SEDIMENTATION OF PARTICLES IN NON-NEWTONIAN FLUIDS

The influence of non-Newtonian flow behavior on sedimentation velocity of particles is investigated using an approximate solution for the motion of an assemblage of solid spheres presented previously by the authors. It is theoretically predicted that the pseudoplaslicity decreases the sedimentation velocity and its reduction is pronounced at large voidage. The present theory is discussed using the available empirical correlations.     

Analysis of Gravity Filtration Behaviors of Waterworks Sludge Based upon Sedimentation Tests

Both gravity filtration experiments under constant pressure conditions and gravity sedimentation experiments were conducted using the municipal waterworks sludge. It was clarified from the theoretical analysis that the effect of sedimentation on the filtration rate was noticeable for the sludge used in this study. The local specific flow resistance at various sludge concentrations was determined by the sedimentation velocity method. The local porosity was related to the local solid compressive pressure by the sedimentation equilibrium method. The decline behaviors in the filtration rate in gravity filtration accompanied by sedimentation were well evaluated only from the sedimentation data based upon the sedimentation velocity method and the sedimentation equilibrium method.     

聚硅酸铝铁、聚合氯化铝、聚合硫酸铁絮凝效果的比较

本文比较了聚硅酸铝铁、聚合氯化铝和聚合硫酸铁三种絮凝剂对钢铁厂、造纸厂废水的絮凝效果。结果表明，聚硅酸铝铁的除浊、除色等效果均好于聚合氯化铝和聚合硫酸铁，其污泥沉降体积最小，沉降时间最短，沉降速度最快。     

Large-scale calculation of hydrodynamic transport properties for random suspensions of hard-sphere particles

A numerical method based on fast multipole summation scheme is used to calculate hydrodynamic interactions in random suspensions of non-colloidal hard-sphere particles. The calculation is carried out for suspensions of 1,024 particles randomly placed in periodic unit cell to determine hydrodynamic transport properties such as permeability of a viscous flow through porous medium, effective viscosity of suspension, and sedimentation velocity of the suspended particles. The particle volume fraction ø ranges from 0.01 to 0.25. Effect of particle number N on the transport properties was examined through the numerical calculations with N=64-1,024. It is shown that sedimentation velocity increases with N approaching an estimate for infinite N, and the finite N effect is negligible in effective viscosity and permeability problems. The present scheme is quite useful for obtaining a statistically-averaged quantity for random suspensions. As an example, ensemble-averaged velocity when position of one particle is fixed is numerically obtained in sedimentation problem. The numerical results are shown to be in excellent agreement with theoretical prediction.     

Particle Deposition on Semiconductor Wafers

The deposition of aerosol particles on semiconductor wafers in the typical manufacturing environment of the clean room has been calculated using the equations of convective diffusion and sedimentation. The result shows that the deposition velocity decreases with increasing particle size in the diffusion regime and increases with increasing particle size in the sedimentation regime, with a minimum deposition velocity occurring in the vicinity of 0.2 μm. The minimum deposition velocity varies from approximately 0.2 × 10^?3 to 0.7 × 10^?3 cm/s, depending on the size of the wafer, the airflow velocity, and whether the wafer is freestanding or placed on top of a workbench.     

Effect of inclination on gas-fluidized beds of fine cohesive powders

Results are presented of an experimental investigation on how bed inclination affects the fluidization and sedimentation behavior of fine cohesive particles. In contrast with the expected Geldart C behavior, and due to self-agglomeration, these fine particles are uniformly fluidized by gas in a vertically oriented bed, displaying a fluid-like regime and expanding smoothly as the gas velocity is increased. When the gas flow supply to the bed is suddenly stopped, the initial sedimentation velocity of the vertically oriented bed is similar to the fluidizing gas velocity as corresponds to uniform fluidization. The main effect of inclination is to induce fluidization heterogeneity. The local gas velocity increases in the adjacent region to the upper wall at the expense of the region adjacent to the lower wall. This situation anticipates the onset of local bubbling in the region adjacent to the upper wall. Meanwhile the region adjacent to the lower wall remains in a solid-like state and does not reach the fluid-like state until values of the gas flow are applied much higher than those needed in a vertical fluidized bed. As a consequence, the expansion and fluidization uniformity of the tilted bed are hindered. If the gas supply to the inclined bed is suddenly stopped, and because of induced heterogeneity, sedimentation takes place at a decreased rate as compared with sedimentation velocity in the uniformly fluidized vertical bed.     

Epoxy/steel fiber composites—A simple model to predict the fiber sedimentation

Sedimentation of short steel fibers (SSFs) is an important phenomenon observed in the manufacture by casting of polymer/metal composites. Modeling of the fiber sedimentation has been a subject of research but hardly applied in the context of injection molds. In this study, the sedimentation velocity of the SSF suspension in nonreactive epoxy resin was evaluated theoretically and experimentally. The sedimentation behavior of single and concentrated SSF in epoxy resin was followed experimentally to obtain the terminal and sedimentation velocities. These data were interpreted using theoretical models that take into account the hindered settling factor and the shape factor. The experimental data can be correlated with the theoretical analyses. The findings were used to propose a simple model to predict the sedimentation of steel fibers in viscous resins that are used in composites for making molding blocks. POLYM. COMPOS., 31:1378–1386, 2010. © 2009 Society of Plastics Engineers     

湿法闭路磨矿斜板分级机分级选择性的生产控制方法

分析提出磷矿石颗粒的垂直沉降速度与磷矿浆的垂直上升速度之间的关系是影响斜板分级机分级选择性的关键,据此找到控制斜板分级机分级选择性的方法,同时进行了相应生产试验。试验结果表明,使磷矿浆的垂直上升速度尽量接近欲分级磷矿石颗粒的垂直沉降速度,是控制斜板分级机分级选择性的有效方法。     

Modification of interparticle forces for nanoparticles using atomic layer deposition

Silica and titania nanoparticles were individually coated with ultrathin alumina films using atomic layer deposition (ALD) in a fluidized bed reactor. The effect of the coating on interparticle forces was studied. Coated particles showed increased interactions which impacted their flowability. This behavior was attributed to modifications of the Hamaker coefficient and the size of nanoparticles. Stronger interparticle forces translated into a larger mean aggregate size during fluidization, which increased the minimum fluidization velocity. A lower bed expansion was observed for coated particles due to enhanced interparticle forces that increased the cohesive strength of the bed. Increased cohesiveness of coated powders was also determined through angle of repose and Hausner index measurements. The dispersability of nanopowders was studied through sedimentation and z-potential analysis. The optimum dispersion conditions and isoelectric point of nanoparticle suspensions changed due to the surface modification. A novel atomic force microscope (AFM) technique was used to directly measure interactions between nanoparticles dispersed on a flat substrate and the tip of an AFM cantilever. Both Van der Waals and electrostatic interactions were detected during these measurements. Long and short range interactions were modified by the surface coating.     

双侧堰辐流式二沉池出水存在的问题及其机制分析

针对合肥市望塘污水处理厂辐流式二沉池集水渠附近有大量小絮凝体出现的情况,对集水渠附近的流速及沿池径方向SS指标进行了监测,分析了沉淀池的运行状况。结果表明,在辐流式二沉池的水面上,SS浓度最低的位置在距离池壁3.0～4.0 m之间;辐流式沉淀池进水流速较大,湍动能较大,不利于污泥沉降;沉淀池内存在明显的异重流,导致池壁处的上升流速较大,易将小絮凝体夹带至出水渠附近,导致出水效果不理想。     

东莞运河采用一级强化处理工艺参数的确定

东莞运河河水的处理采用一级强化混凝处理工艺.在设计和运行上,由于对化学混凝沉淀工艺未能深入的理解和掌握而造成了很大的失误;针对这一存在的问题,利用烧杯试验的原理,具体介绍了化学混凝沉淀工艺的最佳速度梯度值、反应时间、聚合碱式氯化铝铁的投药量以及最佳沉淀时间.     

厌氧反应器内颗粒污泥沉降速率的测定与研究

采用重量沉降法对上流式厌氧污泥床和厌氧颗粒污泥膨胀床生产装置中颗粒污泥的沉降速度进行了测定,将测定结果与沉降速度模型法计算的结果进行了对比,2种方法之间的误差不超过10%。因此认为重量沉降法与沉降速度模型法可作为生产实践中厌氧颗粒污泥沉降速度的确定方法。     

A Numerical Simulation of the Boycott Effect

The lattice Boltzmann method has been used to simulate the velocity field induced and the motion of an ensemble of particles during the sedimentation process in inclined tubes. The simulations show the trajectories and flow behavior of individual particles and particle-particle and particle-wall interactions as well as the formation of particle clusters. The global convection motion that was experimentally observed during such processes and tends to enhance the sedimentation process is also reproduced numerically. In addition we have found that smaller intermittent vortices, formed from the wakes of groups of settling particles, also play an important role in the sedimentation process and the final distribution of particles.     

A Numerical Simulation of the Boycott Effect

The lattice Boltzmann method has been used to simulate the velocity field induced and the motion of an ensemble of particles during the sedimentation process in inclined tubes. The simulations show the trajectories and flow behavior of individual particles and particle-particle and particle-wall interactions as well as the formation of particle clusters. The global convection motion that was experimentally observed during such processes and tends to enhance the sedimentation process is also reproduced numerically. In addition we have found that smaller intermittent vortices, formed from the wakes of groups of settling particles, also play an important role in the sedimentation process and the final distribution of particles.     

Continuous sedimentation theory: Effects of density gradients and velocity profiles on sedimentation efficiency

A theory is presented on continuous sedimentation. In case the solids concentration is small and uniformly distributed over the inlet height, the theory predicts independent sedimentation efficiencies on velocity distributions in a longitudinal vertical plane. A velocity profile in a horizontal plane on the other hand will have a negative effect on the efficiency. Experiments, carried out on a laboratory-scale model, have shown that even smal density differences in the basin can have a significant effect on the velocity distribution. The measured efficiencies are in good agreement with the theory     

Simulation of sedimentation and fluidization of polydisperse suspensions via a Markov model

The particle-based approach to sedimentation is extended to include velocity fluctuations that result in hydrodynamic diffusion. The vector process describing the joint values of position and velocity is Markov. Thus, no integration of velocity is required. Height-velocity “skeletons” for each particle are generated from a bivariate-normal distribution with means, variances, and covariance that depend on three parameters. For each particle, there is a unique region in which the vector of species concentrations determines that particle's parameters and hence its Markov process, but the concentrations in that region depend on the Markov processes of neighboring particles. Though only discrete values of height and velocity are generated, the model ensures that sample paths and particle velocities are continuous. Furthermore, steady-state velocities are normally distributed and velocity autocorrelations decay exponentially. Published experimental results indicate that both are excellent approximations. For polydisperse suspensions, the Markov model is much simpler than the standard hydrodynamic-diffusion model and represents the actual process much better. We simulate the sedimentation and fluidization of polydisperse suspensions and study the effects of two additional parameters: variance and autocorrelation decay rate of particle velocities.     

The effect of brownian motion on particle size analysis by sedimentation

The influence of Brownian motion on the results of particle size measurements obtained by gravity sedimentation methods has been investigated by mathematical simulation of the process. It is shown that the effects can be directly correlated with an effective mean Péclet number (which expresses the ratio of settling velocity to diffusive velocity) for the particular system. For very small Péclet numbers, diffusion effects dominate in all cases and sedimentation methods should not be used. For larger values of the Péclet number, the errors introduced by Brownian motion are most important for very narrow and very broad distributions. In general, the effects can be reduced by increasing the sedimentation height, but it must be remembered that this will lead to increased settling times and may increase errors due to other factors such as thermal convection.     

On the “Rag Layer” and Diluted Bitumen Froth Dewatering

Samples of rag layer were collected using multiple bench‐top, batch centrifugations with optical microslides inserted to a centrifuge cell in the last spinning. Image analysis of the samples provided data for approximate sedimentation rate analysis, based on fractal floc dimension and Smoluchowski coagulation kinetic equation. The analysis shows that, if a flocculating chemical is used as a demulsifier, overdosing will necessarily result in deterioration of the process performance. Optimization of the chemical dosage and accurate process control are thus necessary conditions to attain satisfactory dewatering process performance     

The Presence of Adsorbed Proteins on Particles Increases Aggregated Particle Sedimentation,as Measured by a Light Scattering Technique

Protein conformational changes are often induced when bound to surfaces and can modulate colloidal stability of protein coated particles in dispersion. We evaluated bovine serum albumin (BSA) adsorbed onto polystyrene particles at room temperature. A z-axis translating laser light scattering device (ZATLLS) measured the sedimentation velocity of protein-coated particles tracking aggregation characteristics compared with non-coated ones. Sedimentation velocities of particles moving in the dispersion, and the resulting viscosity and density of the residual solution following sedimentation determined aggregate size in the dispersion using Stoke's law. Our experiments objectively show that albumin-coated polystyrene forms aggregates. Interestingly, coating particles with protein slows the sedimentation velocity which should correspond to a more dispersed system, but it leads to higher aggregate sizes due to the larger influence of proteins in solution raising solution viscosity. Protein-bound particles were observed to fall out of solution in a more controlled and steady manner compared with uncoated particles.     

Effects of polyvinylpyrrolidone and carbon nanotubes on magnetorheological properties of iron‐based magnetorheological fluids

Magnetorheological (MR) fluids based on glycol, iron powder, polyvinylpyrrolidone (PVP), and carbon nanotubes (CNTs) were prepared. Effects of polyvinylpyrrolidone and carbon nanotubes on sedimentation stability and magnetorheological properties were studied. It is found that the synergetic effects of PVP and CNTs improve the sedimentation stability significantly, and the addition of CNTs reduces the sedimentation velocity and increases the equilibrium sedimentation ratio of the magnetizable particles in MR fluids remarkably. The addition of PVP can reduce the sedimentation velocity of the magnetizable particles, but cannot increase the equilibrium sedimentation ratio and will not change the up trend of apparent viscosity with the increasing intensity of the external magnetic field. When the PVP content is lower, the increment of original apparent viscosity of the MR fluids at zero‐intensity of magnetic field is inconspicuous, and their values of apparent viscosity under magnetic field are similar. However, the apparent viscosity of the MR fluids increases tremendously when the contents of PVP increase to certain degree. The results show that the synergetic effects of PVP and CNTs not only improve the sedimentation stability of the MR fluid but also promote its magnetorheological effect. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1653–1657, 2006