结构尺寸和流动条件对混合过程的影响.数值模拟结果表明,在混合通道入口夹角为60°、通道宽度为200μm、注入速度为0.02 m/s的流动条件下,可以取得比较满意的混合效果.利用数值模拟对比了扭曲通道混合器、导流块和直通道结构对混合过程的影响,结果表明使用导流块可以显著提高混合效果.依照模拟计算结果,设计并用MEMS工艺制作了双侧壁有内肋块的通道式微混合器,并进行了流体混合实验,观测了混合过程.拍摄混合实验图像,对比标准浓度-图像灰度关系曲线后识别出拍摄点混合指数.识别计算的结果也证实了所设计混合器性能上的优越性.最后对实验结果误差进行了分析,说明了误差来源并给出了相应的改进措施.     

Rapid microfluidic mixing.

A preformed T-microchannel imprinted in polycarbonate was postmodified with a pulsed UV excimer laser (KrF, 248 nm) to create a series of slanted wells at the junction. The presence of the wells leads to a high degree of lateral transport within the channel and rapid mixing of two confluent streams undergoing electroosmotic flow. Several mixer designs were fabricated and investigated. All designs were relatively successful at low flow rates (0.06 cm/s, > or = 75% mixing), but had varying degrees of success at high flow rates (0.81 cm/s, 45-80% mixing). For example, one design operating at high flow rates was able to split an incoming fluorescent stream into two streams of varying concentrations depending on the number of slanted wells present. The final mixer design was able to overcome stream splitting at high flow rates, and it was shown that the two incoming streams were 80% mixed within 443 microm of the T-junction for a flow rate of 0.81 cm/s. Without the presence of the mixer and at the same high flow rate, a channel length of 2.3 cm would be required to achieve the same extent of mixing when relying upon molecular diffusion entirely, while 6.9 cm would be required for 99% mixing.     

Novel ejector model for performance evaluation on both dry and wet vapors ejectors

A novel ejector model is proposed for the performance evaluation on ejectors with both dry and wet vapor working fluids at critical operating mode. A simple linear function is defined in order to approach the real velocity distribution inside the ejector. Mass flow rates of the primary flow and secondary flow are derived by integrating the velocity function at the inlet section of the mixing chamber. By considering the flow characteristics of the critical-mode operating ejector, the developed model contains only one energy conservation equation and is independent of the flow in the mixing chamber and the diffuser. Experimental data from different ejector geometries and various operation conditions reported earlier are used to verify the effectiveness of the new model. Results show that the model has a good performance in predicting the mass flow rates and the entrainment ratio for both dry and wet vapor ejectors.     

Influence of contact point treatment on the cross flow mixing in a simple cubic packed bed: CFD simulation and experimental validation

The present study numerically investigates the mixing of an axial flow with a cross flow in a structured packed bed. Three-dimensional computational fluid dynamics CFD simulations have been carried out corresponding to the experimental setup. ANSYS software version 14 was used with the standard \(\upkappa \)–\(\upvarepsilon \) turbulence model. The study focuses on the effect of the contact point treatment by using three methods; gap, overlap and bridge to avoid a high skewed element in the near of contact point. A simple cubic packing with spherical particles of 52 mm diameter was used which gives a porosity of 0.48. The mixing is measured by an injection of nitrogen in the box with a structured bed of 924 spheres with an axial flow of air under different operating conditions. The following parameters were measured; height, injection velocity, volume flow rate ratio, flow conditions and location of injection. It is shown that the CFD simulation results can predict the cross flow mixing. The study revealed that the gap method produced the best experimental results.     

The joint mixing action of the static pre-mixer and the rotating drum mixer – Discrete element method approach

In this study, the mixing performance of coupled mixing action of the Komax static mixer (which is used as a pre-mixer) and rotating drum (applied as the final mixer) was explored in the maize meal mixing operation. The main objective of this paper was to predict the behaviour of the previously grinded maize particles, during the mixing process in static mixer and drum mixer, and to explore the possibilities to shorten the mixing time in the main mixer (in order to reduce the energy consumption).Three different experiments were performed: in the first experiment, possibilities of static mixer were explored, second experiment showed the mixing performance of rotating drum, and the combination of these two mixing devices was investigated in the third experiment. Homogeneity of the obtained mixtures was determined experimentally, by the “Microtracers” method.The Discrete Element Method was used for modelling of granular flow in the pre-mixing and final mixing applications, and to predict the inter-particle mixing quality within a static mixer and the rotating drum mixer. The results of the numerical simulation are compared with appropriate experimental results. The possible industrial application of this model could be the optimization of parameters of mixing systems taking into account the quality and the duration of the mixing process.     

Scale-up and flow behavior of cohesive granular material in a four-bladed mixer: effect of system and particle size

Flow of cohesive granular materials with different moisture contents was examined in a four-bladed mixer via the discrete element method (DEM). Firstly, the mixer diameter (D) was increased while keeping the particle diameter (d) constant. It was observed that when the mixer diameter to the particle diameter ratio (D/d) was larger than a certain critical size (D/d ≥ 75), granular flow behaviors and mixing kinetics followed simple scaling relations. For D/d ≥ 75, flow patterns and mixing kinetics were found to be independent of system size, and velocities of particles scaled linearly with the tip speed of the impeller blades and particle diffusivities scaled with the tip speed of the blades and mixer diameter. These results suggest that past a certain system size the flow and mixing of cohesive particles in large-scale units can be predicted from smaller systems. Secondly, system size was kept constant and particle diameter was changed and it was observed that by keeping the Bond number constant (by changing the level of cohesion) the flow behavior and mixing patterns did not change, showing that larger particles can be used to simulate flow of smaller cohesive particles in a bladed mixer by matching the Bond numbers.     

Modeling of Powder Blending Using On-line Near-Infrared Measurements

A model to quantify the degree of mixing in pharmaceutical powder mixing operations was developed. The additive volume mixing model is based on the determination of the characteristic volume of agitation for a given blender, which is dependent on process parameters such as the formulation ingredients, the geometry of the mixer, and the batch load. The calculation of this characteristic volume of agitation is based on the determination of the fitted fraction of formulation mixed after the first blender rotation. A variation of this model, denominated the iterative mixing model, was also developed. On-line near-infrared (NIR) measurements were taken throughout the runs to obtain the mixing profile and the dynamics of the powder bed as a function of blender rotations. Studies were conducted at two scales using two different formulations to study and compare the calculated characteristic volume of agitation for each blender-formulation system. This approach elucidates the existing relationship between the characteristic mixing parameters and critical rotations (required rotations to achieve content uniformity) for a given system and represents a step toward scale-up of solids mixing operations.     

Influence of the geometrical and gasdynamic parameters of a mixer on the mixing of radial jets colliding with a crossflow

Results of investigation of the mixing of a system of axisymmetrical jets with a subsonic carrying flow in a cylindrical channel are presented. The main characteristics of the mixer were varied in wide ranges in the process of experiments. The regime of colliding jets was realized in the majority of experiments. The influence of individual factors on the quality of the mixing was considered. A combination of the mixer parameters within the ranges investigated, optimum for a high-quality mixing, has been determined. The experimental results obtained were compared with the corresponding data of other authors.     

Oscillatory liquid flow in elastic porous tubes

Summary An analytical method of solution of some problems of laminar, oscillatory flow in elastic porous tubes is given. The mathematical model is based on an one-dimensional approach to the liquid motion. The derived formulae describe the main flow characteristics: pressure, velocity, local and surface outflow of fluid. Influence of geometric dimensions, mechanical properties of elastic tubes, local outflow and filtration through the porous wall, on liquid motion durability is analysed. A simple method of experimental determination of functional parameters is shown.     

Stokes flow in a mixer with changing geometry

A slow-flow mixing device that mimics a natural mixing technique is described. Analytical, numerical and experimental results are presented for the `translating, rotating mixer', which illustrate its mixing effectiveness. In large part, this effectiveness is due to the fact that its geometry changes with time, a feature rare in mathematically tractable slow-flow mixing models. The mixer consists of a large circular cylinder filled with fluid, which is stirred by a circular cylindrical `rod' that moves around in the fluid. The stirring rod may also rotate about its axis. The velocity field is calculated explicitly for the mixer, and its mixing action is simulated numerically. Through a complex-variable formulation of the problem, the energy input required for various mixing protocols may readily be determined, and in turn suggestions for efficient mixing using the device are offered. To validate the analytical and numerical results, tracer-advection experiments are performed, using a simple experimental rig and a variety of mixing protocols, providing encouraging agreement with numerical simulation.     

Achieving uniform mixing in a microfluidic device: hydrodynamic focusing prior to mixing

We describe a microfluidic mixer that is well-suited for kinetic studies of macromolecular conformational change under a broad range of experimental conditions. The mixer exploits hydrodynamic focusing to create a thin jet containing the macromolecules of interest. Kinetic reactions are triggered by molecular diffusion into the jet from adjacent flow layers. The ultimate time resolution of these devices can be restricted by premature contact between co-flowing solutions during the focusing process. Here, we describe the design and characterization of a mixer in which hydrodynamic focusing is decoupled from the diffusion of reactants, so that the focusing region is free from undesirable contact between the reactants. Uniform mixing on the microsecond time scale is demonstrated using a device fabricated by imprinting optical-grade plastic. Device characterization is carried out using fluorescence correlation spectroscopy (FCS) and two-photon microscopy to measure flow speeds and to quantify diffusive mixing by monitoring the collisional fluorescence quenching, respectively. Criteria for achieving microsecond time resolution are described and modeled.     

Measurement of desorption rates from octadecylsilyl bonded-phase HPLC particles and its characterization in terms of pore, surface, and film diffusion

An instrument is developed to measure rates of desorption of solutes from particulate HPLC packing materials for processes that are quantitatively complete in a few tenths of a second. The instrument is a modified, pressure-driven, stopped-flow device. The major modifications include positioning a very short (0.6 mm) bed of the particles just upstream of the detector cell, eliminating the mixing chamber, and adding high-speed switching valves in order to allow sequential continuous flow of individual solutions. Instantaneous rate curves are measured for the desorption of 1,2-dimethyl-4-nitrobenzene (DMNB) from 12-microm-diameter porous particles of the bonded-phase packing Luna C-18 employing high linear velocities of the eluting solvent. The same experiment is performed for the nonsorbed compound phloroglucinol (PG) The PG rate curve is used in two ways (i.e., subtraction and deconvolution) in order to correct the observed rate curve of DMNB for experimental artifacts such as bed hold-up volume and instrument band broadening. The cumulative desorption rate curve of DMNB is obtained by integration. It is accurately described (R2 > 0.999) by a theoretical model that invokes both intraparticle diffusion (including both hindered pore diffusion and surface diffusion) and external film diffusion. The surface diffusion coefficient is (3.2 +/- 0.8) x 10(-6) cm(2)/s and the diffusion film thickness is 0.5 microm. The validity, of both the experimental technique and the theoretical model, is demonstrated by excellent agreement between a predicted and an observed chromatographic elution peak for DMNB on a 25-cm-long commercial column of Luna C-18.     

AE型乳化炸药物理敏化连续混拌系统工艺参数研究

文章介绍了AE型乳化炸药连续混拌系统设计思路,给出了合理的工艺参数:混拌分散线速度3.0～3.5m/s 混拌机长度3.0m,混拌机生产能力1.5～3.5t/h.试验证明采用合理的工艺参数,连续混拌机效率高,混拌效果良好.     

3D Printed Microfluidic Mixers—A Comparative Study on Mixing Unit Performances

One of the basic operations in microfluidic systems for biological and chemical applications is the rapid mixing of different fluids. However, flow profiles in microfluidic systems are laminar, which means molecular diffusion is the only mixing effect. Therefore, mixing structures are crucial to enable more efficient mixing in shorter times. Since traditional microfabrication methods remain laborious and expensive, 3D printing has emerged as a potential alternative for the fabrication of microfluidic devices. In this work, five different passive micromixers known from literature are redesigned in comparable dimensions and manufactured using high‐definition MultiJet 3D printing. Their mixing performance is evaluated experimentally, using sodium hydroxide and phenolphthalein solutions, and numerically via computational fluid dynamics. Both experimental and numerical analysis results show that HC and Tesla‐like mixers achieve complete mixing after 0.99 s and 0.78 s, respectively, at the highest flow rate (Reynolds number (Re) = 37.04). In comparison, Caterpillar mixers exhibit a lower mixing rate with complete mixing after 1.46 s and 1.9 s. Furthermore, the HC mixer achieves very good mixing performances over all flow rates (Re = 3.7 to 37.04), while other mixers show improved mixing only at higher flow rates.     

Dispersion and re-agglomeration of graphite nanoplates in polypropylene melts under controlled flow conditions

The kinetics of GnP dispersion in polypropylene melt was studied using a prototype small scale modular extensional mixer. Its modular nature enabled the sequential application of a mixing step, melt relaxation, and a second mixing step. The latter could reproduce the flow conditions on the first mixing step, or generate milder flow conditions. The effect of these sequences of flow constraints upon GnP dispersion along the mixer length was studied for composites with 2 and 10 wt.% GnP. The samples collected along the first mixing zone showed a gradual decrease of number and size of GnP agglomerates, at a rate that was independent of the flow conditions imposed to the melt, but dependent on composition. The relaxation zone induced GnP re-agglomeration, and the application of a second mixing step caused variable dispersion results that were largely dependent on the hydrodynamic stresses generated.     

Analysis of the process of mixing of a passive impurity in a jet mixer

The experimental results for two regimes of mixing of a passive impurity in an axisymmetric jet mixer — the mixing of a turbulent jet and a cocurrent flow to form a recirculation zone behind the nozzle and an analogous mixing without the formation of a recirculation zone (Re_d = 10,000) — have been presented. The velocity field has been measured in the mixer cross sections at different distances from the nozzle (0.1 < x/D < 9.1) with a one-component Doppler laser anemometer, whereas the scalar field (concentration of the passive impurity) has been diagnosed by the laser-induced fluorescence method. Based on the scalar distributions obtained, the autocorrelation function and the integral scale have been computed, the form of the probability density function has been restored, and the distributions of the asymmetry and excess coefficients have been constructed. Visualization of flow in the mixer has been carried out. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 2, pp. 46–59, March–April, 2007.