柱状颗粒沉降过程的CFD-DEM联合仿真和实验研究

针对柱状催化剂颗粒相对于球形颗粒的不同运动特性,选择不同长度直径为2 mm的5种柱状颗粒,采用CFD-DEM数值模拟仿真,研究柱状颗粒在管状容器中沉降的运动行为,并建立柱状颗粒沉降试验台,采用高速摄像拍摄的方法进行实验研究。结果表明,在不同位置释放相同直径和长度的柱状颗粒时,靠近壁面释放的颗粒会在沉降过程中向中心漂移,且比中心释放的颗粒沉降更慢,时间更长;改变柱状颗粒与水平面的夹角,在圆管中心释放颗粒,最终颗粒都会旋转至水平状态,与水平面夹角越大,底部所受阻力越大,转动持续时间随之增加;推导柱状颗粒沉降斯托克斯方程,并通过实验数据对方程中的阻力系数进行修正,将修正后的阻力系数导入用户自定义函数(UDF)计算颗粒沉降末速度,相对误差从原来使用球形颗粒阻力系数的50%下降到17%以内,模拟较为可靠。     

Numerical modelling of filtration with and without sedimentation runs

From experimental batch runs of filtration with sedimentation above the cake formed, a procedure is proposed and applied to experimental data for obtaining the relationships between the settling solids velocity in the hindered settling zone, the local specific resistance and the effective pressure and the solids concentration. Considering the material and momentum balances in the hindered settling zone above the cake, in the cake and in the filter membrane, a simulation program was obtained that satisfactorily reproduces the experimental runs of filtration with sedimentation runs. This simulation work confirms the hypothesis considered in the theory of filtration with sedimentation runs, e.g. the fact that the characteristic lines arise tangently from the cake surface. In addition, the method developed for obtaining the different relationships considered has been demonstrated as correct. The simulation of filtration without sedimentation runs (stirring the suspension above the cake) has also been studied.     

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。     

Analysis of coal-oil mixture sedimentation by modified continuity wave theory

Coal-oil mixtures which have the potential to stretch oil usages have been analyzed as to their sedimentation behavior by applying a modification of the continuity wave theory as developed by Kynch. Three coal-oil systems have been studied and have been shown to follow the pseudo continuity wave theory by assigning a degree of agglomeration to the coal particles. With the presence of surfactants, the sedimentation of the mixtures could be similarly analyzed. The general effects of particle size of the coal and temperature of the system have been considered and presented as guidelines for design.     

黄磷受磷槽两相流场数值模拟

以计算流体力学（CFD）技术为基础,对受磷槽内黄磷沉淀及液液两相流动进行数值模拟。模拟结果表明,受磷槽内水相流动状态十分复杂,且流动趋势不受进口流量的影响。黄磷累积量随进口量改变而改变,但黄磷流动速度变化较小。欧拉多相流及k-ε湍流模型能够较好地描述受磷槽内黄磷分布状态及流体流动状态,模拟结果具有较高的可靠性。     

Numerical simulation of secondary sedimentation tank for urban wastewater

Secondary sedimentation tanks, which are used widely in wastewater treatment, require optimization to improve wastewater treatment efficiency. The flow dynamics is an important parameter for the design of reactors. However, very little literature deals with the flow in secondary sedimentation tanks. Therefore, the flow dynamics in a secondary sedimentation tank is studied in this paper. The solid–liquid two-phase turbulent flow in the tank is modeled with the three-dimensional two-fluid model. Using computational fluid dynamics (CFD), the velocity profile and distribution of solid concentration are given. The use of different baffles in the same tank is also simulated. With the inclusion of the baffle, the distribution of the solid concentration differs greatly. The significant effects of the baffle height and baffle location on the distribution of solid concentrations are also discussed. These results provide useful information for the design of secondary sedimentation tanks.     

Stochastic sedimentation and hydrodynamic diffusion

Molecular collisions with very small particles induce Brownian motion. Consequently, such particles exhibit classical diffusion during their sedimentation. However, identical particles too large to be affected by Brownian motion also change their relative positions. This phenomenon is called hydrodynamic diffusion. Long before this term was coined, the variability of individual particle trajectories had been recognized and a stochastic model had been formulated. In general, stochastic and diffusion approaches are formally equivalent. The convective and diffusive terms in a diffusion equation correspond formally to the drift and diffusion terms of a Fokker–Planck equation (FPE). This FPE can be cast in the form of a stochastic differential equation (SDE) that is much easier to solve numerically. The solution of the associated SDE, via a large number of stochastic paths, yields the solution of the original equation. The three-parameter Markov model, formulated a decade before hydrodynamic diffusion became fashionable, describes one-dimensional sedimentation as a simple SDE for the velocity process {V(t)}. It predicts correctly that the steady-state distribution of particle velocities is Gaussian and that the autocorrelation of velocities decays exponentially. The corresponding position process {X(t)} is not Markov, but the bivariate process {X(t), V(t)} is both Gaussian and Markov. The SDE pair yields continuous velocities and sample paths. The other approach does not use the diffusion process corresponding to the FPE for the three-parameter model; rather, it uses an analogy to Fickian diffusion of molecules. By focusing on velocity rather than position, the stochastic model has several advantages. It subsumes Kynch’s theory as a first approximation, but corresponds to the reality that particle velocities are, in fact, continuous. It also profits from powerful theorems about stochastic processes in general and Markov processes in particular. It allows transient phenomena to be modeled by using parameters determined from the steady-state. It is very simple and efficient to simulate, but the three parameters must be determined experimentally or computationally. Relevant data are still sparse, but recent experimental and computational work is beginning to determine values of the three parameters and even the additional two parameters needed to simulate three-dimensional motion. If the dependence of the parameters on solids concentration is known, this model can simulate the sedimentation of the entire slurry, including the packed bed and the slurry–supernate interface. Simulations using half a million particles are already feasible with a desktop computer.     

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.     

Study on the mechanism of hindered sedimentation by ‘unified theory on solid-liquid separation’

The entire procedure of hindered sedimentation has been calculated by the introduction of new boundary conditions to the ‘unified theory on solid-liquid separation.’ During this study, the hindered sedimentation was defined as ‘a process of sedimentation with transmission of the gravitational force between the particles due to contact.’ The lower concentration limit, on which the ‘unified theory on solid-liquid separation’ is based, could not be applied. To understand the mechanism of hindered sedimentation, variations in the porosity during sedimentation were calculated by using our theory.     

Sedimentation from aerosol flows in finite tubes in random orientation

The sedimentation from aerosol flows in a system of finite tubes in random orientation has been further studied including the case of large sedimentation velocity. The penetration, retreat and deposition efficiencies are obtained for various length-radius ratios of the tubes and various ratios of sedimentation to convection velocities.     

A new method for the control of dilute suspension sedimentation by horizontal movement

This research addresses some factors that control the stability of dilute suspensions in sedimentation processes under a dynamic environment. Experimental and numerical studies were conducted about the sedimentation velocity control of dilute suspensions by horizontal movement for particle concentrations up to 8 wt.%. Nearly monodispersed particles were used as test particles. The effects of horizontal movement speed and amplitude on particle sedimentation process were investigated. Under stationary conditions, particles settle in only one vertical direction because of gravitational force. However, complicated particle motions arise under moving conditions due to circulation flow in horizontal moving conditions. The results show that horizontal movement can reduce the particle settling velocity or maintain the stability of the suspension.     

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.     

The influence of precipitated asphaltenes on sedimentation of particles in coal liquids

Antisolvent addition to coal liquids causes the precipitation of asphaltenes on mineral particles and increases their sedimentation rate. This paper develops a mathematical model of the sedimentation rate of a single particle or agglomerate of particles. It was found that if the difference in density between the solid and the precipitate is more than twice the difference in density between the precipite and the liquid, the sedimentation rate would decrease initially upon precipitation. A simple experiment was performed to confirm the prediction. Further it was found that the agglomeration of particles had a stronger effect upon the sedimentation than the changes in particle size.     

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.     

Influence of aggregates on the sedimentation properties of welan gum

Summary The water-soluble bacterial polysaccharides are used by industry as emulsifying, stabilizing, thickening and suspending agents, and eventually for their gelling properties. It has been observed that several of these properties may be influenced by the presence of disordered intermolecular associations (aggregates or microgels). The influence of aggregates on the sedimentation properties of welan gum were investigated by means of the falling sphere method. The velocity of the particle sedimentation is affected by the type and size of the aggregates.     

Rheology and sedimentation velocity of alkaline suspensions of hematite particles at elevated temperature

This study aims to characterize the sedimentation velocity and the rheology of suspensions of hematite particles suspended in strongly alkaline media at 100 and 110 °C, as done for an alternative electrochemical process in development for iron production by direct electrode reduction of hematite. Considering the medium used in the process, i.e. 12% (v/v) suspension of hematite particles in 50% sodium hydroxide aqueous, the sedimentation velocity of hematite particle at 110 °C is 0.010 mm/s, which is very slow because the average size of the solid particles is around 10 μm and the significant collisions and interactions occuring between the particles in the concentrated suspension. Two geometries were used to characterize the rheological behavior of the apparent viscosity of the suspension of 12% (v/v) (i.e. 33 wt%) at 100 °C: a conventional Couette geometry and a helical ribbon mixer. The suspension was found shear thinning in the range of shear rate studied. The rheological behavior of the suspension can be described by a power-law model. The apparent viscosity of the hematite suspension estimated at a shear rate between 0.5 and 10 s⁻¹ is between 100 and 20 mPa s for the two geometries. The apparent viscosity calculated from the terminal velocity of 10 μm particles is of the same order of magnitude of the results obtained with the two rheometer configurations. The effect of the particle concentration on the sedimentation velocity and viscosity of the hematite suspensions was also studied.     

Applications of the phenomenological theory to several published experimental cases of sedimentation processes

In one space dimension, the phenomenological theory of sedimentation predicts the sedimentation–consolidation behavior of a flocculated suspension in dependence of two constitutive functions describing its material behavior, the solids flux density (or hindered settling function) and the solid effective stress. These functions are assumed to depend only on the local volumetric solids concentration. In this contribution, we review several experimental and theoretical studies of sedimentation in settling columns. We first resume the theories that have been employed to interpret the experimental measurements and then apply the phenomenological model to the available data. The two constitutive functions involved are determined from the published concentration, permeability and effective stress data. The mathematical model is then solved numerically using these functions, and the resulting predictions of settling behavior are compared with the respective authors’ experimental findings and interpretations. In one case, the information obtained from a batch settling experiment is used to simulate continuous sedimentation.     

Continuous sedimentation of a suspension with a nonconvex flux law

The continuous thickening of a solid-liquid suspension by gravity sedimentation is analyzed as an initial-boundary value problem of a nonlinear conservation equation. The concept of a limiting flux, commonly employed in steady state design, emerges as an intrinsic nonlinear phenomenon induced by nonuniform initial conditions and the instability of a certain class of discontinuous solutions. A limiting flux does not obtain for some uniform initial states; instead, the solids concentration follows a hysterisis path as the applied flux increases and then decreases. The model also shows that shock discontinuities caused by the interaction of rarefaction and plane waves dominate the transient response of a thickening column perturbed from its steady state by simultaneously decreasing the applied flux and underflow rate.     

A study of the rate of sedimentation of magnesium hydroxide obtained from sea water

A study has been made of the influence of incomplete precipitation on the rate of sedimentation of magnesium hydroxide obtained from sea water with the use of the following flocculants: Flocculant-B (polyacrylamide) and slag from Fe—Cr of a high carbon content. Incomplete precipitation of magnesium ions from sea water considerably affected the rate of sedimentation of magnesium hydroxide. The rate of sedimentation increased considerably and thus the capacity of the thickener was also increased. At a precipitation of 80%, the capacity of the thickener increased by 86.5% in relation to complete precipitation. Calculations were made according to Kynch's theory. The above procedure leads to a substantial reduction in the content of calcium salt in the product, which can be explained by the law of adsorption, as set out by Paneth, Fajans and Hahn. For optimal conditions of sedimentation, a mathematical expression of the dependence of the quantity of the flocculant, ‘Flocculant-B’ on the degree of completeness of precipitation was determined.