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1.
Nuclear magnetic resonance (NMR) imaging results are presented for the comparative study of sedimentation of the polymethylmethacrylate colloidal suspensions with spherical particle diameters of 475, 350 and 255 nm. The time evolution of the particle volume fraction in the sedimenting system, velocity of the fluid/suspension interface, interface broadening, and sediment growth velocity are measured against the system concentration as well as the container geometry. Using the experimental data the hindered settling function is evaluated. The influence of the container geometry on the parameters defining the hindered settling function in different approaches is evaluated for the first time. The limiting value of the particle diameter is estimated at which the container geometry has no impact any longer. This effect can be explained by the microscale structure persisting at low Peclet numbers. In addition, the influence of the container geometry on interface broadening and sediment formation for different particle diameters and volume fractions is studied. Spontaneous sediment packing induced by a modified container geometry has been found. NMR imaging has proven to be a highly efficient research tool for studying sedimentation at low Peclet numbers. 相似文献
2.
The gravitational settling of inhomogeneously suspended particles in a fluid has been investigated. Of particular interest
is whether collective or individual motion of particles is dominant during their settlings, i.e., whether the particles settle as a continuous suspension or they settle individually relative to the surrounding fluid. We
observed the settling of a stratified suspension which has the lower and upper concentration interfaces in a quasi-two-dimensional
vessel. In some cases, the suspension behaves perfectly as a continuous fluid and the motion of the constituent particle is
subject to bulk flow caused by the interfacial instability. In other cases, the particle behaves individually relative to
the surrounding fluid. The existence of a concentration interface plays a significant role in these extreme behaviors of suspension.
The transition from the collective to individual behaviors can be predicted quantitatively by a parameter which expresses
the border resolution of the concentration interface. 相似文献
3.
Sedimentation has played a key role in the development of colloid science. In fact, it is because of the celebrated experiments by Perrin, yielding a concrete demonstration of molecular reality and giving strong support to Einstein's theory of Brownian motion, that colloids enter the realm of basic physics. Subsequent investigations have shown that a lot more can be learnt both from sedimentation equilibrium and from particle settling dynamics. These advances, together with new experimental approaches, will be reviewed in this paper. Yet, we shall also show that inquiring about gravity settling is far from being a closed matter: for instance, the concept of buoyancy for a settling colloidal mixture is far from being obvious. Moreover, sedimentation holds novel surprises, such as colloidal inflations and settling disasters, showing that a simple external field like gravity may induce mind-boggling, and theoretically challenging effects. 相似文献
4.
In the present paper, we report on the results of an experimental study of the process of gravity sedimentation of a cloud of monodispersed solid spherical particles with initial volume concentration C > 0.03, which was performed in a wide range of Reynolds numbers. An analytical estimate of the settling regimes of spherical particle clouds is presented. A new method for creating a spherical particle cloud with a high concentration of particles is proposed. A qualitative picture of the settling process of a highly concentrated particle cloud under gravity is revealed. A criterial dependence for the drag coefficient of a sedimenting spherical particle cloud as an entity is obtained. 相似文献
5.
We investigate the settling of heavy particles in a steady, two-dimensional random velocity field, and find instances in which particle suspension occurs. This leads to a bimodal velocity distribution that may explain some apparently conflicting results reported in the literature. The bimodal distribution is typically smeared out by a time dependence of the ambient flow but, if the time variation is slow, the settling rates of some particles will be as well. The resulting broadbanded velocity distribution of the settling particles will have significance for processes such as rain drop formation, in which the spread of particle velocities affects the statistics of particle collisions. 相似文献
6.
Thomas Hubner Stefan Will Alfred Leipertz 《Particle & Particle Systems Characterization》2001,18(2):70-78
A new method for the simultaneous determination of the distribution of particle mass density and the distribution of particle size with a technique with only a single measurement is presented. The basic idea of the new optical method is the analysis of gravitational particle settling by a digital image acquisition system. Individual particles illuminated by a laser light sheet are tracked by a continuously operating CCD camera. The projected area, shape factors and the centre of gravity are detected during the sedimentation process from a series of images with a constant time spread. As the algorithm is based on single particle tracking, the heterogeneity of the sample can be taken into account. From these measured particle characteristics, particle size and settling rate are calculated. Thus particle mass density is obtained taking into account also the influence of particle shape on the settling process. This method, which we name sedimentation image analysis (SIA), is particularly suitable for the characterization of heterogeneous material, e.g. soil, in the micrometer range. 相似文献
7.
Sanjeeva Witharana Chris Hodges Dan Xu Xiaojun Lai Yulong Ding 《Journal of nanoparticle research》2012,14(5):851
Nanoparticle suspensions (also called nanofluids) are often polydisperse and tend to settle with time. Settling kinetics in
these systems are known to be complex and hence challenging to understand. In this study, polydisperse spherical alumina (Al2O3) nanoparticles in the size range of ~10–100 nm were dispersed in water and examined for aggregation and settling behaviour
near its isoelectric point. A series of settling experiments were conducted and the results were analysed by photography and
by small angle X-ray scattering (SAXS). The settling curve obtained from standard bed height measurement experiments indicated
two different types of behaviour, both of which were also seen in the SAXS data. But the SAXS data were remarkably able to
pick out the rapid settling regime as a result of the high temporal resolution (10 s) used. By monitoring the SAXS intensity,
it was further possible to record the particle aggregation process for the first time. Optical microscopy images were produced
on drying and dried droplets extracted from the suspension at various times. Dried deposits showed the rapid decrease in the
number of very large particles with time which qualitatively validates the SAXS prediction, and therefore its suitability
as a tool to study unstable polydisperse colloids. 相似文献
8.
A method called “image-sedimentation”, based on the application of Stokes' law to suspension and image techniques, is proposed for the rapid analysis of particle size distributions. A parallel beam of optical light, after passing through a cell containing particles, is measured with a linear image sensor with 2048 photodetectors to determine the attenuated light intensity displayed on a monitor screen. The settling height between any given point and the liquid surface can be measured accurately by use of this image technique without any additional mechanical device to move either the light beam or the glass cell containing the suspension. Consequently, the total time required for the determination of the particle size distribution, typically of a carborundum powder having a median diameter of 5 μm, takes only 5 min as compared with more than 20 min by using other instruments based on the sedimentation principle. 相似文献
9.
A. N. Rubinov A. A. Afanas'ev I. E. Ermolaev Yu. A. Kurochkin S. Yu. Mikhnevich 《Journal of Applied Spectroscopy》2003,70(4):565-572
The amplitude of the gradient force acting on a transparent spherical particle in the field of a zero-order Bessel beam has been calculated in the Rayleigh–Gans approximation. The expression obtained for the gradient-force amplitude takes into account the heterogeneity of the acting radiation in the volume of the particle. The optimal conditions of trapping and transportation of the particle (parameters of the particle, liquid, and of the Bessel beam) to the localization region have been determined using the solution of the kinetic equation of particle motion in a liquid. It is shown that for certain relationships between the particle radius and the Bessel beam width the localization region is shifted relative to the central maximum of the beam. This is due to the equal action of the gradient forces caused by the central maximum and the first interference ring of the Bessel beam. A qualitative comparison of the results obtained with the known experimental data has been performed. 相似文献
10.
Direct imaging of settling, non-Brownian, hard sphere, particles allows measurement of particle occupancy statistics as a function of time and sampling volume dimension. Initially random relative particle number fluctuations, (2)>/ = 1, become suppressed, anisotropic, and dependent. Fitting to a simple Gaussian pair correlation model suggests a minute long ranged correlation leads to strong if not complete suppression of number fluctuations. Calflisch and Luke predict a divergence in velocity fluctuations with increasing sample volume size based on random (Poisson) statistics. Our results suggest this is not a valid assumption for settling particles. 相似文献
11.
V. L. Ganimedov V. V. Shumsky M. I. Yaroslavtsev 《Thermophysics and Aeromechanics》2009,16(2):197-208
An analysis of the losses of heat into the walls of settling chamber in a hypersonic hot-shot tube has been performed. Tests
without diaphragm rupture showed that the fall of settling-chamber pressure during the operating flow regime in the tube was
the consequence of the transfer of heat from working body to wall; this has allowed us to evaluate the heat-transfer coefficient
α and the inner-surface temperature of the wall T
w. An empirical formula relating the coefficient α with the pressure and working-body temperature in the settling chamber in
the range of pressures and temperatures 160 to 540 bar and 700 to 3400 K was obtained. Using the gained dependences of α and
T
w on pressure and temperature, we have developed a physical model for calculating the working-body characteristics in the tube
with allowance for enthalpy losses. We found that by the hundredth millisecond of the operating regime the disregard, in such
calculations, of the wall heat flux in the first settling chamber resulted in overestimation of the stagnation temperature
in the test section in comparison with similar calculations made without allowance for the heat losses by 6–18 % in terms
of the full-scale temperature for aircraft flight in Mach number range 5 to 8. The developed calculation procedure has been
tested in experiments without diaphragm rupture. 相似文献
12.
13.
D. Liu E.E. Keaveny M.R. Maxey G.E. Karniadakis 《Journal of computational physics》2009,228(10):3559-3581
The force-coupling method, previously developed for spherical particles suspended in a liquid flow, is extended to ellipsoidal particles. In the limit of Stokes flow, there is an exact correspondence with known analytical results for isolated particles. More generally, the method is shown to provide good approximate results for the particle motion and the flow field both in viscous Stokes flow and at finite Reynolds number. This is demonstrated through comparison between fully resolved direct numerical simulations and results from the numerical implementation of the force-coupling method with a spectral/hp element scheme. The motion of settling ellipsoidal particles and neutrally buoyant particles in a Poiseuille flow are discussed. 相似文献
14.
15.
One of the advanced strategies in hydrate plug prevention is to obtain an in-depth knowledge of the rheological properties of hydrate slurries. A major challenge in hydrate rheological measurements is that the viscosity profile can be difficult to attribute to physical phenomena, such as particle agglomerate breakup, particle bedding/settling and wall growth. In this work, a novel visual rheometer has been developed to help overcome these previous limitations by enabling the visualisation of the evolution of cyclopentane hydrate slurries during viscosity measurements. Two different model systems were used in this investigation: (1) non-emulsified and (2) emulsified systems. The physical phenomena, including initial hydrate formation, hydrate wall growth, bedding and sloughing were visually observed and directly correlated to the corresponding viscosity profile. For the non-emulsified system, there are four different stages of hydrate slurry development including initial hydrate formation, wall adhesion and growth and sloughing that caused changes in the viscosity profiles. Large fluctuations in the viscosity profile for a non-emulsified system were found to be the result of a sloughing phenomenon. On the other hand, the emulsified system showed a well-dispersed hydrate slurry with minimal wall and impeller growth, corresponding to a smooth viscosity profile. 相似文献
16.
Ning Zhang Zhongquan Zheng Steven Eckels Venkata B. Nadella Xiaoyang Sun 《Particle & Particle Systems Characterization》2009,26(4):199-209
We inject a large number of newly created nano‐particle aggregates into a chamber for the purpose of removing harmful contents in an indoor environment. This study is to experimentally and numerically investigate transient response of particle distributions to particle injections. A room‐sized chamber of 4 m × 2.1 m × 2.4 m is connected to a specially designed particle‐injection system, with two Optical Particle Counters used to simultaneously measure particle‐number densities with the size range from 0.3 μm to 10 μm at the inlet and in the chamber. A velocity probe measures the flow that is up to 1 m/s. An Euler‐type particulate‐phase‐transport model is developed and validated by comparing with experimental data. The study shows that the transient behavior of particle distributions is determined by many factors, including particle size, particle settling speed, sampling location, and velocity distribution. Particle number densities decrease in time more quickly for large particles than for small particles, and locations farther downstream in the chamber correlate more weakly with the inlet injection. 相似文献
17.
We study the settling dynamics of non-Brownian prolate spheroids under steady-state sedimentation. We consider the case of moderate particle Reynolds numbers properly taking into account the hydrodynamic effects. For small volume fractions, we find an orientational transition of the spheroids, characterized by enhanced density fluctuations. Around the transition, the average settling velocity has a maximum which may even exceed the terminal velocity of a single spheroid, in accordance with experiments. 相似文献
18.
气体相与颗粒相混合流场的声速研究, 由于具有重要的基础理论价值与广泛的工程应用背景, 逐渐受到人们重视. 针对稠密可压缩气粒两相流动, 综合考虑颗粒相所占空间体积以及颗粒间相互作用, 推导给出了新的等熵声速计算公式; 新公式包含了已有的纯气体、稀疏气粒两相流情形的计算公式作为其特例, 一方面验证了公式推导的正确性, 另一方面说明新公式更具有通用性; 分析了不同颗粒质量分数条件下的声速变化规律, 相应结果与普朗特的理论分析符合, 特别对于稠密气粒两相流动工况得到了一些新的物理认识; 开展了颗粒间相互作用建模参数的物理分析, 揭示了其对气粒两相流动声速的影响机理. 本文取得的成果为稠密可压缩气粒两相流动研究以及相关工程应用提供理论支撑. 相似文献
19.
《Chinese Journal of Physics (Taipei)》2018,56(4):1427-1438
A circular particle settling in a confined vertical channel with static fluid is investigated by lattice Boltzmann method, the effect of density ratio on the particle motion and flow pattern is discussed. It shows that the particle starting from an initial position off-center displays various moving modes accompanied by different flow patterns for 1.003 ≤ γ ≤ 5.0: when γ < 1.1, the particle is finally in a stable equilibrium near the centerline and settles uniformly with a symmetrical flow, but there are different modes before reaching the centerline at different density ratios, a monotonic approach appears at γ ≤ 1.005 and an transient overshoot appears at 1.005 < γ < 1.1; When γ = 1.1, a weak oscillation appears in the particle moving but the flow is almost symmetrical, it is in a critical state of the particle moving from stable equilibrium with a transient overshoot to oscillation; When 1.1 < γ ≤ 3.0, the particle oscillates regularly around the centerline with the uniform amplitude and frequency, and it turns into irregularity for γ > 3.0. Meanwhile the rotation of the particle depends on the distance from a wall; it rolls up the closer wall and stops at centerline. 相似文献
20.
In separation processes, the knowledge of particle size and density arc often not enough to describe the settling behaviour in a concentrated suspension. Therefore, a direct method for the characterization of the settling behavior of submicron particles in concentrated suspensions is introduced in a centrifugal field by a manometric sedimentation analysis. By means of this cumulative method in a homogeneous suspension, the analyses of both the interfacial settling rate and the settling rate of the particles within the concentrated suspension are possible. This permits a differential examination of settling processes in a broad concentration range. First, the influence of the solid concentration on the settling rate at the interface and within a monodisperse suspension with a range from 0.01 to 30 vol.% is represented. The relationship between the increase in settling rate through particles settling in a cluster and a concentration decrease in the suspension is also represented. Consideration of the possibilities of the analysis of polydisperse suspensions demonstrates the field of applications for this method. 相似文献