Experimental investigations of granular shape effects on the generation of electrostatic charge

In solid processing systems, electrostatic problems are commonly observed for granules of various shapes. However, a complete understanding of the basic dependence of electrostatic charge generation on particle shape has yet to be established. This observation motivated the present study on examining the effect of granular shape on electrostatics. In this study, polyvinyl chloride （PVC） granules （diameter 1.1-4.1 ram, in the shape of a triangle or trapezium） were first discharged to remove any residual charges and sub- sequently their electrostatic charging characteristics were studied by allowing a granule to slide along a pipe wall. Several factors such as granular front-facing angle, length-ratio, sliding area, sliding orienta- tion, sliding times, and relative humidity were considered when studying their effects on the electrostatic charging of granules. It was found that triangular granules with smaller front-facing angles tended to generate more electrostatic charge. The amount of electrostatic charge increased with granular length- ratio and sliding area but decreased with humidity. In addition, granular sliding in the orientation of the front-facing angle （for triangular granules） or the short side （for trapezoidal granules） generated more electrostatic charge than that in the orientation of the long side. For both granule shapes, the elec- trostatic charge increased with granular sliding times and reached a saturated state after around 8-9 sliding movements. The saturated electrostatic charge increased with either granular length ratio or sliding area.     

Influence of liquid layers on energy absorption during particle impact

The influence of the thickness of a covering liquid layer and its viscosity as well as the impact velocity on energy loss during the normal impact on a flat steel wall of spherical granules with a liquid layer was studied. Free-fall experiments were performed to obtain the restitution coefficient of elastic-plastic γ- Al2O3 granules by impact on the liquid layer, using aqueous solutions of hydroxypropyl methylcellulose with different concentrations for variation of viscosity （1-300 mPa s）, In the presence of a liquid layer, increase of liquid viscosity decreases the restitution coefficient and the minimum thickness of the liquid layer at which the granule sticks to the wall. The measured restitution coefficients were compared with experiments performed without liquid layer. In contrast to the dry restitution coefficient, due to viscous losses at lower impact velocity, higher energy dissipation was obtained, A rational explanation for the effects obtained was given by results of numerically solved force and energy balances for a granule impact on a liquid layer on the wall. The model takes into account forces acting on the granule including viscous, surface tension, capillary, contact, drag, buoyancy and gravitational forces. Good agreement between simulations and experiments has been achieved.     

Simulation and calibration of granules using the discrete element method

The discrete element method (DEM), developed by Cundall and Strack (1979) to solve geomechanical problems, is used to simulate the mechanical behavior of granules. According to the DEM, an individual granule can be modeled as a realistic mechanical system consisting of primary particles bonded by interaction forces.Granulometric properties of the model material, zeolite 4A, have been measured to determine their macro properties. To investigate the compression behavior, a compression test was performed using a strength tester on single granules between two pistons. A modeled granule consisting of more than 22,000 primary particles was generated. The micro properties of the modeled granule have been precisely set to allow its macro properties to be equivalent to the macro properties of zeolite 4A granules. To calibrate the mechanical properties, diametrical compression was simulated using two rigid walls stressed at a constant stressing velocity. The force–displacement curve of the modeled granule at compression has been calibrated by the experimental curve of zeolite 4A.     

A novel dual-material probe for in situ measurement of particle charge densities in gas–solid fluidized beds

Particle charge density is vitally important for monitoring electrostatic charges and understanding particle charging behavior in fluidized beds. In this paper, a dual-material probe was tested in a gas–solid fluidized bed for measuring the charge density of fluidized particles. The experiments were conducted in a two-dimensional fluidized bed with both single bubble injection and freely bubbling, at various particle charge densities and superficial gas velocities. Uniformly sized glass beads were used to eliminate complicating factors at this early stage of probe development. Peak currents, extracted from dynamic signals, were decoupled to determine charge densities of bed particles, which were found to be qualitatively and quantitatively consistent with charge densities directly measured by Faraday cup from the freely bubbling fluidized bed. The current signals were also decoupled to estimate bubble rise velocities, which were found to be in reasonable agreement with those obtained directly by analyzing video images.     

A novel dual-material probe for in situ measurement of particle charge densities in gas-solid fluidized beds

Particle charge density is vitally important for monitoring electrostatic charges and understanding particle charging behavior in fluidized beds.In this paper,a dual-material probe was tested in a gas-solid fluidized bed for measuring the charge density of fluidized particles.The experiments were conducted in a two-dimensional fluidized bed with both single bubble injection and freely bubbling,at various particle charge densities and superficial gas velocities.Uniformly sized glass beads were used to eliminate complicating factors at this early stage of probe development.Peak currents,extracted from dynamic signals,were decoupled to determine charge densities of bed particles,which were found to be qualitatively and quantitatively consistent with charge densities directly measured by Faraday cup from the freely bubbling fluidized bed.The current signals were also decoupled to estimate bubble rise velocities,which were found to be in reasonable agreement with those obtained directly by analyzing video images.     

ELECTROSTATIC PHENOMENA IN GAS-SOLIDS FLUIDIZED BEDS

Electrostatic charges are generated by particle-wall, particle-particle and particle-gas contacts in gas-solids transport lines and fluidized bed reactors. High particle charge densities can lead to particle agglomeration, particle segregation, fouling of reactor walls and internals, leading to undesirable by-product and premature shut-down of processing equipment. In this paper, the charge generation, dissipation and segregation mechanisms are examined based on literature data and recent experimental findings in our laboratory. The particle-wall contact charging is found to be the dominant charge generation mechanism for gas-solids pneumatic transport lines, while bipolar charging due to intimate particle-particle contact is believed to be the dominant charge generation mechanism in gas fluidized beds. Such a bipolar charging mechanism is also supported by the segregation patterns of charged particles in fluidized beds in which highly charged particles tend to concentrate in the bubble wake and drift region behind rising bubbles.     

轴承钢早期胶合故障静电在线监测方法及试验

采用自制静电传感器进行润滑条件下轴承钢点接触滑动摩擦胶合故障在线监测试验研究.首先分析了磨损过程静电产生机理和静电感应原理,在此基础上设计研制了磨损区域静电传感器并进行轴承钢胶合故障试验,研究了磨损区域静电信号去噪方法,分析了载荷和转速对摩擦系数和静电信号的影响.研究结果表明:奇异值差分谱理论适用于静电信号去噪,取得了较好的效果;静电信号与摩擦系数同时监测到胶合的发生,具有一致性,并且静电在线监测技术能够在胶合故障发生前监测到异常,为视情维修提供支持;静电信号反映了4个阶段(磨合磨损,稳定磨损,早期胶合以及胶合阶段);在稳定磨损阶段,摩擦系数随载荷的增大而降低,随转速的升高而降低,静电信号均方根值随载荷的变化很小,随转速的升高而增大.     

Variation of granule mass fraction with coordination number in wet granulation process

In granulation, fine particles combine to form a coarse granule in the form of a particle matrix partially or fully saturated with a binder liquid. The final product of granulation possesses a wide variety of granule size distributions with surface mean diameters which differ with operating conditions. The final granule size depends on the operating conditions, e.g. operating gas velocity, inlet air temperature, initial feed particle size, and viscosity of the binder. The objective of this paper is to find out the uniformity in the relation between the granule mass fraction in the final granule size distribution and the number of feed particles present in the granules. The total number of granules obtained depends on the experimental conditions but the granule mass fraction and the number of feed particles forming a single granule are independent of operating variables, feed material and method of granulation. The paper purports further to compare the uniform nature of mass fraction of the granules in final granule size distribution and the primary particles required to form that particular granule size irrespective of experimental conditions of granulation.     

Instability of a charged droplet in an inhomogeneous electrostatic field of a thin rod

The problem of stability of oscillations of a charged droplet in an inhomogeneous electrostatic field of a thin charged rod is investigated in the nonlinear formulation using the asymptotic expansion in two small parameters, namely, the dimensionless equilibrium droplet strain and the ratio of the droplet oscillation amplitude to the droplet radius. It is shown that, when the droplet charge is less than the Rayleigh critical charge, in the inhomogeneous electrostatic field the droplet instability implementation mechanism remains the same as for the charged droplet in the field of a point charge. As the oscillation mode number increases, the critical field parameter reaches saturation tending to the horizontal asymptotics. The longer the rod, the higher the level of the asymptotics. As the rod length increases, the amplitudes of the related droplet oscillations and the increments of the unstable droplet oscillations in the electrostatic field of the rod decrease.     

Equilibrium charge and triboelectric coefficient of spheres in a rotating container

Contact electrification occurs in many granular material systems due to particle-particle and particle-wall contacts. In this paper, we used a simple device to characterize the electrification of different spheres in a rotating spherical container. The charge of the spheres was quantified by a Faraday cup after rotating for various time and the disperse behaviors of charged spheres were recorded by a camera. The effects of the rotational time, sphere size, rotational speed, sphere material, and relative humidity on the equilibrium charge and triboelectric coefficient were analyzed. The results show that the equilibrium surface charge density is independent of the sphere size and the rotating speed, whereas the larger rotating speed enhances the triboelectric coefficient. It is also shown that charge relaxation constants are two orders of magnitude lower than the charge generation constants for all spheres at a low relative humidity. The increase of water molecules in the air was found to result in the decline of charge generation constant and the rise of charge relaxation constant, which together cause the decrease of the equilibrium surface charge density.     

The charged dumbbell model for dilute polyelectrolyte solutions in strong flows

A charged dumbbell model is used to investigate the behavior of dilute polyelectrolyte solutions in a general linear two-dimensional flow. The model studied has a nonlinear spring, conformation dependent friction and a Coulombic repulsive force due to an effective electrostatic charge on the two beads. The relative importance of the electrostatic charge is reflected by an effective charge density parameter,E. Equilibrium properties such as end-to-end distance and intrinsic viscosity are strongly dependent onE. In strong flows, which produce a dramatic increase in the dumbbell dimensions (a coil-stretch transition), the onset behavior is influenced byE. IncreasingE causes the onset velocity gradient to shift to much lower values. Large values ofE change the qualitative behavior to that of rigid (or slightly extensible) macromolecules or fibers. Results are presented for a charged dumbbell at equilibrium, in steady flows, and in transient flows.     

Structural and electrical dynamics of a grating-patterned triboelectric energy harvester with stick–slip oscillation and magnetic bistability

The majority of research work on triboelectric energy harvesting is on material science, manufacturing and electric circuit design. There is a lack of in-depth research into structural dynamics which is crucial for power generation in triboelectric energy harvesting. In this paper, a novel triboelectric energy harvester with a compact structure working in sliding mode is developed, which is in the form of a casing and an oscillator inside. Unlike most sliding-mode harvesters using single-unit films, the proposed harvester utilizes grating-patterned films which are much more efficient. A bistable mechanism consisting of two pairs of magnets is employed for broadening the frequency bandwidth. A theoretical model is established for the harvester, which couples the structural dynamics domain and electrical dynamics domain. This paper presents the first study about the nonlinear structural dynamics of a triboelectric energy harvester with grating-patterned films, which is also the first triboelectric energy harvester integrating grating-patterned films with a bistable magnetic system for power performance enhancement. Theoretical studies are carried out from the perspectives of both structural and electrical dynamics. Surface charge density and segment configuration of the films affect whether the electrostatic force influences the structural dynamics, which can be neglected under a low surface charge density. Differences in structural response and electrical output are found between a velocity-dependent model and Coulomb’s model for modelling the friction in the triboelectric energy harvesting system. The bistable mechanism can effectively improve the output voltage under low-frequency excitations. Additionally, the output voltage can also be obviously enhanced through increasing the number of the hollowed-out units of the grating-patterned films, which also results in a slight decrease in the optimal load resistance of the harvester. These findings enable innovative designs for triboelectric energy harvesters and provide fabrication guidelines in practical applications.

     

静电雾化锥射流模式下液锥形态的研究

本文分析了静电雾化锥射流模式下液锥表面静电应力、表面张力应力分布特性,基于应力平衡建立了液锥力学模型,并对流量、荷电电压及针形喷嘴的内半径等参数对液锥结构形态的影响进行了预测。首先设计了针形喷嘴静电雾化实验装置,应用高速摄影技术观测了静电雾化的典型雾化模式和液锥形态演化特性。实验结果表明:锥射流雾化模式仅在一定的荷电电压范围内才会出现;针形喷嘴的流量增加,液锥锥角减小,液锥长度增长;随着荷电电压或针形喷嘴内半径的增加,液锥锥角增大,液锥长度缩短。实验结果与液锥力学模型的预测结果一致。     

Effect of impeller design on homogeneity,size and strength of pharmaceutical granules produced by high-shear wet granulation

Small mixer impeller design is not tailored for granulation because impellers are intended for a wide range of processes. The aim of this research was to evaluate the performances of several impellers to provide guidance on the selection and design for the purposes of granulation. Lactose granules were produced using wet granulation with water as a binder. A Kenwood KM070 mixer was used as a standard apparatus and five impeller designs with different shapes and surface areas were used. The efficacy of granulate formation was measured by adding an optically sensitive tracer to determine variations in active ingredient content across random samples of granules from the same size classes. It was found that impeller design influenced the homogeneity of the granules and therefore can affect final product performance. The variation in active ingredient content across granules of differing size was also investigated. The results show that small granules were more potent than larger granules.     

Numerical simulation on magnetic confinement characteristics of internal vortex electrostatic cyclone precipitator under different working voltages

Aiming at improving the capture performance of internal vortex electrostatic cyclone precipitator (ECP), a theoretical model with mechanics-electric-magnetic coupling was established, the collection efficiency of magnetic confinement ECP under different working voltages was simulated, and the influence of magnetic flux intensity on the removal performance of submicron particles was explored. Results show that the number of particles escaped from the cyclone is greatly reduced after the introduction of magnetic field and electric field, indicating that charging effect and magnetic confinement are more conductive to trap submicron particles in the internal vortex ECP. The lower the working voltage is, the worse the charging lifting effect is, but the stronger the magnetic confinement characteristics are. Furthermore, the contributions of charging effect to collection efficiency and magnetic confinement characteristics are more obvious at a weaker magnetic flux density. The research results can provide a practical new idea for the innovative design of ECP.     

Investigation on the granulation behavior of iron ore fine in a horizontal high-shear granulator

High-shear granulation is widely used in many particulate industries for its good capability to improve the size, strength and composition uniformity of powder substances. This work conducted an experimental study to investigate the granulation behavior of iron ore fine in a horizontal high-shear granulator, such as granules size distribution, granules growth rate, and permeability of the granules bed. The results show that the granule size and permeability of packed granules bed increase gradually with increasing the granulation time, and the growth of granules can be divided into three stages: the rapid growth stage, the slow growth stage and the relatively stable stage. Both the higher rotational speed and larger number of impellers increase the kinetic energy and collision frequency of the particles, which causes the increase of average granule size, growth rate and permeability of granules packed bed. On the other hand, the shear damage effect of the impellers on the granules is also enhanced with the increase of rotational speed and impeller number, resulting in significant granule size segregation.     

Modeling the electrostatic effect on the hydrodynamic behavior in FCC risers: From understanding to application

A CFD simulation was proposed to investigate the electrostatic effect on the hydrodynamic behavior of turbulent gas–solid flow in FCC risers. The simulation was first verified using the open experimental data with expected electrostatic effects observed in FCC risers. The influences of several operating parameters on the degree of electrification in FCC risers were analyzed, such as surface charge densities, pressure, gas velocity. It was noted that the gas velocity played a highly significant role compared with solid flux, while the effect of pressure was relatively weak. Further analysis showed that a much stronger electrostatic effect was found in small-scale FCC risers than their large-scale counterparts, and in addition, the major regions affected by the electrostatic charge depend on the scale of the riser. Finally, an external electric field was applied to optimize the flow field distribution in the FCC riser. The results of the electrostatic effects on the hydrodynamic behaviors in FCC risers are of great use in providing a reference for the optimization of FCC risers and their scaling.     

Electrostatic beneficiation of fly ash in a free-falling system

A systematic study of fly ash electrostatic beneficiation in a free-falling separation system was carried out to provide fundamental understanding of the separation efficiency for the design of a suitable process for industrial applications. The parameters investigated included feeding position, electric field strength, particle size and moisture content. Particles larger than 105 μm presented the best separation efficiency among four different size fractions, whereas particles smaller than 44 μm showed minimal separation. However, sonication treatments helped separation by liberating more carbon from ash particles, although particle sizes were reduced as well. Experiments also showed that exposure to moisture significantly altered charging behavior of fly ash and its subsequent separation due to more free mobile ion-induced charge exchanges. The optimal feeding position was found to be slightly on the side of the negative electrode, leading to a 30% reduction in loss-on-ignition (LOI) and a 45% recovery in a single pass. A simplified mechanical model based on trajectory analysis for charged particles in an electrical field was in reasonable agreement with experimental results.     

The Motion Behavior of a Group of Charged Droplets in Liquid-Liquid Electrostatic Spray

The droplet sizes and electrical charges under different applied electrical voltages are experimentally measured for a liquid-liquid electrostatic spray system. Considering droplet size and charge distributions, the two-dimensional motion for a group of charged droplets in a liquid-liquid electrostatic atomization system is simulated. From measured droplet size and charge distributions, the simulation can obtain the velocities and positions in a two-dimensional domain for all simulated droplets at different times. The various forces acting on droplet as well as their effects on droplet velocity and trajectory are analyzed and the liquid-liquid electrostatic atomization characteristics are revealed. In addition, for one-dimensional motion trajectory of larger droplet, the comparison between simulation and experiment is also conducted and a general agreement can be obtained.     

Modelling die filling with charged particles using DEM/CFD

The effects of electrostatic charge on powder flow behaviour during die filling in a vacuum and in air were analysed using a coupled discrete element method and computational fluid dynamics (DEM/CFD) code, in which long range electrostatic interactions were implemented. The present 2D simulations revealed that both electrostatic charge and the presence of air can affect the powder flow behaviour during die filling. It was found that the electrostatic charge inhibited the flow of powders into the die and induced a loose packing structure. At the same filling speed, increasing the electrostatic charge led to a decrease in the fill ratio which quantifies the volumetric occupancy of powder in the die. In addition, increasing the shoe speed caused a further decrease in the fill ratio, which was characterised using the concept of critical filling speed. When the electrostatic charge was low, the air/particle interaction was strong so that a lower critical filling speed was obtained for die filling in air than in a vacuum. With high electrostatic charge, the electrostatic interactions became dominant. Consequently, similar fill ratio and critical filling speed were obtained for die filling in air and in a vacuum.