Evaluation of Particle Degradation Due to High-Speed Impacts in a Pneumatic Handling System

Particle degradation can be a significant issue in particulate solids handling and processing, particularly in pneumatic conveying systems, in which high-speed impact is usually the main contributory factor leading to changes in particle size distribution (comparing the material to its virgin state). However, other factors may strongly influence particles breakage as well, such as particle concentrations, bend geometry, and hardness of pipe material. Because of such complex influences, it is often very difficult to predict particle degradation accurately and rapidly for industrial processes. In this article, a general method for evaluating particle degradation due to high-speed impacts is described, in which the breakage properties of particles are quantified using what are known as “breakage matrices.” Rather than a pilot-size test facility, a bench-scale degradation tester has been used. Some advantages of using the bench-scale tester are briefly explored. Experimental determination of adipic acid has been carried out for a range of impact velocities in four particle size categories. Subsequently, particle breakage matrices of adipic acid have been established for these impact velocities. The experimental results show that the “breakage matrices” of particles is an effective and easy method for evaluation of particle degradation due to high-speed impacts. The possibility of the “breakage matrices” approach being applied to a pneumatic conveying system is also explored by a simulation example.     

Visualization of flow structure in a vortex furnace

The spatial structure of a swirling flow in a model vortex furnace with distributed input of fuel-air-mixture jets has been studied. The results of experimental and numerical investigations of a three-dimensional (3D) field of time-averaged velocities in an isothermal laboratory model of a vortex furnace have been used to image the structure of flow. Vortex structures have been identified using λ₂ and Q criteria, as well as the concept of “minimum total pressure.” The vortex core of the flow has a V-shaped 3D structure.     

Gas-particle turbulent flow of foursquare tangential jets in a simplified pulverized-coal firing boiler

An experimental cold-model of a simplified tangential firing boiler was established to investigate the mesoscale turbulent flow behaviors, including gas vortex structures, particle motions and interactions between two phases. A modified PIV technology, employing two pairs of lasers and cameras, was applied to measure the velocity and velocity gradient of turbulent flow in foursquare tangential jets alternatively. At a given initial gas velocity and particle mass loading, the interaction between gas and particles was studied at three different particle sizes. It was found that two main coherent vortex structures, circular eddy and hairpin eddy, distributed mainly in low speed area and heavy impingement area, respectively. The characteristics of particle motion in foursquare tangential jets correlated with gas turbulence dissipation, particle size, particle concentration and particle density. Small particles were easily entrained by gas vortex, so that they consumed more turbulence energy and attenuated the gas turbulence intensity. On the contrary, large particles had more inertia and led to heavier impingement in the chamber center, resulting in particle random distribution and complex momentum transfer between gas and particles. Moreover, large particles stretched the coherent vortex to be narrow and long, while small particles pulled down the vortices rotation intensity.     

A Study of CFD Modeling on Variation of Solid Fraction in a Batch Fluidized Bed

Two-dimensional gas–solid batch fluidized bed is simulated in transient conditions using the Eulerian–Eulerian two-fluid model. The grid independent test has been carried out for three different mesh sizes, and the courant number in the range of 0.2–0.4 has been used. The simulated results are compared with the experimental observations. The study is conducted for different sizes of Geldart group D solid particles, different bed heights, and different air velocities in the bubbling regime of fluidization. Two different models, such as the Gidaspow model and the Wen-Yu drag model, have been tested to model the drag at the phase interaction for Geldart group D solid particles. On comparison with experimental results the Gidaspow drag model matches well with experimental results and hence is used for simulation. At the initial stage of fluidization (where a fountain like phenomena was observed), the bed behaves like a spouting bed, and then the bubbling bed behavior follows. These predictions are compared with the experimentally observed bed height of particles of different sizes and air velocities. It is observed that the Gidaspow drag model predictions are in good accordance with the experimental results for particles of different sizes, both in the initial stage and in the bubbling regime of fluidization.     

Simulation of coupling filtration and flow in a dual scale fibrous media

In this article, numerical simulation of suspension (particles filled-resin) flow through a fibrous media taking into account dual scale porosity in LCM (Liquid Composite Molding) processes is presented. During the flow, a strong interaction between the particle motion and the fluid flow takes place at the porous media wall (the fiber bundle surface). In this study, the Stokes–Darcy coupling is used to describe the resin flow at mesoscopic scale to treat the particles in suspension. A “fluid” model to describe the suspension flow, a “filtration” model to describe the particle capture and a “solid” model dedicated to the modeling of mass particles dynamics was used. The “solid” model is also operated to identify the particles retention.For validation, the numerical results of proposed model were compared with the experimental results from the literature and found in good agreement. Then, other numerical results studying the suspension’s rheological behavior are presented.     

细颗粒物声波团聚的微观机理研究

为拓展声波团聚机理,对声波团聚过程中的声流与声涡作用进行了理论和实验研究。利用声流测试系统,发现在0～1 kHz低频与5 kHz高频时,团聚室内声流现象较为明显;并通过可视化测试,在7 kHz高频时观察到明显的漩涡。结果表明,流场中的声流与声涡对颗粒团聚会产生很大的影响,声流或声涡越强,团聚效果越好。在0～1 kHz低频与5 kHz高频时,声流产生的切应力带动气溶胶颗粒发生碰撞团聚;高频时声涡力矩较大,其产生的轨道角动量带动粒子发生圆周和自旋运动;当声压级大于132 dB时,声涡团聚开始发挥作用,与声流一起促进颗粒团聚,且声压级越大团聚效果越强;与波节相比,波腹处的声流速度更大,声涡现象更明显,团聚效果也更好。     

Sculpting Asymmetric,Hollow‐Core,Three‐Dimensional Nanostructures Using Colloidal Particles

Colloidal elements have historically played a key role in “bottom‐up” self‐assembly processes for nanofabrication. However, these elementary components can also interact with light to generate complex intensity distributions and facilitate “top‐down” lithography. Here, a nanolithography technique is demonstrated based on oblique illuminations of colloidal particles to fabricate hollow‐core 3D nanostructures with complex symmetry. The light–particle interaction generates an angular light distribution as governed by Mie scattering, which can be compounded by multiple illuminations to sculpt novel 3D structures in the underlying photoresist. The fabricated geometry can be controlled by the particle parameters and illumination configurations, enabling the fabrication of a large variety of asymmetric hollow nanostructures. The proposed technique has high pattern versatility, is low cost and high throughput, and can find potential application in nanoneedles, nanonozzles, and materials with anisotropic properties.     

Wavelet multi-resolution analysis on particle dynamics in a horizontal pneumatic conveying

The particle velocities are measured by the high-speed particle image velocimetry (PIV) in the acceleration and fully developed regimes of a horizontal pneumatic conveying. Based on the measured particle fluctuation velocities, continuous wavelet transform and one-dimensional orthogonal wavelet decomposition were applied to reveal particle dynamics in terms of time frequency analysis, the contribution from wavelet level to the particle fluctuation energy, spatial correlation and probability distribution of wavelet levels. The time frequency characteristics of particle fluctuation velocity suggest that the small-scale particle motions are suppressed and tend to transfer into large scale particle motions from acceleration regime to fully developed regime. In the near bottom part of pipe, the fluctuation energy of axial particle motion is mainly contributed from the wavelet levels of relatively low frequency, however, in the near top part of pipe, wavelet levels of relatively high frequency make comparable contribution to the axial particle fluctuation energy in the suspension flow regime, and this contribution decreases as particles are accelerated along the pipe. The low frequency wavelet levels exhibit large spatial correlation, and this spatial correlation increases as the particles flow from acceleration regime to fully developed regime. The skewness factor and kurtosis factor of wavelet level suggest that the deviation of Gaussian probability distribution is associated with the central frequency of wavelet level, and the deviation from Gaussian distribution is more evident as increasing central frequency. The higher wavelet levels can be linked to small sale particle motions, which lead to irregular particle fluctuation velocity.     

FLOW OF SAND-WATER MIXTURES AT VELOCITIES BELOW THE DEPOSITION CONDITION

An experimental study of sand-water flow in a horizontal pipeline has been conducted for the regime in which a stationary deposit was present. The particle diameter ranged between 0.2 mm and 0.01 mm and the particles were not flocculated. Axial pressure gradients and delivered concentrations were measured as functions of mean velocity and in-situ concentration.

A three layer model was found to be useful to predict the pipeline behavior at all but the lowest velocities. The Meyer-Peter sediment transport equation was satisfactory for very low velocities with the larger particles.     

Dynamics of Particle Confined Between Oscillating and Fixed Walls

In this paper, we investigate the motion of a particle confined between the oscillating and fixed walls. As the particle collides with the moving wall in the phase of oscillations, when the wall velocity grows, the wall after collision catches up with the particle. This process can be repeated many times until in finite time interval the particle is found lying on the wall and continues its motion together with the wall. When the sign of the wall acceleration changes the particle detaches from the wall with zero velocity, so that it looks as if the particle “sticks” to the wall. It has been found that if the collision is inelastic, “sticking” leads to convergence of close trajectories except for the case of weak decay. On the contrary, in the case of elastic collision “sticking” of the particle causes even a more rapid divergence of theses trajectories.     

Study of inlet temperature effect on single and double inlets cyclone performance

In this paper, a comprehensive study is performed in order to demonstrate the effect of the flow and particle temperature on cyclone performance. Three main characteristics of the low-mass-loading gas-solid cyclone separators, including: pressure drop, particle separation efficiency and natural vortex length are investigated. Eulerian-Lagrangian approach is employed to solve the unsteady Navier-Stokes and energy equations to model the flow of particles. Because of the strong swirling flow in cyclone, Reynolds stress transport model (RSTM) is used to calculate the Reynolds stresses. Numerical simulation is accomplished at a temperature range of 293–700 K and four inlet velocities. Also, a comparison is conducted between two Stairmand high efficiency cyclones with the same dimensions, one with single inlet and the other with double inlets to declare the effect of the second inlet on cyclone performance. The analysis of results shows that the swirling flow becomes weaker for higher temperature cases and thus, flow pressure drop and particle separation efficiency is noticeably decreased. Increasing in temperature causes decrease in natural vortex length. Also, study of natural vortex length is performed for the studied range of temperature.     

Complex Lattice and Charge Inhomogeneity Favoring Quantum Coherence in High-Temperature Superconductors

The presence of two components in the electron fluid of high-temperature superconductors and the complex charge and lattice inhomogeneity have been the hot topics of the international conference of the superstripes series, SUPERSTRIPES 2015, held in Ischia in 2015. The debate on the mechanisms for reaching room-temperature superconductors has been boosted by the discovery of superconductivity with the highest critical temperature in pressurized sulfur hydride. Different complex electronic and structural landscapes showing up in superconductors which resist to the decoherence effects of high temperature have been discussed. While low-temperature superconductors described by the BCS approximation are made of a single condensate in the weak coupling, the high-temperature superconductors are made of coexisting multiple condensates (in different spots of the k-space and the real space) some in the BCS-BEC crossover regime and others in the BCS regime. The role of “shape resonance” in the exchange interaction between these different condensates, like “the Fano-Feshbach resonance” in ultracold gasses, is emerging as a key term for high-temperature superconductivity.     

The effects of gas flow on granular currents

Currents of particles have been quite successfully modelled using techniques developed for fluid gravity currents. These models require the rheology of the currents to be specified, which is determined by the interaction between particles. For relatively small slow currents, this is determined primarily through friction, which can be controlled and reduced by fluidizing the particles, so that they may become much more mobile. Recent results cannot be predicted using many of the proposed models, and may be defined by the interaction between the particles and the fluid through which they are passing. However, in addition, particles that are only initially fluidized also form currents that are also mobile, but otherwise are different from continuously fluidized currents. The mobility of these currents appears not to be connected to the time taken for them to degas. This suggests that defining the continuous stresses on the particle current may not be sufficient to understand its motion and that a challenge for the future is to understand the structure of these flows and how this affects their motion.     

Study and optimization of flow field in a novel cyclone separator with inner cylinder

This paper presents a study of gas-solid flow in a novel cyclone separator with inner cylinder, compared with that in a conventional cyclone. The Reynolds stress model (RSM) is used to simulate fluid flow, and the discrete phase model (DPM) is selected to describe the motion behavior of particles. The experimental data measured by particle image velocimetry (PIV) is used to verify the reliability of the numerical model. The results show that in the novel cyclone, the cleaned gas can be quickly discharged from the vortex finder, the movement distance and residence time of fine particles are prolonged, the short-circuit flow and vertical vortex under the vortex finder are eliminated, the mutual interference between upflow and downflow in the cylinder is eliminated, and the region of quasi-free vortex in the cone is enlarged. Compared with the conventional cyclone, the novel cyclone has higher collection efficiency and lower pressure drop.     

Revealing the Cluster‐Cloud and Its Role in Nanocrystallization

Elucidating the early stages of crystallization from supersaturated solutions is of critical importance, but remains a great challenge. An in situ liquid cell transmission electron microscopy study reveals an intermediate state of condensed atomic clusters during Pd and Au crystallizations, which is named a “cluster‐cloud.” It is found that nucleation is initiated by the collapse of a cluster‐cloud, first forming a nanoparticle. The subsequent particle maturation proceeds via multiple out‐and‐in relaxations of the cluster‐cloud to improve crystallinity: from a poorly crystallized phase, the particle evolves into a well‐defined single‐crystal phase. Both experimental investigations and atomistic simulations suggest that the cluster‐cloud‐mediated nanocrystallization involves an order–disorder phase separation and reconstruction, which is energetically favored compared to local rearrangements within the particle. This finding grants new insights into nanocrystallization mechanisms, and provides useful information for the improvement of synthesis pathways of nanocrystals.     

Particles-Vortex Interactions and Flow Visualization in 4He

Recent experiments have demonstrated a remarkable progress in implementing and use of the Particle Image Velocimetry (PIV) and particle tracking techniques for the study of turbulence in ⁴He. However, an interpretation of the experimental data in the superfluid phase requires understanding how the motion of tracer particles is affected by the two components, the viscous normal fluid and the inviscid superfluid. Of a particular importance is the problem of particle interactions with quantized vortex lines which may not only strongly affect the particle motion, but, under certain conditions, may even trap particles on quantized vortex cores. The article reviews recent theoretical, numerical, and experimental results in this rapidly developing area of research, putting critically together recent results, and solving apparent inconsistencies. Also discussed is a closely related technique of detection of quantized vortices by negative ion bubbles in ⁴He.     

Velocity Fluctuations in a Particle-Laden Turbulent Flow over a Backward-Facing Step

Dilute gas-particle turbulent flow over a backward-facing step is numerically simulated. Large Eddy Simulation (LES) is used for the continuous phase and a Lagrangian trajectory method is adopted for the particle phase. Four typical locations in the flow field are chosen to investigate the two-phase velocity fluctuations. Time-series velocities of the gas phase with particles of different sizes are obtained. Velocity of the small particles is found to be similar to that of the gas phase, while high frequency noise exists in the velocity of the large particles. While the mean and rms velocities of the gas phase and small particles are correlated, the rms velocities of large particles have no correlation with the gas phase. The frequency spectrum of the velocity of the gas phase and the small particle phase show the -5/3 decay for higher wave number, as expected in a turbulent flow. However, there is a "rising tail' in the high frequency end of the spectrum for larger particles. It is shown that large particles behave differently in the flow field, while small particles behave similarly and dominated by the local gas phase flow.     

The motion of micron-sized particles in He II counterflow as observed by the PIV technique

No Heading The motion of micro-spheres in He II thermal counterflow has been measured using the Particle Image Velocimetry (PIV) technique. Although the tracer particles are able to track the motion of the normal fluid component, a significant discrepancy between the measured particle velocity and theoretical normal fluid velocity is observed. Further analysis of this velocity discrepancy suggests that it may be caused by the interaction between particles and vortex lines in the superfluid component. A semi-empirical correlation for the interaction force is developed and compared to the experimental results, from which new dynamic behavior of particles in He II is presented in the form of an effective kinematic viscosity of the superfluid.PACS numbers: 67.40.Pm, 67.40.Vs, 47.80.+v     

The Next Generation of Colloidal Probes: A Universal Approach for Soft and Ultra‐Small Particles

The colloidal probe technique, which is based on the atomic force microscope, revolutionizes direct force measurements in many fields, such as interface science or biomechanics. It allows for the first time to determine interaction forces on the single particle or cell level. However, for many applications, important “blind spots” remain, namely, the possibility to probe interaction potentials for nanoparticles or complex colloids with a soft outer shell. Definitely, these are colloidal systems that are currently of major industrial importance and interest from theory. The here‐presented novel approach allows for overcome the aforementioned limitations. Its applicability has been demonstrated for 300 nm sized carboxylate‐modified latex particles as well as sub‐micron core–shell particles with a soft poly‐N‐isopropylacrylamide hydrogel shell and a rigid silica core. For the latter, which until now cannot be studied by the colloidal probe technique, determined is the temperature dependency of electrosteric and adhesion forces has been determined on the single particle level.     

Particle dynamics in the axisymmetric toroidal flow in thermocapillary liquid bridges: towards understanding PAS

The motion of small suspended particles in the two- dimensional stationary vortex flow arising in the half-zone model of crystal growth is investigated. The particle dynamics is modeled by a modified Maxey-Riley equation assuming one-way coupling. The calculations for gravity as well as microgravity conditions are performed for configurations similar to the one used in the experiments. The computed trajectories and their analysis shed some light on the formation mechanism of the particle accumulation structures (PAS) which have been observed in half-zone experiments.