Central difference solutions of the kinematic model of settling of polydisperse suspensions and three-dimensional particle-scale simulations

The extension of Kynch's kinematic theory of sedimentation of monodisperse suspensions to polydisperse mixtures leads to a nonlinear system of conservation laws for the volume fractions of each species. In this paper, we show that a second-order central (Riemann-solver-free) scheme for the solution of systems of conservation laws can be employed as an efficient tool for the simulation of the settling and the separation of polydisperse suspensions. This is demonstrated by comparison with a published experimental study of the settling of a bidisperse suspension. In addition, we compare the prediction of the one-dimensional kinematic sedimentation model with a three-dimensional particle-scale simulation.     

超声法纳米颗粒悬浮液粒径测量中温度影响的实验研究

针对医药、化工领域高浓度纳米悬浮液颗粒粒径超声检测中温度影响,采用超声衰减谱法(UAS)对体积浓度30%的纳米铟锡金属氧化物(ITO)水性悬浮液在循环流速800 r/min,温度298～358 K时颗粒粒径分布进行实验。结果表明:温度升高,超声幅值A减小,超声衰减系数增大,颗粒中位径D50增大,颗粒系分布曲线整体朝大颗粒方向偏移,但是分布宽度保持稳定的趋势。同时,将室温(298K)测量结果与CPS离心沉降颗粒测量仪对比,结果较吻合。通过线性回归的方法修正温度对测量结果的影响,超声衰减法能够应用于358K的高温下高浓度纳米颗粒检测。     

Emergence of coherent structures and large-scale flows in motile suspensions

The emergence of coherent structures, large-scale flows and correlated dynamics in suspensions of motile particles such as swimming micro-organisms or artificial microswimmers is studied using direct particle simulations. A detailed model is proposed for a slender rod-like particle that propels itself in a viscous fluid by exerting a prescribed tangential stress on its surface, and a method is devised for the efficient calculation of hydrodynamic interactions in large-scale suspensions of such particles using slender-body theory and a smooth particle-mesh Ewald algorithm. Simulations are performed with periodic boundary conditions for various system sizes and suspension volume fractions, and demonstrate a transition to large-scale correlated motions in suspensions of rear-actuated swimmers, or Pushers, above a critical volume fraction or system size. This transition, which is not observed in suspensions of head-actuated swimmers, or Pullers, is seen most clearly in particle velocity and passive tracer statistics. These observations are consistent with predictions from our previous mean-field kinetic theory, one of which states that instabilities will arise in uniform isotropic suspensions of Pushers when the product of the linear system size with the suspension volume fraction exceeds a given threshold. We also find that the collective dynamics of Pushers result in giant number fluctuations, local alignment of swimmers and strongly mixing flows. Suspensions of Pullers, which evince no large-scale dynamics, nonetheless display interesting deviations from the random isotropic state.     

Average pressure and velocity fields in non-uniform suspensions of spheres in Stokes flow

A widely used method for the approximate numerical simulation of the bulk behavior of particle suspensions consists in filling the entire space with copies of a fundamental cell in which N particles are arranged according to some probability distribution. Until now this method has only been used for suspensions that are spatially uniform in the mean. The case of spatially non-uniform systems, on the other hand, has not been considered. Here the average velocity and pressure fields for such a non-uniform suspension of identical rigid spheres in Stokes flow are calculated, and analytic solutions expressed in terms of multipole coefficients are presented. The results match and extend others obtained by the authors in parallel work using a completely different approach. In particular, the definition of a quantity to be identified with the mixture pressure is fully supported by the present results. An explicit result for the structure of the viscous stress in the suspension is also found. It is shown that, for spatially non-uniform systems, the stress contains a non-symmetric contribution analogous to a baroclinic source of vorticity.As a byproduct of the analysis, certain integrals of two periodic functions introduced by Hasimoto are calculated. These integrals would arise in similar problems, e.g. the electric field produced by electric multipoles in a periodic cubic structure.     

Shear-induced orientational dynamics and spatial heterogeneity in suspensions of motile phytoplankton

Fluid flow, ubiquitous in natural and man-made environments, has the potential to profoundly impact the transport of microorganisms, including phytoplankton in aquatic habitats and bioreactors. Yet, the effect of ambient flow on the swimming behaviour of phytoplankton has remained poorly understood, largely owing to the difficulty of observing cell–flow interactions at the microscale. Here, we present microfluidic experiments where we tracked individual cells for four species of motile phytoplankton exposed to a spatially non-uniform fluid shear rate, characteristic of many flows in natural and artificial environments. We observed that medium-to-high mean shear rates (1–25 s⁻¹) produce heterogeneous cell concentrations in the form of regions of accumulation and regions of depletion. The location of these regions relative to the flow depends on the cells'' propulsion mechanism, body shape and flagellar arrangement, as captured by an effective aspect ratio. Species having a large effective aspect ratio accumulated in the high-shear regions, owing to shear-induced alignment of the swimming orientation with the fluid streamlines. Species having an effective aspect ratio close to unity exhibited little preferential accumulation at low-to-moderate flow rates, but strongly accumulated in the low-shear regions under high flow conditions, potentially owing to an active, behavioural response of cells to shear. These observations demonstrate that ambient fluid flow can strongly affect the motility and spatial distribution of phytoplankton and highlight the rich dynamics emerging from the interaction between motility, morphology and flow.     

Ultrasonic determination of the particle concentration in model suspensions and ice slurry

The development of ice slurry for refrigeration systems and the enhancement of its efficiency depend on an accurate control of the ice concentration. We present here an ultrasonic method capable to measure precisely the particle concentration in ice slurry. To calibrate the ultrasonic measurement, we first determine the sound velocity and attenuation in two model suspensions (glass beads/polyethylene glycol and polyethylene beads/vaseline oil) for different particle volume fractions. The experimental results show a good agreement with the predictions of the two-component models in the long-wavelength limit. Additionally, the sound attenuation reveals a clear signature of the aggregate formation in the nearly iso-dense suspension. We next conduct the measurement of the sound velocity in the polypropylene glycol ice slurry where the ice concentration changes with temperature. The ice concentrations extracted from our sound velocity measurements are well consistent with the values determined from the binary phase diagram.     

DEM simulation analysis of the improvement in particle discharge flowability using adhesive force distribution models based on admixed particle coating

Particle flowability can be improved by admixing particles smaller than the original particles (main particles). However, the details of the mechanism of this improvement are not yet fully understood. In this study, we used a discrete element method simulation to investigate the effects on the particle flowability of the adhesive force distribution at each contact point based on the admixed particle coating. We used the non-uniform, random, and uniform surface adhesive force distribution models and calculated the discharge flow rates. The non-uniform models had a larger discharge flow rate compared with the other models because the non-uniform adhesive force distribution destabilized the force balance in the bed, and thus destabilized the particle arching structure, which generated discontinuous layers more frequently and improved the flowability. Consequently, in a smaller particle admixing system, the adhesive force distribution at each contact point would help to improve the flowability.     

基于稳态运行模式影响NBI真空空间粒子分布特性的研究

EAST-NBI真空空间粒子分布状态是影响中性束再电离损失的重要因素,是研究EAST-NBI系统的结构设计和提高加热效率的重要课题。根据EAST-NBI的工作原理、结构特点以及系统的气源特性,利用Monte-Carlo方法建立模型,运用Matlab软件对EAST-NBI稳态运行模式下真空空间内任意时刻的粒子分布进行编程模拟计算。研究结果表明气源的种类、气体量大小和气体粒子的发射方向是影响EAST-NBI真空空间粒子分布的主要因素,为减小再电离损失、提高加热效率和EAST-NBI的研制提供理论依据。     

Role of base plate rotational speed in controlling spheroid size distribution and minimizing oversize particle formation during spheroid production by rotary processing

The occurrence of material adhesion and formation of oversize particles in the product yield during one-pot spheroid production by rotary processing leads to a less predictable process and a decrease in the usable portion of the total product yield obtained from each production run. The use of variable speeds of the rotating frictional base plate during the spheronization run was investigated for achieving optimal spheroid production. When the base plate speed was increased during liquid addition, the greater centrifugal forces generated improved liquid distribution and the mixing of the moist powder mass, resulting in a decrease in the amount of oversize particles formed. When the base plate was maintained at a high speed throughout the run, the amount of oversize particles and mean spheroid size increased, and a greater “between batch” mean spheroid size variability was also observed. The findings showed that, when higher speeds were used, the residence time must be adjusted accordingly to avoid excessive coalescence and growth while maintaining even liquid distribution. A “low-high-low” speed variation during rotary processing may be used to produce spheroids with a narrow size distribution and with a minimal amount of oversize particles in the total product yield.     

Numerical investigation of dilute aerosol particle transport and deposition in oscillating multi-cylinder obstructions

The transport and deposition of aerosol particles through a fibrous filter is encountered in many natural and industrial processes. As the filtration performance for a stationary filter has been extensively studied in the literature, the present work focuses on the effect of fiber oscillation in a filter where the fibers are allowed to vibrate periodically. The transport and deposition of dilute aerosol particles in such a system is simulated using an efficient numerical model, where an iterative immersed-boundary lattice Boltzmann method is applied to solve the background flow with finite-size moving fibers, and the motion of aerosol particles is then tracked by a one-way coupling Lagrangian approach. In the present scheme, the no-slip boundary condition at the fiber surface can be exactly enforced with an iterative approach and the numerical stability is improved by adopting the MRT collision model. After the model validation in the two special cases of flow over an oscillating fiber in a quiescent fluid and particle capture by a stationary fiber, the filtration performance of an oscillating multi-fiber filter is investigated to study the effects of fiber number, arrangement and vibration mode. It is found that the oscillating motion of fiber has significant influence on the filtration performance. For a single fiber, with larger oscillation amplitude, the distribution ranges of the release position and impact angle of captured particles both increase. On the other hand, a larger fiber oscillation frequency tends to reduce the width of release position but increase the width of impact angle of deposited particles. Furthermore, the collection efficiency is found to be linearly related to the oscillation amplitude or frequency. For multiple fibers, the collection efficiency always increases with larger fiber number, but it is a non-monotonic function of the arrangement parameters, i.e., the longitudinal and transverse spacings, and the vibration parameters such as the amplitude, frequency and vibration mode. It is interesting to find that the in-phase mode can usually lead to excellent collection efficiency.     

Gender and age distribution of occupational fatalities in Taiwan

This study analyzed fatal occupational injuries in Taiwan. One thousand eight hundred ninety work-related accident reports filed in the years 1996-1999 were extracted from the annual publication of the Council of Labor Affairs (CLA). These data were analyzed in terms of gender, age and work experience of the accident victim as well as accident type and the work-related source of injury to identify significant contributing factors. The CLA data showed that work-related falls were the leading cause of work-related fatalities in both male and female workers (38.2% of male victims and 39.2% of female victims). Gender differences were also noted in the accident type and age of the injured workers. Male workers had a significantly higher prevalence of fatal occupational injuries than female workers throughout the analyzed period (7.4 compared to 0.9 per 100,000 full-time workers). Young males aged 24 years or less had the highest rate of fatal occupational injuries. The finding that gender and age are major factors in occupational injuries is a significant finding in the field of occupational safety and may be helpful for developing accident prevention programs.     

Measurements of ultrafine particle concentrations and size distribution in an iron foundry

The number and surface area concentration of ultrafine particles in an iron foundry is of interest as freshly generated ultrafine particles are produced by metal melting, pouring and molding processes. This study measured the number and surface area concentrations of ultrafine particles and their size distributions in an iron foundry using a scanning mobility particle sizer (SMPS). The 10-100 nm ultrafine particle number concentrations (NC(0.01-0.1)) and surface area concentrations (SC(0.01-0.1)) measured at the iron foundry were 2.07 x 10(4) to 2.82 x 10(5)particles cm(-3) and 67.56 to 2.13 x 10(3)microm(2)cm(-3), respectively. The concentrations changed dramatically depending on on-site manufacturing conditions. The NC(0.01-0.1) levels in the iron foundry were approximately 4.5 times higher on average compared with those in the outdoor ambient environment. These measurement results indicate that the presence of extra particles in the workplace air is within the ultrafine range. Additionally, the analytical results suggest that the number mode diameter can be used to estimate the SC(0.01-0.1) levels using the NC(0.01-0.1) levels. Moreover, the ultrafine particle number mode diameter was found to be about 46.1 nm in the iron foundry.     

Numerical and Experimental Investigation to Reduction of Sedimentation of Particles in MRFs with Permanent Magnets

A method to reduce the sedimentation of the ferromagnetic particles in magnetorheological fluids (MRFs) is studied with numerical simulation and experiment. It shows that, making use of the magnetic field generated by a permanent magnet put simply above an MRF, the sedimentation of the particles in the MRF can be reduced remarkably. The magnetic force on a ferromagnetic particle and that on a particle chain are computed with the finite element (FE) code ANSYS. It reveals that the magnetic force on a particle-chain is much larger than the sum of the magnetic force on each individual particle in the chain without considering the interaction between the magnetized particles. The improvement of the sedimentation stability of MRF samples with permanent magnets is also investigated experimentally with a specially designed testing device, and it is found that a proper choice of the permanent magnet and the distance between the MRF sample and the permanent magnet can efficiently improve the sedimentation stability of MRFs.     

Effect of particle size distribution on performance and particle kinetics in a centrifugal slurry pump handling multi-size particulate slurry

A significant variation in particle size distribution (PSD) is generally encountered in slurry transportation. The goal of this work is to establish the effect of variation in PSD on the centrifugal slurry pump (CSP) performance and particle kinetics. Computational fluid dynamics (CFD) modeling of a CSP with multi-size particulate slurry has been performed with a sliding mesh approach using the granular Eulerian-Eulerian model. The numerical model is validated with the experimental data of the pump performance for multi-size particulate fly ash slurry. The maximum deviations in the predicted head and efficiency compared to the measured values are of the order of ±2% and ±3.5%, respectively. Simulations with a single representative particle size for multi-size particulate slurry using median and weighted mean diameter approach are also carried out to understand the difference in performance prediction with equi-size and multi-size slurry. The predicted trend of pump performance variation with PSD is linear and non-linear with equi-size and multi-size slurries, respectively. The median and weighted mean approaches showed error in capturing the effect of variation in PSD on pump performance. The variation in PSD significantly affects the flow of particles inside the impeller and casing flow passages due to particle kinetics. Reduction in the intensity of granular pressure, maximum granular viscosity, and the head loss due to friction in impeller and casing flow passages are found with the increase in the fine size particles.     

Numerical simulation of sedimentation in the presence of 2D compressible convection and reconstruction of the particle-radius distribution function

Numerical simulation of the sedimentation of a polydisperse suspension in a convectively unstable medium is presented. For the simulation of 2D compressible convection, the full system of hydrodynamic equations is solved by the explicit MacCormack scheme. Velocities and positions of suspension particles are calculated simultaneously with the solution of the equations. Initially, the particles are randomly distributed in the computational region. The total weight of sedimented matter is recorded during the numerical experiment. The results are compared with the sedimentation of the same suspension without convection. To reconstruct the particle-radius distribution function from the sedimentation curve, a new method is used. This method is based on the solution of the sedimentation integral equation by the Tikhonov regularization method and was recently developed by the author. To illustrate this technique, sedimentation of cement powder in air is simulated. The suspension contains 50000 particles. The particle radii are assumed to be log-normally distributed. Heat-driven convection is completely determined by the top and bottom boundary temperatures of the computational region and lateral boundary conditions. It is shown that convective motions of a medium with sedimented particles lead to the following effect: the fine disperse fraction of the suspension remains suspended much longer than without convection. Some particles will not sediment at all. The maximum radius of the particles of this fraction depends on the convection parameters (e.g. on convection cell size and convection velocities). These parameters, in their turn, depend only on the temperature difference of the top and bottom boundaries. The results of these calculations can be applied in geology and meteorology for studying dust sedimentation in air as well as in technology. Heat-driven convection can be used for separation of suspensions with the cut-off particle radius depending on temperature difference only.     

Inline method of droplet and particle size distribution analysis in dilute disperse systems

In the present article a measurement method of particle size distributions (PSD) in industrial installations which use a dispersed phase of low concentration (like spray dryers or spray scrubbers) is introduced. A new type of inline-measurement system has been developed and designed to work in spray drying conditions. A standard digital camera is used to record shadows of flowing particles inside the spray drying chamber. Collected images were analyzed by a newly developed software which recognizes particles only in the focus area and eliminates several types of artifacts. The constructed prototype of the PSD inline-analyzer was installed and used to monitor large laboratory scale spray dryer. All data collected by the designed system during the spray drying experiments were compared with data measured with an offline reference system to show accuracy of the new measurement technique.     

DP600双相钢单向拉伸的交叉颈缩现象与有限元分析

目的探究DP600双相钢单向拉伸时交叉颈缩现象的产生原因。方法在单轴拉伸实验的基础上,利用ABAQUS有限元软件对拉伸过程进行模拟,对交叉颈缩区域进行应力、应变分析。结果即将发生断裂时刻,试样标距内中心位置应力达到最大值,临近中心位置两侧由于"局部卸载"而出现应力大幅减小的情况。结论试样标距内两侧距离中心越远的位置,越接近于单轴应力状态。DP600板材裂纹萌生时,与断裂发生部位越接近,其应力状态与平面应变状态越相似,沿着颈缩线方向基本没有应变产生,材料主要由颈缩线区域的板材厚向减薄颈缩线加宽补偿垂直于颈缩线方向上的拉伸。     

The movement law and orientation control of rectangular particles in the viscous fluid domain based on IS-FEM

Understanding the movement law and orientation control mechanism of non-spherical particles are significant for industrial applications. In this work, the flow characteristics of rectangular particles, in the uniform and wedge viscous fluid domain, are simulated by the immersed smoothed finite element method (IS-FEM). The influences of mesh resolution and time-step on particle velocity are analyzed, and the numerical procedure is validated by the published model and sedimentation experiments. The operating parameters that affect the particle flow are systematically studied, including Reynolds number, initial angle, channel offset distance, and aspect ratio. Moreover, the particle angles are adjusted by the velocity gradient of fluid domains. The result indicates that the velocities, angle, and drag of rectangular particles are closely related to the working conditions. The long axis of rectangular particles is consistent with the flow direction in shrinking fluid domains and is perpendicular to the flow direction in expanding fluid domains. The angle distribution law of rectangular particles in moving wedge fluid domains is determined. These findings provide a theoretical foundation for particle sedimentation and suspension flow, which is helpful for the further separation and orientation control of mixed particles.