Measurement of the electrostatic powder coating properties for corona and triboelectric coating guns

Measurements have been made of the deposited powder layer for conventional electrostatic corona powder guns and triboelectric guns. By selectively removing the powder layer under computer control conditions, measurements of the powder thickness, and the adhesive properties of the powder layer have been obtained for variation in the coating process and detailed comparisons have been obtained for both coating systems.     

Direct numerical simulations of turbulent flame expansion in fine sprays

Direct Numerical Simulations of expanding flame kernels following localized ignition in decaying turbulence with the fuel in the form of a fine mist have been performed to identify the effects of the spray parameters on the possibility of self-sustained combustion. Simulations show that the flame kernel may quench due to fuel starvation in the gaseous phase if the droplets are large or if their number is insufficient to result in significant heat release to allow for self-sustained flame propagation for the given turbulent environment. The reaction proceeds in a large range of equivalence ratios due to the random location of the droplets relative to the igniter location that causes a wide range of mixture fractions to develop through pre-evaporation in the unreacted gas and through evaporation in the preheat zone of the propagating flame. The resulting flame exhibits both premixed and non-premixed characteristics.     

Particle size,cohesiveness and charging effects on electrostatic and nonelectrostatic powder coating

Ten powders, differing in protein, carbohydrate and salt contents and ranging from 19 to 165 μm were coated by nonelectrostatic and electrostatic coating. Nonelectrostatic transfer efficiency (TE) increased to a maximum before leveling off with increasing particle size. Electrostatic TE either decreased or increased then decreased with increasing particle size. Powders became more free flowing as particle size increased. Since TE increases as powders become more free flowing, TE increased with particle size for both nonelectrostatic and electrostatic coating. For electrostatic coating, the effect of charge decreases with increasing particle size. Thus, the conflicting effects of ability to pick up charge and flowability caused an increase then decrease in the TE for powders coated electrostatically, and can also explain the exceptions. The average improvement in TE by electrostatic coating was 20%, with the improvement increasing as particle size decreased.     

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.     

X射线菲涅耳波带板成像和投影式相衬成像的数值模拟比较

通过解析分析和数值模拟,比较了钛K线（4.5 keV）与铜K线（8.0 keV）等X射线源背光透视物体情况下,菲涅耳波带板直接成像与投影式相衬成像对被透视物体的空间分辨能力。结果表明,波带板成像可实现优于1 m的高空间分辨能力,而且使用较大尺度背光源更有利于成像。对于高透射或弱吸收的透视物体,波带板难以成像,可采用投影式相衬成像实现m级空间分辨。计入了以前文献没有考虑到的更高阶影响后,解析给出了点光源照射下相衬像的强度分布与对比度。模拟了微焦点X射线源照射存在厚度起伏的薄膜靶以及密度空间调制靶的相衬成像,点光源情况下模拟结果和解析结果相符。讨论了光源大小、成像距离等参数对相衬成像对比度和空间分辨能力的影响,结果表明,通过减小光源尺度和调节物体到探测面的距离,空间分辨能力可优化到1～4 m。     

The influence of particle size on separation and dustiness in powder mixtures during nonelectrostatic and electrostatic coating

Common food powders and their mixtures, consisting of two powders with the same composition but different in particle size: fine (51–95 μm) and coarse (244–401 μm) NaCl, KCl, sucrose, rice starch, maltodextrin, whey protein, casein and soy protein, were coated on a target at 0 and −25 kV. Over half of the mixtures showed separation due to a difference in particle size. Separation was caused by the difference in individual transfer efficiency of the powders and interactions during coating. Both composition and differences in size were found to be important. Being in a mixture did not change the amount of dust formed.     

A full 3D numerical model of the industrial electrostatic coating process for moving targets

In this paper, a full three dimensional FLUENT numerical model of the electrostatic coating process as used in the automotive industry is presented and selected parametric studies on the numerical model have been conducted. The model incorporates a moving mesh capability in order to simulate movements of the target. The target follows piecewise linear motion and the entire area of the target has been coated in multiple passes. All the dominant mechanical and electrical phenomena were taken into account and cases with both neutral and charged particles were examined. It was confirmed that the transfer efficiency increases when the electrical forces are present in the model and that the coating uniformity improves with the movement of the target. The deposition pattern corresponds to the motion of the target and gradually builds up in a cumulative manner. Due to the aggregation effect associated with the target movement, the deposition uniformity is dramatically improved.     

3D numerical model of the electrostatic coating process with moving objects using a moving mesh

In this paper, a full three dimensional FLUENT numerical model of the electrostatic coating process with the embedded moving mesh capability and piecewise linear type target motion is presented. The model includes target geometries that do not exhibit symmetry. All the dominant mechanical and electrical phenomena are taken into account. Mechanical phenomena include shaping air effects, downdraft effects and the motion of the polydispersed particles. Electrical phenomena include the particle space charge distribution, corona discharge and the electrohydrodynamic flow effects. It was demonstrated that the numerical model can accurately mimic the type of the motion used in real world applications.     

从MODIS数据反演粗细粒子气溶胶

陆地气溶胶一般由粗粒子和细粒子气溶胶组成,二者的光学特性并不相似,如何利用卫星观测数据获取粗粒子和细粒子气溶胶是反演中所面临的重要问题.基于AERONET/PHOTONS北京站地面观测数据,本文分析得到了北京地区的气溶胶模式(包括气溶胶复折射指数和谱分布函数),结果表明北京地区的气溶胶呈明显的粗细粒子混合的双峰分布,粗...     

Molecular dynamics simulations of electrostatic layer-by-layer self-assembly

Electrostatic assembly of multilayered thin films through sequential adsorption of polyions in layer-by-layer fashion utilizes the strong electrostatic attraction between oppositely charged molecules. We perform molecular dynamics simulations of multilayers of flexible polyelectrolytes around a charged spherical particle. Our simulations establish that the charge reversal after each deposition step is a crucial factor for the steady layer growth. The multilayers appear to be nonequilibrium structures.     

Summation of electrostatic interactions in quasi-two-dimensional simulations

An error in our previous paper (Clark, A. T., Madden, T. J. and Warren, P. B., 1996 Molec. Phys., 87, 1063) is corrected. This brings the results into agreement with the recent work of Grønbech-Jensen et al. (Grønbech-Jensen, N., Hummer, G. and Beardmore, K. M., Molec. Phys., 1997, 92, 941).     

静电放电火花产生的电磁场树枝模型

 静电放电火花产生的电磁脉冲会对电子系统的正常工作造成严重的干扰，甚至造成系统的损伤。利用时域有限差分法建立了静电放电火花产生的电磁场的数值模型，模型中充分考虑了放电电极上的静电荷对电场的影响。把由此模型放电计算的电磁场值与由此解析方法得到的场值进行了比较，结果吻合良好。由此可以用此模型来研究静电放电火花产生的电磁场与电子系统的能量耦合问题。     

静电放电火花产生的电磁场树枝模型

静电放电火花产生的电磁脉冲会对电子系统的正常工作造成严重的干扰,甚至造成系统的损伤。利用时域有限差分法建立了静电放电火花产生的电磁场的数值模型,模型中充分考虑了放电电极上的静电荷对电场的影响。把由此模型放电计算的电磁场值与由此解析方法得到的场值进行了比较,结果吻合良好。由此可以用此模型来研究静电放电火花产生的电磁场与电子系统的能量耦合问题。     

Three-dimensional numerical studies on the effect of the particle charge to mass ratio distribution in the electrostatic coating process

Charge to mass ratio is a crucial parameter that governs the behavior of particle trajectories in a charged cloud of particles. The complex nature of the charging process limits our ability to accurately determine the charging level when particles of varying size are present. Using a numerical approach, it is possible, however, to take into account predefined values for this parameter. In this paper, the average charge to mass ratio and the distribution of the charge to mass ratio in the coating of a flat target were systematically varied to demonstrate their effect on the motion of the charged particles. The results show that the transfer efficiency increases as the average charge to mass ratio increases. It was found that the transfer efficiency is a weak function of the average particle size in the range tested and that it increases as the width of the size distribution increases.     

A finite volume method for electrostatic three species negative corona discharge simulations with application to externally charged powder bells

A three species model for steady-state negative corona discharge is studied. Ionization, attachment, and recombination reactions are modeled. A novel unstructured finite volume algorithm to solve the equations is presented, using the secondary emission of electrons from the cathode to set boundary values for the electrons.To show the usefulness of the method for industrial applications it is used to characterize the electrostatic properties of an externally charged rotary powder bell used in the automotive industry. Experimental current density profiles are reconstructed with good accuracy, which validates the model and the method with real experimental data.     

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.     

Novel wet electrostatic precipitator for collection of fine aerosol

A novel wet electrostatic precipitator (WESP) is designed for effective control of fine aerosol from humid gases. It operates on the principle of unipolar particle charging in the corona discharge and particle precipitation under the field of their own space charge. The new precipitator is characterized by high gas velocity in the ionizing stage. Tests were carried out for gas with (NH₄)₂SO₄, HCl and (NH₄)Cl aerosol at particle number concentration up to 5·10⁷#/cm³ and mass concentration 10–1000 mg/Nm³. For test conditions one-field WESP ensures mass collection efficiency 90–97% and two-field electrostatic precipitator up to 99%.     

Preparation of carbon nitride fine powder by laser induced gas-phase reactions

3 N₄ structure, with an α-C₃N₄-like form being prevalent. The powders were found to be slightly contaminated by SF₆ sensitizer products. Our results suggest that by improving the experimental parameters this contamination might be reduced and that the laser pyrolysis method offers possibilities for production of C_xN_y materials, with controlled composition. Received: 4 February 1997/Accepted: 7 April 1997     

Long fine single-wall carbon nanotube growth by Nano-spin-threading: model schematics and simulations

We have investigated model schematics for a long fine single-wall carbon nanotube growth inside a larger diameter nanotube. Our proposed schematics are as follows: fullerenes are encapsulated into the nano-channel connected with fullerene storage tank; and then a inner nanotube grows via fullerene coalescence under 1200 °C in the nano-channel. Then the grown carbon nanotube is extracted from the nano-channel by mechanical control. We have investigated fullerene mergence inside single-wall carbon nanotube using classical molecular dynamics simulations based on the Tersoff–Brenner potential and the Lennard–Jones potential. During fullerene-encapsulating, since the fullerenes naturally have the kinetic energies due to the suction force and can be also accelerated by external force fields to improve the fullerene encapsulation rate, they can be migrated toward the other side of the nano-channel with kinetic energies. Our molecular dynamics simulations showed that the structural relaxation of dynamically free atoms affected on the growth of inner carbon nanotube rather than the Stone-Wales transformations. Since the broken bonds make the structural relaxation during merging to be easily achieved from the migration of carbon atoms or carbon chains, the inner nanotube grows via the re-bonding-reactions of dynamically free carbon atoms or chains as well as the Stone-Wales transformations. We could conclude that the growth rate of the inner CNT could be increased when bond-breakings between carbon atoms of fullerenes were easily achieved.     

Progressive formation of fine and coarse ripples on SiC surface by repeated irradiation of femtosecond laser pulses

In this work, we report the progressive formation of first nanoparticles, next fine ripples, and eventually coarse ripples during the irradiation of single-crystal 6H-SiC surfaces with increasing number of femtosecond laser pulses (λ = 515 nm, τ = 250 fs, repetition rate = 100 kHz). At laser fluences greater than the single-pulse ablation threshold, nanoparticles were produced on the surface by the first few pulses over which fine ripple patterns overlapped at increased pulse numbers. As the pulse number was further increased over ten, the surface was gradually transformed into a coarse ripple–covered one. At laser fluence below the threshold, however, only fine ripples were formed nonuniformly.