A novel mixing method for levitated particles using electrostatic fields

A continuous particle mixing system using electrostatic forces and vibrations was developed. The system consists of two symmetrically arranged devices. The same or different types of charged particles were continuously fed from each device in a dispersed state and mixed instantaneously in the space between devices. When charged particles with opposite polarities were fed from each device by changing the direction of the electric field, the particles were homogeneously mixed. The electric field and particle trajectories were numerically calculated to elucidate the particle-mixing mechanism. Furthermore, the mixing state of the particles was evaluated quantitatively using Shannon entropy.     

Tracking particles in sands based on particle shape parameters

The acquisition of particle kinematics of deforming sands generally requires tracking the particles within the specimen. For this purpose, one particle tracking method based on particle polar radius functions (PR-Track) and a second method based on the SH spectrum of particle spherical harmonics (SH-Track) are presented. Both methods are applied to the acquisition of particle kinematics of a Leighton Buzzard sand sample undergoing shearing in a miniature loading apparatus using X-ray micro-tomography. The results are compared to those from a particle volume-based particle tracking method (PV-Track). It is found that PR-Track and SH-Track return particle tracking results with a high precision which is comparable to PV-Track. For the tested sample, PR-Track is about 0.4 times faster than SH-Track. Furthermore, PR-Track and SH-Track have a much lower computational efficiency than PV-Track. However, PR-Track and SH-Track do not require particles to be displaced in a similar manner to their neighbours, which is assumed by PV-Track. Particle tracking results from PR-Track and SH-Track are not sensitive to search region sizes. These features imply that PR-Track and SH-Track are capable of tracking particles of granular media undergoing a large and complicated deformation. This capability is verified in a simple case study by applying the methods to track particles undergoing particle column collapse.     

Experimental and CFD study on a cyclonic classifier with new flow pattern

Physical principle of conventional top-inlet classifier (CTC) with reverse-flow pattern leads to the heavily fine particles entrainment in coarse fraction. Present work concentrates on the flow-field design for less downward airflow at near-wall region of the classifier. A new middle-inlet classifier (NMC) is proposed and analyzed using computational fluid dynamics (CFD) method and powder classification experiments. The results showed that new flow pattern characterized by a pair of vortexes was created in the new classifier. The upper vortex with 80% of the total air volume moves upward and forms the washing effect at near-wall region, which effectively reduces the fine particles entrainment in coarse fraction. The downer vortex with reverse-flow pattern discharges the coarse particles timely. The radial centrifugal sedimentation combined with the axial counter-current washing effect dominates the particle classification in the NMC. Compared to the CTC, classification accuracy index of the NMC with double-vortex averagely increases by 27% with a pressure drop reduction of more than 38%. This work offers a new principle for high-efficiency particle classification and new strategy for improving the classification performance of turbo air classifiers and hydrocyclones.     

Influence of particle packed pattern on the transient granular flow in a wedge-shaped hopper

A hopper has very wide and vital applications in handing the granular materials in daily life and industrial production, and the full understanding of the granular flow inside a hopper is of great importance to control and optimize the discharge process. By employing experimental and numerical methods, the influence of particle packed pattern on the transient granular flow is investigated in terms of the particle-scale kinetics and structure. For the mono-sized particles packed pattern, despite the similar particle-scale structure, smaller particles achieve greater kinetic energy conversion efficiency, which helps shorten the discharge time. For the binary-sized particles uniform mixing pattern, the interaction between particles increases the individual kinetic energy and transient average coordination number (CN) of large particles, while decreases that of small ones. Then the in-between kinetic energy and the disperse structure are reached. For the layer by layer mixing pattern, the strong percolation effect caused by the upper small particles hinders the increase of the individual kinetic energy at the beginning of the discharge process, and the transient average CN at the layer interface abruptly reaches 8. By contrast, when the small particles are placed at the bottom, more particles are active in the larger space, and subsequently, a looser structure is achieved in a shorter period.     

Crystal structure of very small particles of chromium and iron

Very small crystals of chromium with diameters less than about 30 Å, formed by vacuum deposition onto a substrate at room temperature, gave rise to an electron diffraction pattern consisting of diffuse haloes. It was concluded that these haloes could not be interpreted in terms of the ordinary b.c.c. structure of chromium but could be interpreted in terms of the new modification of chromium called δ-Cr. This conclusion was drawn by comparing the observed intensity profiles of the diffraction haloes with those calculated from the scattering formula for gas molecules, assuming that the molecules had given numbers of atoms arranged in the δ-Cr structure. Particles of iron also showed similar halo patterns when the particles were very small. The iron and chromium particles all had their normal b.c.c. structures when their sizes exceeded 20 and 100 Å, respectively.     

Experimental and numerical investigations of particle suspension suppression mechanisms for a particle-suppression device in a stirred liquid bath

Particle suspension in a turbulent flow can seriously affect the performance of manufactured products in many industrial processes in which the motion of particles cannot be modeled using the numerical method because of the enormous number of particles. Therefore, in this study, a full-scale computational fluid dynamics (CFD) simulation and a 1/5 scaled-down water model experiment were employed to investigate the flow pattern and dynamic behavior of particles in a continuously stirred vessel system. Based on the understanding of the suspension mechanism of settling particles, a particle-suppression device was designed to realize the harmless movement and deposition of particles. The results showed that the flow guidance and division mechanisms of the particle-suppression device led to the inhibition of particle suspensions. In addition, the optimal parameter combination for the device from the water model experiment combined with the orthogonal experimental design, resulted in a 98.3% reduction in the concentration of suspended particles. The suspension of particles was effectively suppressed, which improves product quality and production efficiency. Reliable results can be achieved by combining CFD simulations and water model experiments.     

Alternative approach for defining the particle population requirements for static image analysis based particle characterization methods

For image based particle characterisation approaches one of the most common discussion points is determining the number of particles required to have statistical confidence that the measurement is able to adequately describe the distribution of the sample. This topic becomes significantly more challenging when applied to the extraction of single component size distributions from multi-component samples.The aim of this work was to propose a means to accurately assess the particle number requirements using a method specific approach. The method applies a sub-sampling method to the original imaged dataset in order to provide an understanding of the impact of sub-sampling on the ability to accurately reproduce the original distribution.The method was applied to understand the particle number requirements for two batches of theophylline anhydrous with varied particle size distributions, using the input size distribution to guide the requirements for the subsequent multi-component samples of both materials.The results demonstrate the utility of the method to determine the appropriate number of particles required to recreate the size distributions. Whilst the minimum number of particles required to be sampled can be estimated, how those particles are sampled can also affect the validity of the measurement and must be taken into consideration.     

Numerical Model for particle deposition and loading in electret filter with rectangular split-type fibers

A simulation model for electret filter made of split type fibers has been developed to study the filtration efficiency as well as the particle loading process. The filter was assumed to be composed of rectangular fibers arranged in staggered array in which the flow field, the electrostatic field and the collection mechanisms were determined by numerical simulation. Single fiber efficiencies under various filtration conditions were calculated and compared with results obtained from semi-empirical expressions derived from experimental results. Influences of particle charge, fiber charge and orientation of fiber on the collection efficiency were evaluated. Finally the particle loading process was studied using the present model. Dendrite growth of particles in equilibrium charge state was simulated. The mechanical efficiency compensation effect was studied by a series of simulations. It is found that the loading of 1.5 m or larger particles has a significant mechanical collection compensation to the loss in electrostatic efficiency; while for 0.4 m particles such compensation is slow and insignificant.     

Effects of mixing ratio and order of admixed particles with two diameters on improvement of compacted packing fraction

To improve particle flowability, a technique is used in which fine particles are admixed with the main particles. However, the effects of coating structure on the improvement in flowability are not yet fully understood. Thus, predicting the improvement resulting from this technique is difficult. In this study, we focused on the effects of the particle diameter distribution of the admixed particles on coating structures and improvement of flowability in terms of the compacted packing fraction in a particle bed. Main particles of size 397 nm with admixed particles of sizes 8 and 104 nm were used. Bimodal particle diameter distributions were adjusted by changing the mixing ratios of the two admixed particles. Furthermore, the main and admixed particles were mixed in various orders. We examined the compacted packing fractions for these different mixing ratios and orders. Scanning electron microscopy images were obtained in order to analyze the coating structures on the main particle surfaces. The results show that the main particle packing fraction was most greatly improved by pre-mixing the two admixed particles. This can be explained by a linked rigid-3-bodies model with leverage based on increasing the apparent diameter of the main particles.     

Computational simulation of thermally sprayed WC–Co powder

WC–Co cemented carbides are a class of hard composite materials of great technological importance. They are widely used as tool materials in a large variety of applications that have high demands on hardness and toughness, including mining, turning, cutting and milling. The HVOF (high velocity oxygen fuel) technology has been very successful in spraying wear resistant WC–Co coatings with higher density, superior bond strengths and less decarburization than many other thermal spray processes, attributed mainly to its high particle impact velocities and relatively low peak particle temperatures. The degree of decomposition and bond strength is directly related to relevant particle parameters such as velocity, temperature and state of melting or solidification. These are consecutively related to process parameters such as powder particle size distribution, carrier gas flow rate, and fuel type employed. To obtain detailed particle data important for thermal spraying, mathematical models are developed in the present paper to predict the particle dynamic behavior in a liquid fuelled HVOF thermal spray gun. The particle transport equations are coupled with the three-dimensional, chemically reacting, turbulent gas flow, and solved in a Lagrangian manner. The melting and solidification within the particles as a result of heat exchange with the surrounding gas flow is solved numerically. The in-flight characteristics of WC–Co particles are studied and the effects of carrier gas parameters on particle behavior are examined. The results demonstrate that WC–Co particles smaller than 5 μm in diameter undergo melting and solidification prior to impact while most particles never reach liquid state during the HVOF thermal spraying. The flow rate of carrier gas has considerable influence on particle dynamics as well as deposition on substrate. At higher flow rate the powder particles are redirected further away from the substrate center, while smaller flow rate results in better heating, higher impact velocity and deposition closer to the substrate center.     

Investigation on tectonically deformed coal particle crushing by DEM under triaxial loading: Implication for coal and gas outburst

Tectonically deformed coal is composed of loosely bonded and brittle particles. The effect of particle crushing on tectonically deformed coal was studied through triaxial loading tests using the discrete element method. Coal particles are highly sensitive to crushing under confining pressure. When confined pressure at 2.5 MPa, the sample behaves similarly to an uncrushed sample. The majority of boundary work is consumed by particle friction and crushing during loading. The porosity increases obviously, the granular system is transferred into fluid-like state. With an increase in confining pressure, particle crushing intensifies, resulting in a growth in weak force chains and crushing energy, and a decrease in porosity. The pattern of particle crushing is closely linked to specific force chains. Particle rotation is a significant factor affecting particle crushing, and it hinders the connection of shear zones. Crushed particles decrease porosity by filling the gaps between larger particles. After compaction, crushed particles form a crushing and compaction belt, which creates local gas sealing conditions and makes gas extraction difficult in tectonically deformed coal areas. These characteristics play a crucial role in understanding how in-situ stress impacts gas outbursts and the difficulty of gas extraction.     

Experimental and CFD-DEM numerical evaluation of flow and heat transfer characteristics in mixed pulsed fluidized beds

Pulsed fluidized beds can make gas-solid mix and contact more uniform, therefore obviously improving heat transfer efficiency. The mixed pulsed fluidized bed, whose total gas flow is composed of stable gas flow and pulsed gas flow, is proposed in this research. Firstly, the experimental device for drying particles in a mixed pulsed fluidized bed is established. Pressure signals with different frequencies and gas flow ratios are collected, and flow pattern diagrams are obtained through a high-speed camera. Secondly, the CFD-DEM parallel numerical simulation method is constructed to research the mixed pulsed fluidized bed performance. Particle mixing, motion and heat transfer characteristics under different pulse frequencies and flow ratios are studied. Results show that particles in the mixed pulsed fluidized bed exhibit regular periodic motion, thereby promoting the mixing effect of particles. Moreover the bubble nucleation point moves to the bottom of the bed with the increasing pulse frequency. When the total gas velocity is relatively low, particle mixing effect can be enhanced by increasing the proportion of pulsed gas. However, when the velocity is relatively high, particle mixing effect will be enhanced by decreasing the proportion.     

DEM simulation of the shear behaviour of breakable granular materials with various angularities

Particle shape is an important factor that affects particle breakage and the mechanical behaviour of granular materials. This report explored the effect of angularity on the mechanical behaviour of breakable granular materials under triaxial tests. Various angular particles are generated using the quasi-spherical polyhedron method. The angularity α is defined as the mean exterior angle of touching faces in a particle model. A breakable particle is constructed as an aggregate composed of coplanar and glued Voronoi polyhedra. After being prepared under the densest conditions, all assemblies were subjected to triaxial compression until a critical state was reached. The macroscopic characteristics, including the shear strength and dilatancy response, were investigated. Then, particle breakage characteristics, including the extent of particle breakage, breakage pattern and correlation between the particle breakage and energy input, were evaluated. Furthermore, the microscopic characteristics, including the contact force and fabric anisotropy, were examined to probe the microscopic origins of the shear strength. As α increases, the peak shear strength increases first and then remains constant, while the critical shear strength generally increases. Assemblies with larger angularity tend to cause more serious particle breakage. The relative breakage is linearly correlated with α under shear loading. Compared with unbreakable particles, the peak shear strength and the critical volumetric strain decline, and the degree of decline linearly increases with increasing α.     

Development of a method for estimating particles mixing curves in short DEM simulation time

A new method for estimating particles mixing curves by simulating the particles behavior for a short period of time using discrete element method (DEM) was developed. The mixing curve is the time variation of the mixing degree, and can be divided into the following two stages: one is the mixing degree increasing stage, and the other is the mixing degree stagnation stage. The mixing curves could be estimated from time variations of mixing and de-mixing rates by assuming that the stagnation occurs when the mixing and de-mixing are balanced. Assuming the mixing and de-mixing rates are represented by the first-order exponential functions, each rate was estimated by the simulations of particles behavior for a short period of time. The estimated mixing curves agree with experimental ones and the proposed estimation method reduced the calculation time to approximately one-fifth of the conventional one that does not use a method to reduce the mixing time. Therefore, the proposed estimation method could estimate the mixing curve in a short period of time.     

Simulation of intense particle beams with regularly distributed Gaussian subbeams

Techniques for the simulation of intense particle beams are investigated with respect to the required number of simulation particles. It is shown that for nonchaotic systems it is advantageous if the particles initially are not distributed in a statistical manner but rather arranged in a regular pattern in phase space. This reduces the number of required simulation particles drastically. In the case of such an initially regular arrangement of particles the algorithm which assigns the charges of the particles to the computation mesh becomes of prime importance. The performances of different commonly used algorithms are investigated. The Gaussian assignment algorithm proved far superior to other more commonly used techniques, allowing simulations even at the theoretical limit of 1 particle per cell. Examples for very accurate simulations of beam dynamics with very few particles using an initially regular mesh of particles and Gaussian assignment are given.     

异形粒子测量中粒子有序排列问题的研究

由前向衍射图样可以测量粒子尺寸，但是难于获得关于粒子的全部信息，文中在反向散射理论分析的基础上提出了一种能精确测量红细胞几何尺寸和表面凹陷深度的测量仪器，并说明了被测粒子有序排列的方法及意义。     

单层曲面复眼成像系统的优化设计

与传统曲面复眼结构不同,提出了在曲面基底上设计非均一微透镜阵列的构想.整个透镜阵列呈环状对称分布,沿径向排列的各级透镜其焦距由所处位置决定,与基底到光探测阵列的距离相吻合.根据几何光学成像原理计算了各级透镜的设计参数并通过光线追迹加以验证.仿真结果表明,这种设计可对全视场在光探测阵列上聚焦,解决了传统曲面复眼边缘视场成像质量急剧下降的问题.同时还论证了采用光刻胶热熔法在曲面基底上制作非均一微透镜阵列的可行性.     

Magnetic coupling properties of multiple metal wear particles for high-precision electromagnetic debris detection applications

The electromagnetic wear particle detector is one of effective method to monitor wear debris in lubricating oil for assessing the wear condition of mechanical equipment. However, the motion of wear particles, especially the aggregation behavior in both fluid field and magnetic field, may make the particle detector generate false wear signals. Therefore, to estimate the impact on the detection accuracy of wear debris by the particle aggregation effect, the magnetic coupling model of multiple wear particles is established for studying the magnetic coupling effect between adjacent metal particles. The research results show that the effect changes the total magnetic energy variation induced by the particles and then affects the amplitude of particle signal. Meanwhile, the variation degree is closely associated with the frequency of magnetic field of particle detectors. Overall, the aggregation of wear particles with the same magnetic properties (both ferromagnetic or non-ferromagnetic) leads to false alarms of particle detectors; and the aggregation of wear particles with different magnetic properties causes missing alarms of particle detectors. Meanwhile, increasing the field frequency may increases the probability of missing alarm failure of particle detectors.     

Numerical study of the mixing process of binary-density particles in a bladed mixer

Discrete element method (DEM) simulations of binary mixing of particles with different densities were conducted to study the influence of density ratio, blade speed, and filling level on the particle dynamics and mixing performance in a bladed mixer. Four particles with different densities at different locations were tagged to discuss the influence of three factors on the particle trajectory and velocity field in the mixer. A method based on cubic polynomial fitting of relative standard deviation was used to determine the critical revolution during the mixing process. It was found that the non-dimensional tangential velocity decreases with the increase of the blade speed and filling level, the fluctuation of vertical velocity increases with the radial location, blade speed, and filling level, and it is more pronounced than the fluctuation of tangential and radial velocity during the mixing process. Results obtained indicate that the mixing performance of particles with different density increases with the decrease of density ratio and filling level, while it increases with the increase of blade speed.     

Host-guest composites containing ultrasonically arranged particles

Ultrasonic particle arrangement method was used for the fabrication of composite materials with layer or lattice structures. Guest particles were periodically arranged in a host solution by using ultrasonic standing waves, and then the solution was solidified to obtain a solid composite material. The sample cell was rotated during the solidification process to prevent particle sedimentation. Polymer, glass, or metal particles with various shapes were ultrasonically arranged in a polysiloxane resin, which was useful as a host material due to its simple solidification process and suitability for forming fine structures. Composite materials with periodic structures were successfully fabricated by this method.