Numerical simulation of motion of rigid spherical particles in a rotating tumbler with an inner wavelike surface

The present work is a numerical simulation of motion of rigid spherical particles within a 2-D tumbler with an inner wavelike surface. The rotation of the tumbler is simulated as a traveling sine wave around a circle. The discrete element method (DEM, a hard sphere approach) is used. The particle-wall interactions are taken into account in a changed numerical approach of hard sphere model. The effects of two basic factors of the rotating velocity (phase velocity) and the wave numbers are separately investigated. A simple but useful method for cluster identification is provided and used. The energy-based analysis of particle clusters and the motion pattern study indicate the existence of a pulsed variation in the kinetic energy of the clusters at low wave numbers and a cyclic bulk motion of the clusters at high wave numbers. The necessary conditions for the pulsed variation of motion of particle clusters at low wave number are analyzed and a mode for industrial application, e.g. coal grinding process in power plant, is demonstrated.     

Nonoclusters of iron prepared by different methods and analyzed by various spectroscopic techniques

Two different preparative methods for loading zeolites with zerovalent iron clusters are compared: in the first case Fe(CO)₅ is adsorbed by NaX zeolite at low temperatures and decomposed under vacuum conditions up to 673 K. The iron clusters inside the cages of the zeolite are investigated by ferromagnetic resonance and Mössbauer spectroscopy. The second case deals with a sample prepared by ion exchange of NaA zeolite by means of a calculated amount of ferrous ions followed by reduction under an excess of sodium vapor at 573 K. Here the iron clusters are analyzed by Mössbauer and magnetization measurements. In both cases cluster composition and cluster sizes are comparable. The clusters are termed “nanoclusters” because their diameter is in the nanometer range ( 2 nm). We propose using these clusters for Fischer-Tropsch reactions, and we have achieved promising preliminary results.     

Electrical properties of multicomponent CMM in correlation with their microstructure

In this work, electrical properties (resistance, resistance temperature coefficient, thermoelectric voltage, current–voltage curve) of multi-component ceramic matrix materials have been investigated. The influence of the structure of discrete particles on the microstructure and properties relations of CMM composites has been studied. Three groups of resistive materials were analyzed: (a) high resistance matrix and percolation clusters; (b) ionic or (c) covalent ceramic matrix and metal-like percolation clusters.     

Axial and lateral dispersion of fine particles in a binary-solid riser

Axial and lateral mixing of fine particles in a binary-solid riser have been investigated using a phosphor tracer method. The measured bimodal residence time distribution (RTD) demonstrated two types of axial dispersions of the fines: the dispersion of discrete particles and that of clusters. A proposed one-dimensional, bimodal dispersion model describes the bimodal RTDs very well. The axial Peclet number of the fines is not sensitive to the fraction of coarse particles, gas velocity and solids circulation rate. Lateral solids dispersion was determined by measuring the solids RTD at different radii using a point source tracer injection. A two-dimensional dispersion model describes the measured RTDs satisfactorily. Lateral solids mixing decreased as coarse particles were added into the riser. Correlations of the axial and lateral Peclet numbers obtained fit the experimental data well.     

基于EDEM的双卧轴搅拌机合理充盈率探究

基于EDEM离散元软件,对双卧轴搅拌机在不同充盈率下物混合料搅拌均匀性进行仿真分析,探究合适的充盈率。首先从理论上分析充盈率对搅拌均匀性的影响;其次在EDEM中设置3种不同充盈率进行运动仿真,最后通过对比不同充盈率下骨料和粉料离散系数随时间的变化情况,分别计算各自的有效生产量。综合仿真情况得出:较合理的充盈率为0.35,此时有效出料量最大,总体上阻力相对较小。     

Phase Behavior and Micro-Structure of Fat–Oil Mixtures: Engineering the Shape of Fat Clusters

The mechanical properties of fat–oil mixtures are dictated by the morphology of the fat crystal network, which in turn is strongly affected by the processing conditions. In this work the effect of uniform (linear) cooling rates on the size and shape of tribehenin (fat) clusters in isopropyl myristate (oil) was studied. The state of the fat–oil mixtures was comprehensively analyzed by establishing pseudo‐phase diagrams via visual observations, brightfield and polarization microscopy at various cooling rates and fat concentrations. The microstructure of the constituent fat clusters undergo a shape transition from bundles of needles at low cooling rates to spherical clusters at high cooling rates. The network structures formed with these different shaped fat clusters were examined using rheology. At a given cooling rate, the storage modulus (G?) of fat–oil mixtures versus fat concentration followed a power law relationship. For a given fat concentration, the fat crystal networks comprising of bundles of needles showed higher G? values as compared to fat networks made of spherical clusters. While the fractal dimension of networks cooled at different cooling rates varied over a small range of 2.78–2.86, considerable change in the pre‐exponential factor (γ) was observed. The results show the critical importance of processing conditions on the shape and size of fat clusters and its impact on the rheological properties of the fat crystal network.     

Extraction of cadmium by TODGA–dodecane and TBP–dodecane: A comparative study by MD simulation

Molecular dynamics (MD) simulation of [Cd(II)] along with nitrate ions and water in dodecane was carried out for different nitric acid concentrations. The extraction process using N,N,N’,N’-tetraoctyldiglycolamide (TODGA) and tributyl phosphate (TBP), in biphasic systems, is also simulated at three nitric acid concentrations. In the TBP-based system, the formation of a third phase was observed at 3 M nitric acid concentration. Cd(II) ions form reverse micelles-like clusters with TODGA as an extractant in dodecane. The mass percentage of TODGA in these clusters decreases with increase in the acid concentration while increasing the size of the aggregates at the same time.     

Comparison of fibre optical measurements and discrete element simulations for the study of granulation in a spout fluidized bed

Spout fluidized beds are frequently used for the production of granules or particles through granulation. The products find application in a large variety of applications, for example detergents, fertilizers, pharmaceuticals and food. Spout fluidized beds have a number of advantageous properties, such as a high mobility of the particles, which prevents undesired agglomeration and yields excellent heat transfer properties.A discrete element model is used describing the dynamics of the continuous gas phase and the discrete droplets and particles. For each element momentum balances are solved. The momentum transfer among each of the three phases is described in detail at the level of individual elements.The results from the discrete element model simulations are compared with local measurements of time time-averaged particle volume fractions as well as particle velocities by using a novel fibre optical probe in a fluidized bed of 400 mm I.D. Simulations and experiments were carried out for three different cases using Geldart B type aluminium oxide particles: a freely bubbling fluidized bed; a spout fluidized bed without the presence of droplets and a spout fluidized bed with the presence of droplets. It is found that the experimental and numerical results agree in a qualitative manner.It is demonstrated how the discrete element model can be used to obtain information about the interaction of the discrete phases, i.e. the growth zone in a spout fluidized bed. Additional analysis of the numerical results indicates that liquid breakthrough does not take place for the studied test case.     

Defect Structures of Tin-Doped Indium Oxide

Defect structures associated with tin doping of indium oxide, an optically transparent conductor, have been characterized by atomistic simulations and first-principles density functional calculations. A comprehensive survey of defect clusters containing up to three tin dopants in the first and second cationic coordination shells of an oxygen interstitial has been conducted. The analysis of energetically favorable defects gives insights into the role and nature of defect clusters in the material. In particular, the origins of the experimentally postulated b-site preference of tin dopants have been examined. Our results show that b-site preference occurs only in defect clusters with oxygen interstitials and is not intrinsic to dopants. In contrast, in nearest coordination to an interstitial, a strong d-site preference is found. Density functional calculations in the discrete variational-embedded cluster approximation have been conducted on selected defect structures to illuminate the effect of clustering on partial atomic charges, bond-orders, and ¹¹⁹Sn Mössbauer parameters.     

A CFD-DEM study of the cluster behavior in riser and downer reactors

This paper presents a numerical study of the particle cluster behavior in a riser/downer reactor by means of combined computational fluid dynamics (CFD) and discrete element method (DEM), in which the motion of discrete particles is obtained by solving Newton's equations of motion and the flow of continuum gas by the Navier-Stokes equations. It is shown that the existence of particle clusters, unique to the solid flow behavior in such a reactor, can be predicted from this first principle approach. The results demonstrate that there are two types of clusters in a riser and downer: one is in the near wall region where the velocities of particles are low; the other is in the center region where the velocities of particles are high. While the extent of particle aggregation appears to be similar, the duration time for the first type in a downer is shorter than in a riser. Furthermore, it is demonstrated that the formation of clusters is affected by a range of variables related to operational conditions, particle properties, and bed properties and geometry. The increase of solid volume fraction, sliding and rolling friction between particles or between particles and wall, or damping coefficient can enhance the formation of clusters. The use of multi-sized particles can also promote the formation of clusters. But the increase of gas velocity or use of a wider bed can suppress the formation of clusters. The van der Waals force may enhance the formation of clusters when solid concentration is high but suppress the formation of clusters near the wall region when solid concentration is low.     

Numerical simulation of hydrodynamics in downers using a CFD-DEM coupled approach

The gas-solid flows in a two-dimensional downer of 10 m in height and 0.10 m in width were simulated using a CFD-DEM method, where the motion of particles was modeled by discrete element method (DEM) and the gas flow was described by Navier-Stokes equations. The simulations revealed a rich variety of developing flow structures in the downer under different operating conditions. The two-phase flow development can be clearly characterized by the micro-scale particle distributions in the downer. Near the inlet, the particle distribution is dominated by the distributor design. Then, the particles disperse in the column, forming a homogeneous transit region. After that clusters start to form and modulate the gas-solid flow field till the fully-developed state. The particle-scale simulation disclosed that the clusters are composed of loosely collected particles, and these particles have the same flow direction as the bulk flow so that no particle backmixing can be observed. As the particles in the downer have the tendency to maintain the inertia, the capability of lateral transfer of particles is relatively weak, which was illustrated by tracking the movement of the single particles and clusters. The simulations of the inlet effect on the hydrodynamics in the downer showed that the gas-solid flow structure and the mixing behavior are sensitive to the inlet design. An inappropriate design or operation would probably cause the undesired flow phenomena such as the wide distribution of residence times. The time-averaged hydrodynamics based on the transient simulations showed good agreement with the experimental findings in the literature. The simulation based on the CFD-DEM coupled approach provides a theoretical way to comprehensively understand the physics at micro- to macro-scales in the co-currently downward gas-solid flows.     

垂直并流向上气固两相流中流动结构的分析及曳力系数的计算

This study investigates the heterogeneous structure and its influence on drag coefficient for concurrent up gas-solid flow.The energy-minimization multi-scale (EMMS) model is modified to simulate the variation of structure parameters with solids concentration,showing the tendency for particles to aggregated to form clusters and for fluid to pass around clusters.The global drag coefficient is resolved into that for the dense phase,for the dilutephase and for the so-called inter-phase,all of which can be obtained from their respective phase-specific structure parameters.The computational results show that the drag coefficients of the different phases are quite different,and the global drag coefficient calculated from the EMMS approach is much lower than that from the correlation of Wen and Yu.The simulation results demonstrate that the EMMS approach can well describe the heterogeneous flow structure,and is very promising for incorporation into the two-fluid model or the discrete particle model as the closure law for drag coefficient.     

An investigation of the shape of small platinum clusters using the embedded atom method

The equilibrium shape and structure of small platinum clusters with 5–60 atoms was investigated using the embedded atom method (EAM). An analysis of the stability of the clusters with polyhedral symmetries indicated that, except at the magic number sizes, the clusters spontaneously reconstructed to more stable shapes at 0 K. Even the magic number icosahedra and cubo-octahedra were unstable upon annealing at 500 K. The clusters which reconstruct to lower energy structures appear to be highly asymmetric and irregular. Many different structures were found to have similar energies. The annealed clusters also display surface features not found in bulk platinum.     

The adsorption site of ammonia at copper surfaces

The adsorption of ammonia at copper surfaces was studied using Hartree-Fock-Slater LCAO calculations to investigate ammonia interaction with copper clusters. Important factors influencing ammonia adsorption were established from trends in its behaviour at clusters of different size. Adsorption at different coordination sites was examined in light of these results and it was found that a strong electrostatic contribution directs the ammonia towards lower coordination sites.     

Verification of Ozone Clusters (O6 & O9)

The use of silica gel for the storing of ozone was investigated. The ozone gas that was discharged from the silica gel was found to be different from normal ozone which is generated by the ozone generator that makes ozone with electric discharge. Ozone clusters could be synthesized by this method, which detaches ozone from silica get without requiring any additional energy. These ozone clusters didn't decompose at room temperature and atmospheric pressure over a half-day. We confirmed that the cluster was heavier than a heavy standard gas, the presence of clusters with an analysis using GC/MS, and the existence of ozone clusters by the bonding energy using a computer simulation by MOPAC.     

排尘口尺寸对单管气相流场影响的模拟研究

应用计算流体力学软件Fluent，对三级旋风分离器内的单根PSC－250型旋风管进行了模拟计算。在计算过程中采用经过验证的计算模型和离散方法：湍流模型采用各向异性的RSM模型，离散方式采用对流项的QUICK格式，压力梯度项采用PRESTO格式，对两种不同排尘口结构的单管的流场进行了详细分析，为以后单管结构的进一步改进和优化提供参考。     

Controlled surface topography of nanostructured particles prepared by spray‐drying process

Nanostructured particles (clusters) with complex and periodic topography at the microscopic scale show unique structural patterns. Hence, good properties should be obtained when the surface topography of such clusters, especially those containing nanoparticles (NPs) of different sizes, can be carefully tuned. The coffee‐ring structure is one of the most interesting structures for catalyst and photonic crystal applications or porous particle molds. Here, well‐defined clusters with a coffee‐ring structure were prepared by spray drying. The complexity of the NP distribution in the resulting two‐ and three‐component systems is discussed. A better understanding of how finite groups of different NPs self‐organize in a moving droplet to form a confined geometry may aid in controlling the structure of matter at multiple length scales. Interestingly, the configuration of the large microsphere clusters was found to be influenced by the presence of the small particles, which formed a ring‐like structure. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1503–1511, 2017     

微粒群算法参数的理论分析

微粒群算法是近年来兴起的一种智能优化算法,而算法参数是影响算法性能和效率的关键。用基于常系数非齐次差分方程求解的分析、基于动态系统理论的分析和基于离散系统稳定判据的分析三种不同的方式对微粒的位置和速度两个变量进行了深入理论分析,最终得出了一个共同的结论,即保证微粒收敛的参数取值区域约束在一个直角梯形的内部,这将对算法的实际应用起到很重要的作用。     

Precision characterization of three ultrafine condensation particle counters using singly charged salt clusters in the 1–4 nm size range generated by a bipolar electrospray source

Abstract

Molecular singly charged clusters generated by electrospray sources are commonly used for calibration measurements of ultrafine Condensation Particle Counters (CPCs) in the sub-2?nm size range. This technique has been limited to the smallest singly charged clusters. In this study, we used a bipolar electrospray source combining two electrosprays of opposite polarities to generate singly charged clusters of ammonium salts dissolved in acetonitrile, i.e., tetra-heptyl ammonium bromide (THABr) and tetra-butyl ammonium iodide (TBAI) clusters. A high-resolution UDMA acts as a classifier for positively and negatively charged clusters which are used as mobility standards for the measurement of the complete ascent of the detection efficiency curve of three ultrafine CPCs. The CPCs characterized in this work are two laminar flow-type CPCs using n-butanol (TSI UCPC Model 3776, TSI Inc., Minneapolis, MN USA) and diethylene glycol (DEG) (TSI UCPC Model 3777, TSI Inc.) as working fluid and the turbulent mixing-type DEG-based Particle Size Magnifier (Airmodus A10 PSM). The clusters generated by the bipolar electrospray source are analyzed for their elemental composition using the Atmospheric Pressure interface Time-Of-Flight mass spectrometer (ioniAPi-TOF, Ionicon Analytik GmbH, Austria) in positive and negative ion mode. We present mobility-dependent mass spectra from experiments using the UDMA and the ioniAPi-TOF in series for the analysis of positively charged clusters when operating the electrospray source in unipolar and in bipolar mode to demonstrate the charge reduction and purity of the sample.     

Numerical simulation of particle motion in vibrated fluidized beds

Flow behavior of particles is simulated in a two-dimensional vibrated bubbling fluidized bed. The motion of particles is simulated by discrete element method (DEM). The distributions of velocity and concentration of particles are analyzed at the different amplitudes and frequencies of vibration. In the case with vertical vibration, the bed consists of three different regimes along bed height: a low particle concentration regime with a vibration gap near the bottom, a high concentration regime in the middle of the bed and a transition regime at the bed surface. The gas pressure losses are reduced with the vibration. The phase delay time between the time at the maximum pressure loss and the time at the maximum displacement of vibrated distributor in one cycle decreases with the increase of amplitudes. The contacting interaction between the vibrated distributor and particles increases as the distributor moves up, and trends to zero as the distributor moves down with the increase of frequency of vibration. The vibrating energy is transferred by collisions between the vibrated distributor and particles at the period of acceleration of the distributor.