Dynamic broadening of the crystal-fluid interface of colloidal hard spheres

We investigate the structure and dynamics of the crystal-fluid interface of colloidal hard spheres in real space by confocal microscopy. Tuning the buoyancy of the particles allows us to study the interface close to and away from equilibrium. We find that the interface broadens from 8-9 particle diameters close to equilibrium to 15 particle diameters away from equilibrium. Furthermore, the interfacial velocity, i.e., the velocity by which the interface moves upwards, increases significantly. The increasing gravitational drive leads to supersaturation of the fluid above the crystal surface. This dramatically affects crystal nucleation and growth, resulting in the observed dynamic broadening of the crystal-fluid interface.     

Collective effects in settling of spheroids under steady-state sedimentation

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.     

Settling velocity of equiaxed dendrites in a tube

The settling velocity of equiaxed dendrites can cause macrosegregation and influence the structure of the equiaxed zone during the casting solidification process. So an understanding of the settling characteristics is needed to predict the structure and segregation in castings. The settling velocity of NH4Cl equiaxed dendrites of non-spherical geometry was studied experimentally in an NH4Cl-70wt.%H2O solution. A calculation formula was proposed to calculate the settling velocity of sediment equiaxed dendrites in a tube filled with saturated solution at a moderate Reynolds number region. The retardation effects of the wall and morphology of the equiaxed dendrite on the settling velocity were taken into account in the development of the calculation formula, and the correction function B of the drag coefficient with consideration of the retardation effects of the wall and morphology of the equiaxed dendrite on the settling velocity of the equiaxed dendrite was calibrated according to the experimental results. A comparison showed that the formula has a good accordance with the experimental results.     

Kinetic Monte Carlo simulation of nanoparticle film formation via nanocolloid drying

A kinetic Monte Carlo simulation of nanoparticle film formation via nanocolloid drying is presented. The proposed two-dimensional model addresses the dynamics of nanoparticles in the vertical plane of a drying nanocolloid film. The gas–liquid interface movement due to solvent evaporation was controlled by a time-dependent chemical potential, and the resultant particle dynamics including Brownian diffusion and aggregate growth were calculated. Simulations were performed at various Peclet numbers defined based on the rate ratio of solvent evaporation and nanoparticle diffusion. At high Peclet numbers, nanoparticles accumulated at the top layer of the liquid film and eventually formed a skin layer, causing the formation of a particulate film with a densely packed structure. At low Peclet numbers, enhanced particle diffusion led to significant particle aggregation in the bulk colloid, and the resulting film structure became highly porous. The simulated results showed some typical characteristics of a drying nanocolloid that had been reported experimentally. Finally, the potential of the model as well as the remaining challenges are discussed.     

Settling statistics of hard sphere particles

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.     

Scholte波与含泥沙两相流介质属性关系的分析及仿真验证

基于四种超声悬浮液模型Urick, Urick-Ament, HT, Mcclements分析了Scholte波在两相流体与多孔介质固体界面处的传播特性. 结合各模型的复波数表达式建立含泥沙流体-多孔介质固体界面波特征方程, 分析了Scholte波速与两相流体积含量、粒径等介质属性的关系. 通过仿真实验获得界面波信号, 运用时延估计获得Scholte波速与泥沙含量、粒径的关系, 发现所得的波速与Urick-Ament和HT理论有相对好的一致性. 关键词： Scholte波 两相流体 多孔介质 泥沙含量     

Shadow Doppler Velocimetry for Simultaneous Size and Velocity Measurements of Irregular Particles in Confined Reacting Flows

We report the application of the Shadow Doppler Velocimeter (SDV) for spatial precise, simultaneous measurement of the size and velocity to assess the particle retention performance of a laboratory, 1/6 scale, 10 kW vertically-fired atmospheric model of the pressurised pulverised-coal furnace of Reichert et al. [1]. The SDV is based on the imaging of a conventional LDV probe volume onto a linear photodiode array and has the advantage over other sizing methods for irregular particles that it is tolerant of the optical misalignment and fouling which are inevitable when passing laser beams through windows in such furnaces. The size and two components of velocity of burning coal particles were measured in the present geometry which has 172 mm furnace diameter and 40 mm lateral exit duct diameter and a calculated exit bulk velocity of 4 m/s, evaluated at 300 K. The Sauter mean diameter of the particles is, within the experimental error, uniform at about 40 μm in the vertical profile normal to the axis of the exhaust pipe, 34.5 mm upstream of the exit. Coal particle velocities in the near-exit region are directed towards the exit, closely following the gas-phase velocities. Both these observations imply that particle retention efficiency due to streamline curvature is low and extrapolation suggests that there will be even less at large scales.     

On high velocity impact of micro-sized metallic particles in cold spraying

In this study, a systematic examination of particle deformation behaviour in cold spraying was conducted for Cu particle using both the Lagrangian and Arbitrary Lagrangian Eulerian (ALE) methods. It is found that the meshing size in modelling by Largrangian method influences significantly the localized shear instability at interface areas. With refining the meshing size the onset velocity for interface shear instability decreases. The extrapolation of these results yields a reasonable critical velocity comparable to the actual one in cold spray practice. The results indicate that both the flattening ratio and compression ratio of the deformed particles increase with the increase in particle velocity, which are in good agreement with the experiment results. The ALE method provides a suitable way to examine the particle deformation in cold spraying. Moreover, the numerical results also show that there exists the similarity for the deformation of particles of different diameters.     

Particle-laden viscous thin-film flows on an incline: Experiments compared with a theory based on shear-induced migration and particle settling

Particle settling in driven viscous films is a complex physical process involving different physical effects. A recent analysis by Cook (2008) [10] has identified a balance between hindered settling and shear-induced migration as the dominant large scale physics for particle/liquid separation. However, experimental data for this has been lacking. This paper presents new data including the role of particle size and liquid viscosity showing clear agreement with the theory. We discuss the role of timescales in the dynamics of the experiment and present results from a dynamic model.     

An interferometric technique for measuring gas flow velocity at large peclet numbers

A technique for determining the gas flow velocity at large Peclet numbers using an interferometric approach has been developed. It is shown that if a heat source is introduced in a uniform gas flow, areas under curves describing shifts of interference maxima are approximately equal to each other and tend to a certain limiting value. The limiting area is inversely proportional to the gas flow velocity and does not depend on the heat conduction and the type of flow. A theoretical expression has been derived that gives the relative error of velocity determinations by the method developed.     

Diffusion-broadened velocity spectra of convection in variable-temperature BP-LED experiments

When NMR diffusion experiments are performed at temperatures different from ambient temperature, temperature gradients due to probe design can cause thermal convection and therefore significantly affect the signal amplitude. Fourier transformation of the signal amplitude gives rise to a diffusion-broadened velocity spectrum, which contains information about the convection velocity. It is shown that when the diffusion broadening factor is smaller than the maximum velocity, the broadening has little effect on the determination of the maximum velocity. Thus, convection velocity can be determined in the presence of diffusion.     

Gravitational settling of a highly concentrated system of solid spherical particles

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.     

Measurement of unstable burning velocities of iso-octane–air mixtures at high pressure and the derivation of laminar burning velocities

A new technique is reported for measuring burning velocities at high pressures in the final stages of two inwardly propagating flame kernels in an explosion bomb. The flames were initiated at diametrically opposite spark electrodes, close to the wall, in quiescent mixtures. Measurements of pressure and flame kernel propagation speeds by high-speed photography showed the burning velocities to be elevated above the corresponding laminar burning velocities as a result of the developing flame instabilities. The enhancement increased with increase in pressure and decreased with increase in Markstein number. When the Markstein number was negative, instabilities could be appreciable, as could the enhancement. For the iso-octane–air mixtures investigated, where the mixtures had well-characterised Markstein numbers or critical Peclet numbers at the relevant pressures and temperatures, it was possible to explain the enhancement quantitatively by the spherical explosion flame instability theory of Bechtold and Matalon, provided the critical Peclet number was that observed experimentally, and allowance was made for the changing pressure. With this theoretical procedure, it was possible to derive values of laminar burning velocity from the measured values of burning velocity over a wide range of equivalence ratios, pressures, and temperatures. The values became less reliable at the higher temperatures and pressures as the data on Markstein and critical Peclet numbers became less certain. It was found that with iso-octane as the fuel the laminar burning velocity decreased during isentropic compression.     

Physical phenomena affecting quantitative imaging of ultrasound contrast agents

Microbubbles stabilized by a surfactant or polymer coating are the most effective form of contrast agent available for ultrasound imaging. They have shown great potential as a means of quantifying tissue perfusion, in particular determining physiologically significant parameters such as relative vascular volume and flow velocity. Clinical implementation of quantitative imaging procedures, however, has been hindered by poor characterisation of the complex relationship between microbubble concentration, scattering and image intensity. The aim of this paper is to describe theoretical and experimental investigations of the physical phenomena underlying these effects, such as the time, pressure and frequency dependence of microbubble behaviour, the influence of the bubble coating, size distribution and concentration; and to discuss the challenges involved in developing accurate quantitative imaging protocols.     

Velocity measurement of a settling sphere

We study experimentally the motion of a solid sphere settling under gravity in a fluid at rest. The particle velocity is measured with a new acoustic method. Variations of the sphere size and density allow measurements at Reynolds numbers, based on limit velocity, between 40 and 7 000. At all Reynolds numbers, our observations are consistent with the presence of a memory-dependent force acting on the particle. At short times it has a t ^-1/2 behaviour as predicted by the unsteady Stokes equations and as observed in numerical simulations. At long times, the decay of the memory (Basset) force is better fitted by an exponential behaviour. Comparison of the dynamics of spheres of different densities for the same Reynolds number show that the density is an important control parameter. Light spheres show transitory oscillations at Re∼ 400, but reach a constant limit speed. Received 12 April 2000 and Received in final form 13 July 2000     

Hydrodynamic and brownian fluctuations in sedimenting suspensions

We use a mesoscopic computer simulation method to study the interplay between hydrodynamic and Brownian fluctuations during steady-state sedimentation of hard sphere particles for Peclet numbers (Pe) ranging from 0.1-15. Even when the hydrodynamic interactions are an order of magnitude weaker than Brownian forces, they still induce backflow effects that dominate the reduction of the average sedimentation velocity with increasing particle packing fraction. Velocity fluctuations, on the other hand, begin to show nonequilibrium hydrodynamic character for Pe>1.     

反向碰撞法测量Bi的低压Hugoniot数据

在火炮和二级轻气炮上利用反向碰撞技术,通过测量飞片击靶速度以及飞片/窗口的界面粒子速度,获得了金属铋(Bi)在10-45 GPa压力范围内的Hugoniot数据. 该方法克服了电探针法在测量低压Hugoniot数据时由于导通一致性差而不能准确得到冲击波速度的难题,同时又避免了精确测量样品中冲击波走时的问题. 实验获得的冲击波速度(D)-波后粒子速度(u)Hugoniot数据表明,Bi在粒子速度u=0.9 km/s附近D-u曲线发生了明显拐折,产生这一拐折的原因推测与冲击导致的Bi的固-液相变有关. 关键词： Hugoniot数据 反向碰撞 冲击相变 铋     

Settling Behavior Characterization of Submicron Particles in Dilute and Concentrated Suspensions

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.