搅拌槽中固体颗粒离底悬浮机理的研究

The minimum fluid velocity to maintain particles just suspended was deduced, and the theoretical analysis shows that the minimum velocity is influenced by the properties of the solid and liquid, not by the operational conditions. For justification, the local minimum velocity at the bottom of the tank was measured by a bi- electrode conductivity probe, in a square-sectioned stirred tank (0.75 m×0.75m×1.0m) with the glass beads-water system. The experiments showed that the fluid velocities for the same suspension state were identical despite that the power dissipated per unit mass was not the same under different configuration and operation. Both theoretical analysis and experimental results indicate that the off-bottom suspension is controlled by the local fluid flow over the bottom of the stirred tank.     

ON THE EFFECTIVE CONDUCTIVITY OF A DILUTE SUSPENSION OF SPHERICAL DROPS IN THE LIMIT OF LOW PARTICLE PECLET NUMBER

In this paper we consider the effective conductivity of a dilute suspension of neutrally bouyant spherical drops which is undergoing a simple shear flow. The thermal conductivity, viscosity and specific heat capacity of the drops are assumed to be different from those of the suspending fluid, though it is assumed that the local Peclet and Reynolds numbers are small both inside and outside the drop. The analysis consists of three parts: a derivation of the relationship between bulk heat flux on the one hand and the thermal and momentum fields at the microscale of the suspended particles on the other; a calculation of the local temperature field near a single neutrally buoyant spherical drop in shear flow with an imposed transverse temperature gradient at large distances; and a synthesis of the general relationship for bulk heat flux and the calculated local temperature field to determine an effective conductivity for a dilute suspension of spherical drops.     

Particle motion at the wall of a circulating fluidized bed

The motion of alumina particles of mean size 74.9 μm in the region near to the wall of the 305 mm diameter riser of a cold model circulating fluidized bed has been studied using a high-speed video camera employing normal and magnifying lenses. Particles in this region were found to move predominantly downwards, against the main gas flow. High density groups or swarms of particles typically arch-shaped were observed to descend in contact with the wall at velocities in the range 0.3–0.4 m s⁻¹. Tfie distribution of swarm descent velocities was shown to be little affected by changes in superficial gas velocity over the range 3–5 m s⁻¹ and imposed mean solids mass flux over the range 2 to 80 kg m⁻² s⁻¹. A region of steady bulk downflow of solids with a velocity of approximately 1.0 m s⁻¹ was observed to appear a few millimetres from the wall at mean suspension densities greater than 5.6 kg m⁻³. Motion of particles in contact with the riser was analysed by identification of three flow forms; dilute, dense and swarm flow. Results of the analysis are linked with the observations of other workers concerning the onset of the so-called ‘similar profiles’ regime. The relationship between the measured effective particle swarm length and the cross-sectional mean suspension density was established and the implications for modelling suspension-to-wall heat transfer discussed.     

Design and performance evaluation of hydrocyclones for removing micron particles suspended in water

A hydrocyclone for collecting micron-sized hydrosols efficiently has been studied through experimentation. Hydrocyclones separate particles of the dispersed phase from the liquid on the basis of the density difference between the phases. The purpose of this study was to design and evaluate the performance of a high-capacity hydrocyclone for the removal of submicron-sized particles suspended in liquid. Furthermore, the performance of three types of hydrocyclone was evaluated with regard to solid particle density using fly ash and coagulation sludge. The particle cut-size decreases with reducing inlet area and increasing inlet velocity in the hydrocyclone. The hydrocyclones have good performance, which is demonstrated by the optimal cut-size of 20 μm in mass median diameter at the inlet diameter per body diameter ratio of 0.21 and the pressure drop of 72.5 kPa with a particle density of 2,500 kg/m³.     

Motions of a fluid drop in linear and quadratic flows

A theoretical analysis is presented of the flow field near a spherical fluid drop immersed in an incompressible Newtonian fluid which, at large distances from the drop, is undergoing an undisturbed flow. The undisturbed flows considered here are relevant to studies of drop motions near a phase boundary, and to some aspects of the coalescence of liquid drops. Exacl solutions in closed form have been found using the harmonic function expansion in spherical coordinates. Calculation of the hydrodynamic force on the drop leads to a generalization of Faxen’s law to afluid particle in anarbitrary undisturbed creeping-flow. The solutions are then expressed in terms of she fundamental singularity solutions for Stokes flow in anticipation of future analysis of the drop coalescence. In addition, the deformed shapes are determined for a fluid drop freely suspended in an axisymmetric Poiseuillian flow.     

基于煤粉分层流动的水平管压降模型

基于质量守恒,研究了水平管分层流动不同条件下煤粉在悬浮层和滑动层的分布情况,建立了水平管中分层流动模型,并利用该模型计算了水平管压降,分析了悬浮层和滑动层对总压降的影响,计算结果与实验结果吻合较好。     

细小尺度下潜热型功能热流体压降与传热特性

实验研究了相变微胶囊颗粒(囊芯材料为正十六烷,壳材为尿素-甲醛树脂)和去离子水混合制成的潜热型功能热流体流过等热流细小圆管的流动与传热特性,同时以去离子水作为传热工质在相同条件下进行了对比实验。得到了压降随质量流量的变化规律,实验段出、入口温度以及量纲1出口温度随Reynolds数变化规律,量纲1壁面温度沿轴向的分布规律,平均Nusselt数随Reynolds数的变化关系。结果表明,相变微胶囊颗粒的加入会导致流动压降增大,但随着流量增加,流动压降逐渐与单相液体的接近;出口温度及壁面温度要比相同条件下单相液体的低;含有较小相变微胶囊颗粒浓度的潜热型功能热流体的平均Nusselt数是相同条件下单相液体的2.0～4.0倍。     

Prediction of pressure drop with steady state pneumatic conveying of solids in horizontal pipes

Current relationships for determination of the pressure drop with pneumatic conveying of solids in pipes are not of general validity. The theoretical considerations underlying these relationships do not take into account the influence of the rotatory motion of the particles. On the other hand, extremely high rotatory speeds of the particles due to wall collision are observed.Therefore a new concept is presented which takes into account the rotatory motion of the particles. A further aim was to represent the data in the form of nondimensional groups which allow meaningful, physical interpretation of the results obtained.A state diagram for the prediction of pressure drop with pneumatic conveying in the form of sliding particles strands is described.Calculation of power loss per particle and of the force and the moment acting on a particle leads to a nondimensional representation of pressure drop in which a normalized pressure drop is combined with a particle Froude number and a Froude number which contains the particle fall velocity and the pipe diameter. This combination of nondimensional groups defines fully suspended flow.The normalized pressure drop is defined in such a way that it represents the nondimensional slip of the particles, too.Comparison with pressure drop measurements for pneumatic transport in horizontal pipes which included changes in particle size, particles density and pipe diameter confirms the physical significance of the parameters used with regard to the prediction of pressure drop and of particles slip velocity.A simple procedure for the prediction of pressure drop in the full range of steady-state pneumatic conveying is proposed.     

Growth of electrodeposited γ-manganese dioxide from a suspension bath

The addition of solid particles of manganese oxides to the electrolyte of a manganese dioxide production bath (suspension bath process=SBP) allows the anodic current density to be increased to values more than twice that used in the conventional EMD process. This is possible even with the use of titanium anodes which are sensitive to passivation if the current density exceeds critical values. Analysis of deposition conditions and properties of the SBP-EMD shows that the SBP-EMD resembles and EMD which has been deposited at very low current densities. These low current densities are realized by the adsorption of the suspended particles on the anode surface, thereby promoting dendritic crystal growth on the adsorbed particles and increasing the real anode surface by up to a factor of ten. This assumption is able to explain most of the properties found for the SBP-EMD, e.g. X-ray pattern, potential drop, hardness of the deposit, low specific surface area, low content of combined water and high electronic conductivity.     

Measurement of Solids Distribution in Suspension Flows using Electrical Resistance Tomography

Neutrally buoyant particles in low Reynolds number, pressure‐driven suspension flows migrate from regions of high to low shear. When the particle density differs from the suspending fluid, buoyancy forces affect this particle migration. The ratio between the buoyancy and viscous forces, as quantified by a dimensionless buoyancy number, determines the phase distribution of the fully developed suspension. Electrical resistance tomography (ERT) was used to visualize and quantify particle migration in low Reynolds number pressure‐driven pipe flows of dense and light particles suspended in a viscous fluid. The measured phase distributions are compared to the predictions of a modified suspension balance model.     

搅拌釜内固-液悬浮体系的CFD-DEM模拟(英文)

Computational fluid dynamics-discrete element method （CFD-DEM） coupled approach was employed to simulate the solid suspension behavior in a Rushton stirred tank with consideration of transitional and rotational motions of millions of particles with complex interactions with liquid and the rotating impeller. The simulations were satisfactorily validated with experimental data in literature in terms of measured particle velocities in the tank. Influences of operating conditions and physical properties of particles （i.e., particle diameter and density） on the two-phase flow field in the stirred tank involving particle distribution, particle velocity and vortex were studied. The wide distribution of particle angular velocity ranging from 0 to 105 r·min-1 is revealed. The Magnus force is comparable to the drag force during the particle movement in the tank. The strong particle rotation will generate extra shear force on the particles so that the particle morphology may be affected, especially in the bio-/polymer-product related processes. It can be concluded that the CFD-DEM coupled approach provides a theoretical way to under-stand the physics of particle movement in micro-to macro-scales in the solid suspension of a stirred tank.     

圆管中中性悬浮液流动阻力和流变性的实验测定

对高浓度的液固悬浮液在圆管中的流变特性进行了实验研究,实验体系为聚苯乙烯颗粒在NaCl水溶液中的中性悬浮液。测定了在层流状态下,固体颗粒的体积分数变化（20%～50%）、悬浮液流速变化（0.031～0.22 m•s^-1）以及颗粒粒度变化对悬浮液压降的影响规律。实验结果表明,固体颗粒的浓度会影响悬浮液的流变性质,颗粒粒径对悬浮液流变性影响微弱。悬浮液的压降随颗粒体积分数和流速的增大而增大,悬浮液的流动特性在较高颗粒浓度范围内符合幂率流体模型。     

Flow of a gas-solid two-phase mixture through a packed bed

This paper is concerned with an upward co-current flow of a gas-solid two-phase mixture through a packed bed, a system employed in a number of industrial processes. Experimental work was carried out by using glass balls for packed bed, and both glass beads and FCC as suspended particles. The effects of solids loading and gas velocity on the pressure drop as well as the static and dynamic solid hold-ups within packed bed were examined. Experimental results showed different behaviour of the FCC from glass beads. At a given gas velocity, pressure drop increased approximately linearly with solids loading with a slope for FCC much higher than that for glass beads. The static hold-up of glass beads was much lower than corresponding dynamic hold-up at a given gas velocity, and it did not seem to change much with solids loading under the conditions of this work. At a given gas velocity, the static hold-up of FCC, however, was found to be comparable with the corresponding dynamic hold-up. An analysis was conducted on the pressure drop using a modified version of the Ergun equation by taking into account the effects of suspended particles on the viscosity and density, as well as the gravitational force. It was found that the modified Ergun equation agreed well with the experimental results of both this work and those reported in the literature. Effort was also made to develop relationships for the dynamic hold-up and the interaction coefficient between the suspended and the packed particles, the so-called solid-phase friction factor in the literature. The dynamic hold-up was found to increase with an increase in the product of velocity ratio of the solid to gas phases and the square root of the diameter ratio of the suspended to packed particles, whereas the interaction coefficient increased in general with increasing Froude number but with significant scattering.     

Two-dimensional transient numerical simulation of solids and gas flow in the riser section of a circulating fluidized bed

A computational fluid dynamics software (CFX) was modified for gas/particle flow systems and used to predict the flow parameters in the riser section of a circulating fluidized bed (CFB). Fluid Catalytic Cracking (FCC) particles and air were used as the solids and gas phases, respectively. Two-dimensional, transient, isothermal flows were simulated for the continuous phase (air) and the dispersed phase (solid particles). Conservation equations of mass and momentum for each phase were solved using the finite volume numerical technique. Two-dimensional gas and particle flow profiles were obtained for the velocity, volume fraction, and pressure drop for each phase. Calculations showed that the inlet and exit conditions play a significant role in the overall mixing of the gas and particulate phases and in the establishment of the flow regime. The flow behavior was analyzed based on the different frequency of oscillations in the riser. Comparison of the calculated solids mass flux, solids density and pressure drop with the measured pilot-scale PSRI data (reported in this paper) showed a good agreement.     

新型自吸式多相搅拌槽临界悬浮特性的实验研究

在不通气的情况下,对新型多相搅拌槽进行实验,得出了其固相含量w、颗粒密度r、搅拌槽内径D与临界悬浮转速Nc的关系,在相同条件下,与给定结构尺寸的标准搅拌槽对比了临界悬浮转速. 结果表明,Nc随固相分率j增加和粒径dp增大而增大,不利于达到良好的悬浮效果;固相颗粒密度越大,临界悬浮转速随之增大;槽径增大使达到悬浮状态的临界转速降低. 该新型搅拌槽临界悬浮转速的关联式是Nc=6.3dp0.21(gDr/rL)0.45w0.19/D0.55;同一固相分率下,新型多相搅拌槽的临界悬浮转速比标准搅拌槽降低了30%,在较低转速下就能使颗粒悬浮.     

Partikeltrennung in laminaren Rohrströmungen mit nachgeschalteter Strömungsaufweitung

The classification of particles with minor or negligible density difference to the fluid, for example separation of cells from a cell suspension, or clots from polymers is feasible by laminar flow in a circular and a downstream tube expansion containing the collector tube for the large particles. Experiments have been performed in a laboratory equipment. A numerical simulation of the particles trajectories following the stream‐lines in the downstream expansion was executed.     

An analysis of pressure drop fluctuation in a circulating fluidized bed

The characteristics of pressure drop fluctuation in a 5.0 cm I.D.×250cm high circulating fluidized bed with fine polymer particles of PE and PVC were investigated. The measurements of time series of the pressure drop were carried out along the three different axial locations. To determine the effects of coarse particles and relative humidity of air on the flow behavior of polymer powders-air suspension in the riser, we employed deterministic chaos analysis of the Hurst exponent, correlation dimension and phase space trajectories as well as classical methods such as standard deviation, probability density function of pressure drop fluctuation. From a statistical and chaos analysis of pressure fluctuations, the upper dilute region was found to be much more homogenous flow compared to that in the bottom dense region at the same operating conditions. It was also found that the addition of coarse particles and higher humidity of air reduced the pressure fluctuations, thus enhancing flow stability in the riser. The analysis of pressure fluctuations by statistical and chaos theory gave qualitative and the quantitative information of flow behavior in the circulating fluidized bed.     

Zur Bewegung feiner Partikeln,die in turbulent strömenden Medien suspendiert sind

Motion of fine grained particles, suspended in turbulent flow . This article considers the motion of particles, suspended in turbulent flow. If the particles are sufficiently small to respond to turbulence, their motion includes stochastic components. Concerning processes like air classification or separation of fine powders the stochastic contribution – characterized by the conception of a particle diffusivity – the particle motion exhibits a detrimental influence. Sharpness of cut and separation efficiency are reduced. The paper aims to present the state of the art in particle diffusion. First, theoretical investigations are reported, attention being focused on the equation of motion of the particle which is the link between the motion of the fluid and the motion of the particle. Then, experimental results are reviewed. The following tendencies can be seen: Particles which response to turbulence of fluid flow show increasing diffusivity with increasing inertia. Field forces like gravity or electrical field forces exhibit a damping effect on diffusivity.     

The use of a mixing-sensitive chemical reaction for the study of pulp fibre suspension mixing

The competitive, consecutive chemical reactions between 1-naphthol and diazotized sulfanilic acid were utilized to study the mixing of a pulp fibre suspension in a 22 L stirred tank reactor. Mixing quality was determined from the distribution between the mono and bis substituted reaction products once a correlation was made for the adsorption of the product dyes onto the suspended fibres. The technique was found to be adequate for assessing micromixing and turbulence intensity within a fibre suspension provided the measured product distribution, Xs, was between 0.4 and 0.01. Thus the mixing conditions that could be assessed depended on both the energy dissipation within the mixer and the amount of the fibre present. For the experimental conditions chosen for this study energy dissipation rates would typically have to be less than 80 W/kg and the suspension mass concentration less than 2.5%. When compared with water, a reduction in turbulence levels at both the impeller zone and a remote zone in the stirred vessel was observed for fibre mass concentrations as low as 0.5%. The turbulence decreased as the suspension mass concentration was increased. This decrease is attributed to energy dissipation by friction at fibre-fibre contact points as the fibres move relative to one another in the flow. This sink removes energy from the turbulence cascade which never shows up as small-scale fluid deformations leading to better mixing.     

Flow of particles suspended in a sheared viscous fluid: Effects of finite inertia and inelastic collisions

We investigate in this article the macroscopic behavior of sheared suspensions of spherical particles. The effects of the fluid inertia, the Brownian diffusion, and the gravity are neglected. We highlight the influence of the solid‐phase inertia on the macroscopic behavior of the suspension, considering moderate to high Stokes numbers. Typically, this study is concerned with solid particles O (100 μm) suspended in a gas with a concentration varying from 5% to 30%. A hard‐sphere collision model (with elastic or inelasic rebounds) coupled with the particle Lagrangian tracking is used to simulate the suspension dynamics in an unbounded periodic domain. We first consider the behavior of the suspension with perfect elastic collisions. The suspension properties reveal a strong dependence on the particle inertia and concentration. Increasing the Stokes number from 1 to 10 induces an enhancement of the particle agitation by three orders of magnitude and an evolution of the probability density function of the fluctuating velocity from a highly peaked (close to the Dirac function) to a Maxwellian shape. This sharp transition in the velocity distribution function is related to the time scale which controls the overall dynamics of the suspension flow. The particle relaxation (resp. collision) time scale dominates the particulate phase behavior in the weakly (resp. highly) agitated suspensions. The numerical results are compared with the prediction of two statistical models based on the kinetic theory for granular flows adapted to moderately inertial regimes. The suspensions have a Newtonian behavior when they are highly agitated similarly to rapid granular flows. However, the stress tensors are highly anisotropic in weakly agitated suspensions as a difference of normal stresses arises. Finally, we discuss the effect of energy dissipation due to inelastic collisions on the statistical quantities. We also tested the influence of a simple modeling of local hydrodynamic interactions during the collision by using a restitution coefficient which depends on the local impact velocities. © 2010 American Institute of Chemical Engineers AIChE J, 2010