Critical thicknesses of electrostatic powder coatings from inside

A simple electrostatic model is applied to the charge powder coating of a grounded conductor eventually covered by insulating layers. The electric field inside the powder coating and its evolution during the process are established with also the corresponding evolution in the dielectric layer and some practical consequences are also discussed. The thickness of the charged powder layer is limited by two types of process: a self-limiting process related to the vanishing field in the air gap and a field strength process occurring on one of the two sides of the coating–dielectric interface. Inside the powder coating, the electric field induces an increased electrostatic pressure on the powder grains at the substrate–coating interface and a vanishing pressure on the grains at the coating–air interface. This internal field is amplified into air bubbles and it may be responsible for the back ionisation process and of the formation of moon craters via the ionisation of the air molecules followed by the pressure exerted by the ions pushed in the direction of the free surface. For each of these limits, analytical expressions are established and they permit to identify the role of physical properties of the deposited powder (particle size and dielectric constant) as well as the role of thickness, structure, and dielectric constant of the insulating substrate. The present approach explains the difficulty in obtaining thick coatings on thick insulating substrates or thick coatings from the use of too fine powder grains. Finally, different behaviours as a function of the size of the deposited powder grains are deduced from the contribution of the electric field to the velocity of the particles sticking to the surface. Then and for the first time, the present contribution underlines the important role of the subsurface composition and the need to characterize the structure, dimension and dielectric constant of this subsurface for various applications concerning electrostatic spraying and powder painting of plastics and of insulating coatings on metallic work pieces.     

Corona discharge ion sources for fine particle charging

Charging of aerosol droplets and solid particles is applied in many industrial processes such as electrostatic painting, particle separation and electrostatic precipitation. In most of charging devices, electrical discharges are used as a source of ions, which are deposited onto the particles. In the present paper, the charging process by ionic current in alternating electric field was optimized experimentally. Alternating electric field charger was used as a charging device in these experiments. The current voltage characteristics of electrical discharge in this device, and the charge imparted to the particles were determined. The level of charge was measured at the outlet of the charger and was compared to the Pauthenier limit for different supply voltages, and frequencies. MgO powder was used as a source of particles in these experiments. It was noticed that higher supply voltage of the charger gives higher level of particle charge, but at the same time, the particle deposition on the charger elements was increased, decreasing the particle penetration. A compromise between these two tendencies is therefore necessary. As a result we have proposed a criterion maximizing the total charge born by the particles which is a product of relative particle charge and particle penetration.     

Charged dust grain acceleration in tokamak edges

It is shown that the force associated with the normal component of the space charge electric field at the plasma surface, arising from the charge separation, can accelerate a charged dust particle that is sitting in the scrape-off layer (SOL) close to the chamber walls in tokamaks. The acceleration of dust particle is found to be proportional to the strength of the space charge electric field and inversely proportional to the square root of the dust mass density.     

Behaviour of charged insulating particles in contact with a rotating roll electrode

The overall economic efficiency of standard industrial roll-type separators for granular materials can be improved by operation at higher velocities of the rotating roll electrode. The aim of this paper is to estimate how high this speed could be and still have a good separation. The answer to this question implied the calculation of the electric image force, which opposes the centrifugal force and sticks the corona-charged insulating particles to the rotating roll electrode. This force depends on the residual charge carried by the particles. By estimating the decay of this charge from surface potential measurements carried out on granular layers of insulating materials dispersed on grounded plate electrodes, it was possible to simulate the particle lift-off from the rotating roll electrode under various operating conditions. The results presented in the paper were obtained for fly-ash particles, but the numerical simulation methodology employed by the authors can be successfully applied for the optimisation of other electrostatic separation applications.     

Effect of spark discharges on the trajectories of insulating particles in roll-type corona-electrostatic separators. Experimental and numerical study

Corona-electrostatic separation is a multi-variable process that has been thoroughly studied in connection with its various applications in the recycling industry. The aim of the present paper is to point out one parasitic phenomenon that adversely affects the efficiency of the separation: the sparks generated at the passage of conductive particles through the electric field zone. The experiments were carried out on a laboratory roll-type corona electrostatic separators, and the sparks were generated by introducing 16 calibrated copper pins in 40-g samples of granular insulating material (PVC; typical granule size: 1.5 mm) that were fed at a constant rate onto the surface of the grounded rotating roll electrode. The distribution of the PVC granules in the 14 boxes of the collector was altered by the occurrence of the spark discharges, as they were accompanied by the annealing of the electric field between the electrodes. The numerical simulation of insulating granules charging and movement under the action of the electric field enabled a better understanding of the interactions between the spark discharges and the other factors that influence their trajectories and affect the efficiency of the separation: roll-speed, particle size and ambient humidity. The particle dynamics equations were solved using an iterative scheme by using the electric field calculated in any point with the commercial software TRICOMP. The good agreement between the predictions made by these simulations and the experimental findings confirms the ability of the mathematical model to reflect the complexity of the physical phenomena.     

Effect of particle size distribution on the polarity of triboelectric charging in granular insulator systems

Triboelectric charging occurs in granular insulating systems even when all particles are composed of identical material. A simple model is used here to address triboelectric charging in such systems. The basis of the model is the existence of electrons trapped in high-energy states, which can be released during collisions with another particle and transferred to the other particle. This model shows that triboelectric charging in insulator systems composed of particles of identical material can be attributed to a distribution of particle sizes, such that smaller particles tend to charge negatively and larger particles tend to charge positively. This polarity of charging has been observed in field studies of sand storms, dust devils and volcanic plumes, and most laboratory experiments on triboelectric charging in granular systems.     

Carbon nanotube/polyaniline nanocomposites and their electrorheological characteristics under an applied electric field

A conducting polyaniline (PANI) was synthesized via an oxidative dispersion polymerization technique, using poly(vinyl alcohol) (PVA) as a polymeric stabilizer, in the presence of multi-walled carbon nanotubes (MWNT) purified in acidic solution, and dispersion stability of the MWNT in an aqueous solution of PVA was studied for different PVA concentrations. Their morphology was confirmed by a scanning electron microscope. Its electrorheological (ER) characteristics were also investigated by dispersing the PANI/MWNT composite particles in an insulating silicone oil. Its ER properties were examined using a rotational rheometer under varying applied DC electric field strengths, in which the ER fluid is generally composed of a suspension of conducting particles dispersed in an insulating fluid, which shows a rapid and reversible change in shear viscosity with an applied electric field. Synthesized PANI/MWNT composite particles are observed to enhance interparticular interactions, since the degree of polarization of PANI/MWNT composite particle increases with applied electric field strengths. The shear stresses of the PANI/MWNT nanocomposite based ER fluid increase with the electric field strength for a broad range of shear rates.     

Nuclear-magnetic-resonance evidence for charge inhomogeneity in stripe ordered La(1.8-x)Eu0.2Sr(x)CuO4

We report 17O nuclear-magnetic-resonance (NMR) results in the stripe ordered La(1.8-x)Eu0.2Sr(x)CuO4 system. Below a temperature T(q) approximately 80 K, the local electric field gradient and the absolute intensity of the NMR signal of the planar O site exhibit a dramatic decrease. We interpret these results as microscopic evidence for a spatially inhomogeneous charge distribution, where the NMR signal from O sites in the domain walls of the spin density modulation are wiped out due to large hyperfine fields, and the remaining signal arises from the intervening Mott insulating regions.     

Particle dynamics simulations of triboelectric charging in granular insulator systems

Particle dynamics simulations are carried out to study triboelectric charging in granular systems composed of a single insulating material. The simulations implement a model in which electrons trapped in localized high energy states can be transferred during collisions to low energy states in the other particle. It is shown that this effect alone can generate electrostatic charging in the system, and cause net electron transfer from larger particles to smaller particles. The magnitude of charging is small for systems of a single particle size but becomes much greater for a system with polydispersal particle sizes, due to the net electron transfer from larger to smaller particles. The negative charge of smaller particles, and positive charge of larger particles has been observed in field studies and laboratory experiments of granular systems.     

DC conductivity of a suspension of insulating particles with internal rotation

We analyse the consequences of Quincke rotation on the conductivity of a suspension. Quincke rotation refers to the spontaneous rotation of insulating particles dispersed in a slightly conducting liquid and subject to a high DC electric field: above a critical field, each particle rotates continuously around itself with an axis pointing in any direction perpendicular to the DC field. When the suspension is subject to an electric field lower than the threshold one, the presence of insulating particles in the host liquid decreases the bulk conductivity since the particles form obstacles to ion migration. But for electric fields higher than the critical one, the particles rotate and facilitate ion migration: the effective conductivity of the suspension is increased. We provide a theoretical analysis of the impact of Quincke rotation on the apparent conductivity of a suspension and we present experimental results obtained with a suspension of PMMA particles dispersed in weakly conducting liquids.     

Interaction between glow discharge plasma and dust particles

The effect of dust particle concentration on gas discharge plasma parameters was studied through development of a self-consistent kinetic model which is based on solving the Boltzmann equation for the electron distribution function. It was shown that an increase in the Havnes parameter causes an increase in the average electric field and ion density, as well as a decrease in the charge of dust particles and electron density in a dust particle cloud. Self-consistent simulations for a wide range of plasma and dust particle parameters produced several scaling laws: these are laws for dust particle potential and electric field as a function of dust particle concentration and radius, and the discharge current density. The simulation results demonstrate that the process of self-consistent accommodation of parameters of dust particles and plasma in condition of particle concentration growth causes a growth in the number of high-energy electrons in plasma, but not to depletion of electron distribution function.     

Charge dynamics in a model for grains electrization

In this work we investigate a model for the dynamics of granular electrization using event-driven simulations and approximate calculations. The model is defined as a mixture of isolating grains of different species confined in a cubic box. During the collisions, the grains and the walls can acquire electric charge via tribocharging. We focus on the dynamics of charge exchange, and calculate the time evolution of the total charge in each species, that presents a double exponential behavior in the case of zero gravitational field. For non-zero field, a stretching of the curve is present, caused by the resulting density and velocity profiles.     

Electromagnetic scattering of charged particles in a strong wind-blown sand electric field

Some field experimental results have shown that the moving sands or dust aerosols in nature are usually electrified,and those charged particles also produce a strong electric field in air, which is named as wind-blown sand electric field.Some scholars have pointed out that the net charge on the particle significantly enhances its electromagnetic(EM) extinction properties, but up to now, there is no conclusive research on the effect of the environmental electric field. Based on the extended Mie theory, the effect of the electric field in a sandstorm on the EM attenuation properties of the charged larger dust particle is studied. The numerical results indicate that the environmental electric field also has a great influence on the particle's optical properties, and the stronger the electric field, the bigger the effect. In addition, the charged angle, the charge density, and the particle radius all have a specific impact on the charged particle's optical properties.     

The effect of electrostatic force on the evolution of sand saltation cloud

In a wind-blown sand layer, it has been found that wind transport of particles is always associated with separation of electric charge. This electrification in turn produces some electrostatic forces in addition to the gravitational and fluid friction forces that affect the movement of saltating sand particles, further, the wind-blown sand saltation. To evaluate this effect quantitatively, this paper presents a simulation of evolution of wind-blown sand grains after the electrostatic forces exerted on the grains are taken into account in the wind feedback mechanism of wind-blown saltation. That is, the coupling interaction between the wind flow and the saltating sand particles is employed in the simulation to the non-stationary wind and sand flows when considering fluid drag, gravitation, and a kind of electrostatic force generated from a distribution of electric field changing with time in the evolution process of the sand saltation. On the basis of the proposed simulation model, a numerical program is given to perform the simulation of this dynamic process and some characteristic quantities, e.g., duration of the system to reach the steady state, and curves of the saltating grain number, grain transport rate, mass-flux profile, and wind profile varying with time during the non-stationary evolution are displayed. The obtained numerical results exhibit that the electrostatic force is closely related to the average charge-to-mass ratio of sand particles and has obvious influence on these characteristic quantities. The obtained results also show that the duration of the system to reach the steady state, the sand transport rate and the mass flux profile coincide well with experimental results by Shao and Raupach (1992) when the average charge-to-mass ratio of sand particles is 60 μC/kg for the sand particles with average diameter of 0.25 mm. When the average charge-to-mass ratios of sand particles are taken as some other certain values, the calculation results still show that the mass flux profiles are well in agreement with the experimental data by Rasmussen and Mikkelsen (1998) for another category of sand particles, which tell us that the electrostatic force is one of main factors that have to be considered in the research of mechanism of wind-blown sand saltation.     

工业中粉体颗粒的荷电机理及数值模拟方法

工业过程中粉体颗粒不可避免地会相互摩擦碰撞而荷电. 荷电颗粒的存在可能会危害正常的工业生产过程, 也可能对工业过程起促进作用. 因此, 荷电粉体颗粒及其特性受到了广泛的关注, 但目前对粉体颗粒的荷电机理依然缺乏透彻的了解, 尤其是在气固两相流动中的粉体颗粒荷电现象. 事实上, 工业中存在的粉体颗粒的运动都受到流体的影响, 是典型的气固两相流系统, 流体对粉体颗粒的作用使粉体颗粒接触的荷电现象变得更为复杂, 因此从两相流动的观点来研究粉体颗粒荷电的物理本质就显得越来越重要. 本文介绍了工业过程中的几种不同类型的粉体颗粒荷电行为, 回顾了颗粒的荷电机理与描述颗粒荷电的数学模型. 对于工业过程中颗粒的荷电现象及颗粒在多相流体中的动力学行为, 介绍了研究颗粒受流体影响时荷电特性的数值模拟方法. 本文旨在对粉体颗粒的荷电机理、应用以及研究方法进行梳理与探讨, 为正确认识工业过程中粉体颗粒的荷电现象并加以控制利用提供理论借鉴.     

Influence of wall motion on particle sedimentation using hybrid LB-IBM scheme

We integrate the lattice Boltzmann method (LBM) and immersed boundary method (IBM) to capture the coupling between a rigid boundary surface and the hydrodynamic response of an enclosed particle laden fluid. We focus on a rigid box filled with a Newtonian fluid where the drag force based on the slip velocity at the wall and settling particles induces the interaction. We impose an external harmonic oscillation on the system boundary and found interesting results in the sedimentation behavior. Our results reveal that the sedimentation and particle locations are sensitive to the boundary walls oscillation amplitude and the subsequent changes on the enclosed flow field. Two different particle distribution analyses were performed and showed the presence of an agglomerate structure of particles. Despite the increase in the amplitude of wall motion, the turbulence level of the flow field and distribution of particles are found to be less in quantity compared to the stationary walls. The integrated LBM-IBM methodology promised the prospect of an efficient and accurate dynamic coupling between a non-compliant bounding surface and flow field in a wide-range of systems. Understanding the dynamics of the fluid-filled box can be particularly important in a simulation of particle deposition within biological systems and other engineering applications.     

Dynamics of induction charging for multiple particle agglomerations with a thin conducting surface layer

In electrostatic applications, particles are typically stacked in an arbitrary array. In this paper, multiple particle agglomerations with a finite volume conductivity, surface conductivity and permittivity have been simulated. Upon exposure to the electric field, electric shielding can occur due to the proximity of other particles, which greatly reduces the maximum accumulated charge and affects the charging time. All results have been obtained using the COMSOL commercial software. The simulation results show that shielding the electric field from a given particle reduces its saturation charge and the rate of charge accumulation was mainly affected by the volume and surface conductivities.     

Charge Separation at the Evaporation (Condensation) Front of Water and Ice. Charging of Spherical Droplets

Charge separation at evaporation (condensation) front of water and ice is analyzed. Relatively low distribution coefficient of protons and hydroxide ions between vapor and condensed phase that is less than the distribution coefficient of water molecules leads to accumulation of protons and hydroxide ions at the phase front upon evaporation and a decrease in the amount of such species upon condensation. Interphase charge separation is caused by the subsequent diffusion of nonequilibrium protons and hydroxide ions. The charge separation is also affected by the double electric layer generated by orientation defects at the water and ice surfaces. Dependences of electric field at a plane surface of water and ice on the rate of phase transformation are calculated. Electric charges of spherical water droplets are estimated at different field strengths and droplet radii.     

The influence of dust particle geometry on its charge and plasma potential

In this paper, a self‐consistent numerical model describing the behaviour of plasma around isolated highly charged dust particles with different shapes of rotation figures is presented. Dust particles in the form of a sphere, oblate ellipsoids (disk‐like particles), and elongated ellipsoids (rod‐like particles) are considered in the presence of an external electric field. Using the developed model, self‐consistent distributions of a space charge and plasma potential are obtained around non‐spherical dust particles. These distributions are carefully analysed by decomposing them in a series of Legendre polynomials. Decompositions of these distributions are compared with particles of different geometry. In addition, for different geometries of dust particles, the dependencies of the charge of a dust particle on geometry in the absence of an external field are investigated.     

Numerical simulation of the electrostatic powder coating process with a corona spray gun

In this paper results of investigations are described aiming to numerically simulate the electrostatic powder coating process using an extended commercial computational fluid dynamics (CFD) code. The fully three-dimensional turbulent flow was calculated. Based on the Lagrangian approach the trajectories of the powder particles were modelled considering electric and aerodynamic forces. In the calculations of the particle propagation both the particle size distribution and the particle charge distribution obtained through experiments have been applied. The model accounts for the space charge effect of the charged particles and the turbulence dispersion on the particle trajectories. It was found that the space charge plays an important role for the final spray pattern shape, also increasing the transfer efficiency. The numerical results, such as velocity profiles, static and dynamic film thickness on the target were in good agreement with experiment.