DETERMINATION OF INERTIAL EFFICIENCIES AND DUST FREE ZONES OF ELECTRICALLY ENHANCED COLLECTORS

The determination of particle trajectories when electrostatic forces are present is an important part of analyzing collection efficiency. For small Stokes numbers, an asymptotic expansion of the Fuchs trajectory equation yields a first order differential equation which depends only on spacial variables. Using this equation, trajectories of particles about collectors with an axis of symmetry parallel to the direction of air flow can then be determined by a single numericalintegration. The geometry of the direction field of this equation is analyzed to numerically determine the limiting trajectories which, in turn, yield the efficiency. The same kind of analysis of the saddle points of this direction field in combination with numerical integration can be used to determine those solution paths which bound the dust free zones about the collector.

These ideas are demonstrated by applications to the case of a spherical collector on which there is an electrostatic charge and which is subject to an external electrical field.     

DETERMINATION OF INERTIAL EFFICIENCIES AND DUST FREE ZONES OF ELECTRICALLY ENHANCED COLLECTORS

ABSTRACT

The determination of particle trajectories when electrostatic forces are present is an important part of analyzing collection efficiency. For small Stokes numbers, an asymptotic expansion of the Fuchs trajectory equation yields a first order differential equation which depends only on spacial variables. Using this equation, trajectories of particles about collectors with an axis of symmetry parallel to the direction of air flow can then be determined by a single numericalintegration. The geometry of the direction field of this equation is analyzed to numerically determine the limiting trajectories which, in turn, yield the efficiency. The same kind of analysis of the saddle points of this direction field in combination with numerical integration can be used to determine those solution paths which bound the dust free zones about the collector.

These ideas are demonstrated by applications to the case of a spherical collector on which there is an electrostatic charge and which is subject to an external electrical field.     

Modeling of Cross-Flow Across an Electrostatically Charged Monolith Filter

The flow field and filtration efficiency of electrostatically charged micro-channel filters under cross-flow conditions were modeled. In our simulations, the fluid flows tangentially to the filter face (cross-flow). Particles with diameters larger than 2 μm were considered in this study, hence, the effects of Brownian motion were not included in the simulations. The influence of particle size, pressure drop, and electrostatic charge on the filtration efficiency was investigated. Measurements from performing electrostatic force microscopy (EFM) on the monolith sample confirmed the presence of charge and gave a qualitative measurement of the charge distribution. Results from the flow simulations indicate that the electrostatic forces increased the particle capture efficiency only at lower pressure drop. At higher pressure drops, electrostatic forces did not significantly increase the capture efficiency of the particles. Also, the capture efficiency of relatively small particles is found to be more dependent on the pressure drop across the filter than that of larger particles.     

Electrostatic control of particle deposition

A two-dimensional simulation has been conducted in an electrostatic powder-coating booth for controlling particle deposition. In the absent of electrostatic forces, most of particles flow along steamlines and cannot deposit on the target. When electrostatic forces (coulombic and image forces) are added in the motion of the particle, the trajectories change significantly near the target. It is found that image force hardly affects the deposited position of the particle. From the calculated force distribution near the aimed region on the target, coulombic force is found to be larger than drag force when the particle is very close to the target. Particles with small Stokes number having small specific charge deposit exactly on the aimed regions. Furthermore the width of the aimed region also affects the accuracy of the particle deposition.     

Numerical investigation of powder dispersion mechanisms in Turbuhaler and the contact electrification effect

Powder dispersion in dry powder inhalers (DPI) is affected by factors such as device design and flow rate, but also electrification due to particle–particle/device collisions. This work presented a CFD-DEM study of powder dispersion in Turbuhaler®, aiming to understand the effect of electrostatic charge on the dispersion mechanisms. The device geometry was reconstructed from CT-scan images of commercial Turbuhaler device. Different work functions were applied to the active pharmaceutical ingredient (API) powder and the device wall. Electrostatic charges were accumulated on the API particles due to contact potential difference (CPD) between the particles and the device wall. Results showed that both the chamber and the spiral mouthpiece played an important role in de-agglomeration of powders caused by particle–wall impactions. With increasing flow rates, the performance of the device was improved with higher emitted dose (ED) and fine particle fractions (FPF). The electrostatic charging of the particles was enhanced with higher CPD and higher flow rates, but the electrostatic charging had a minimum effect on powder dispersion and deposition with slight reduction in ED and FPF. In conclusion, the van der Waals force is still the dominant adhesive inter-particle force, and the dispersion efficiency is affected by the flow rate rather than contact electrification of particles. Future work should focus on the effect of highly charged particles emitted from the inhaler on the deposition in the airway.     

A Lagrangian approach for quantum-mechanical electrostatic analysis of deformable silicon nanostructures

Semiconductor mechanical components of nanoelectromechanical systems (NEMS) typically undergo deformations when subjected to electrostatic forces. Computational analysis of electrostatic NEMS requires an electrostatic analysis to compute the electrostatic forces acting on the nanomechanical structures and a mechanical analysis to compute the deformation of the nanomechanical structures. Typically, the mechanical analysis is performed by a Lagrangian approach using the undeformed position of the structures. However, the electrostatic analysis is performed by using the deformed position of the nanostructures. The electrostatic analysis on the deformed position of the nanostructures requires updating the geometry of the structures during each iteration. In this paper, based on a recently proposed hybrid BIE/Poisson/Schrödinger approach, we propose Lagrangian formulations for the BIE/Poisson/Schrödinger equations and solve the coupled Lagrangian BIE/Poisson/Schrödinger's equations self-consistently using the undeformed position of the semiconductors to compute the charge distributions on the deformed semiconductors. The proposed approach eliminates the requirement of updating the geometry and, consequently, significantly simplifies the procedure of coupled electromechanical analysis of NEMS.     

Improvement on the first pass transfer efficiency of fine polymer coating powders for corona spraying process

Corona charge spraying has been widely used in the powder coating application, thanks to its outstanding charging performance leading to high coating efficiency. Yet, this spraying technology has not been working very well with fine powders that started to be used by powder industry in recent years. Fine powders are known as the powders that have median sizes smaller than 30 μm. Utilizing fine powders can improve coating quality and reduce film thickness. However, it has been experienced that the fine powders have much lower first pass transfer efficiency (FPTE). This study provided a solution for the problem by humidifying the powder coating particles. The gained moisture on the particle surfaces can reduce the powder resistivity. As a result, the electric field strength inside the deposited particle layer on the target is reduced, allowing more charged particles to be deposited. Therefore the FPTE of the fine powder is increased. Discovered by the experiments, a maximum of 17% increase in the FPTE was achieved with the humidified fine powder. The improved FPTE of the fine powder was comparable to the regular powder. The study also evaluated the influence of the increased humidity on the fine powder flowability. Based on the results obtained from the powder characterization tests, the suggested humidification process would not significantly deteriorate the fine powder flowability.     

Sunil de Silva-A Scientist and a Visionary

The objective of this work was to analyze the behavior of good conducting granules in a newly designed plate-type electrostatic separator. Commercial software was employed for the numerical analysis of the electric field generated by an original electrode configuration. This enabled the evaluation of the electric charge and forces on conducting particles. The trajectory of a single particle was calculated over a set of 1300 points. The simulated results show that the trajectories depend on the applied voltage, the radius, and the mass density of the granules. They were found in good agreement with the experiments carried out on a laboratory electrostatic separator.     

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.     

Shape effect of the matrix on the capture cross section of particles in high gradient magnetic separation

A high gradient magnetic separator consists of a region of a high and approximately uniform magnetic field and a ferromagnetic matrix of fine wires which distort the field and produce large local gradients. As a particle is carried through the separator by a carrying fluid, both magnetic forces and drag forces are exerted on it. In order to gain insight into the capture mechanism, the drag and magnetic forces on a spherical paramagnetic particle were examined. The equilibrium of these forces defines the path of the particle as it passes by a matrix element. It is shown that for any geometry the particle motion is a function of two dimensionless variables. A computer with a plotter was used to compute the particle paths. In order to provide for most flexibility the magnetic field is that of a magnetized elliptical cylinder with any orientation with respect to the background field and flow stream, while the flow velocities are those corresponding to another elliptical cylinder of different configuration and orientation which allows computation of the change of capture cross section as the matrix element collects material. Examples of particle orbits and changes of capture cross section are given inthe paper for various aspect ratios of the original matrix element.     

Electrostatic Effects on Dispersion,Transport, and Deposition of Fine Pharmaceutical Powders: Development of an Experimental Method for Quantitative Analysis

Currently, there is no standard method for testing the electrostatic properties of pharmaceutical powders. The objective of this study was to develop a method of characterizing the dispersion, charging, and transport properties of fine powder flowing through tubes of different materials. Powders of known composition and size distribution were dispersed pneumatically and transported through a short section of tubing containing spiral baffle inserts of the same material to simulate powder flow in long sections of horizontal and vertical tubes with bends. The test powder was dispersed using ring jet suction and passed through the baffled tube to a sampling chamber, from which the powder cloud was sampled for particle size and electrostatic charge distribution measurement using an Electrical Single Particle Aerodynamic Relaxation Time (E-SPART) analyzer. Experimental data on the tribocharging and transport properties of different powders are presented along with an explanation of the charging mechanisms. Analyses of particle size and electrostatic charge distributions in real time and on a single particle basis using the E-SPART analyzer coupled with surface structure analyses with XPS and UPS showed that: (1) most powders are charged bipolarly with relatively high charge-to-mass ratio (Q/M) values that would have a strong effect on transport and deposition of powders; and (2) surface structures, particularly adsorbates, influence the work function and tribocharging of powder. Different methods, including plasma treatment, with minimal changes or contamination of the bulk properties of the powders are also suggested. pharmaceutical powders tribocharging dispersion work function charge distributions charge decay plasma treatment     

A coarse-grained parcel method for heat and mass transfer simulations of spray coating processes

The Discrete Element Method (DEM) is commonly used for modeling the flow of particulate materials. Unfortunately, such detailed simulations are computationally very demanding, restricting its use for industrially-scaled processes. The number of particles in a simulation can be reduced by introducing parcels (i.e., “coarse graining”), which – in essence – relies on the increase of the particle diameter for interaction calculations. However, sophisticated models are necessary to preserve the original behavior of the material when using such an approach. Our present contribution extends available coarse-graining concepts by introducing models for (i) particle–fluid mass transfer and (ii) the deposition rate of spray droplets on particles. Our mass transfer model is based on an existing model for heat transfer. For the spray deposition model, we introduce an effective particle diameter to compute the correct amount of droplets that impact particles. We show that these models can be used with confidence up to a coarse-graining level of 5, which we demonstrate for a simple-shaped fluidized bed. The models proposed by us are critical for detailed simulations of spray coating processes since they enable precise particle-droplet-air interaction modeling at low computational cost.     

Electrostatic Effects on Dispersion, Transport, and Deposition of Fine Pharmaceutical Powders: Development of an Experimental Method for Quantitative Analysis

Currently, there is no standard method for testing the electrostatic properties of pharmaceutical powders. The objective of this study was to develop a method of characterizing the dispersion, charging, and transport properties of fine powder flowing through tubes of different materials. Powders of known composition and size distribution were dispersed pneumatically and transported through a short section of tubing containing spiral baffle inserts of the same material to simulate powder flow in long sections of horizontal and vertical tubes with bends. The test powder was dispersed using ring jet suction and passed through the baffled tube to a sampling chamber, from which the powder cloud was sampled for particle size and electrostatic charge distribution measurement using an Electrical Single Particle Aerodynamic Relaxation Time (E-SPART) analyzer. Experimental data on the tribocharging and transport properties of different powders are presented along with an explanation of the charging mechanisms. Analyses of particle size and electrostatic charge distributions in real time and on a single particle basis using the E-SPART analyzer coupled with surface structure analyses with XPS and UPS showed that: (1) most powders are charged bipolarly with relatively high charge-to-mass ratio (Q/M) values that would have a strong effect on transport and deposition of powders; and (2) surface structures, particularly adsorbates, influence the work function and tribocharging of powder. Different methods, including plasma treatment, with minimal changes or contamination of the bulk properties of the powders are also suggested.

pharmaceutical powders tribocharging dispersion work function charge distributions charge decay plasma treatment     

螺旋送粉器内流场的三维数值模拟

为了给螺旋送粉器内流道优化提供理论依据,根据气-固两相流输送的相关理论,运用Eulerian-Lagrange数学方式,采用FLUENT软件,通过离散相模型对螺旋送粉器内固体颗粒的轨迹进行数值模拟,主要研究螺旋送粉器的转速对其输送性能的影响,及不同直径粉体颗粒轨迹。结果表明:气体速度和粉体颗粒大小是影响螺旋送粉器送粉性能的关键因素。     

Dynamic electrostatic charge of lactose-salbutamol sulphate powder blends dispersed from a Cyclohaler®

Context: Electrostatic forces have been claimed to be a mechanism for aerosol deposition in the lungs. However, the extent of its influence on aerosol performance is not clear, particularly for carrier-drug formulations.

Objectives: To prepare lactose-salbutamol powder blends, varying in blend ratio, and identify any relationships between salbutamol dose, electrostatic characteristics and in vitro aerosol performance.

Methods: Decanted lactose and micronized salbutamol sulfate was mixed to produce five blends (equivalent to 50, 100, 200, 300 and 400 µg salbutamol per 33?mg of powder). 33?±?1?mg of a blend was loaded into a Cyclohaler? and dispersed into the electrical Next Generation Impactor (eNGI) at an air flow rate of 60?L/min. This was conducted in triplicate for all five lactose-salbutamol blends.

Results: Fine particle fraction increased with salbutamol dose, from 5.89?±?1.42 to 21.35?±?2.91%. Specific charge (charge divided by mass) distributions for each blend were greatest in magnitude for the 50 µg blend and similar in magnitude between all other blends. However, in eNGI Stage 1 (>8.06?µm), specific charge decreased from 100 µg (?170.4?±?45.8 pC/µg) to 400 µg (?10.0?±?9.1 pC/µg).

Conclusions: The improvement in fine particle fraction with increased salbutamol dose was indicative of fine drug binding to high and low energy sites on the lactose carrier surface. This finding was supported by electrostatic charge results, but the aerosol charge itself was not found to influence aerosol performance by electrostatic forces.     

Ballistics Model for Particles on a Horizontal Plane in a Vacuum Propelled by a Vertically Impinging Gas Jet

In previous work, a time-stepped numerical algorithm to compute particle trajectories consisting of lunar soil blown by the engine exhaust of a lunar lander was developed. The time-stepped integration method relies on the gas density, velocity, and temperature fields, calculated by computational fluid dynamics simulations, to compute the forces and accelerations acting on single noninteracting particles. In this paper, a computationally efficient particle ballistics model is presented where the trajectory is estimated by computing the vertical position (axial coordinate x) as a function of the horizontal position (radial coordinate y) using a constant horizontal velocity and a vertical acceleration approximated as a power-law. The unknown parameters of the model are determined by fitting the ballistics trajectory path to a matrix of trajectories generated by the time-step integration method using the rocket exhaust gas properties predicted by computational fluid dynamics software. Also in previous work, a strictly empirical trajectory model was developed to satisfy the need for a computationally efficient method of computing particle trajectories. This new model (like the previous model) is expressed as a time-independent trajectory path function. However, the method of this current work is based on physical laws of motion, unlike the previous empirical model.     

High gradient magnetic particle separation in viscous flows by 3D BEM

The boundary element method was applied to study the motion of magnetic particles in fluid flow under the action of external nonuniform magnetic field. The derived formulation combines the velocity-vorticity resolved Navier–Stokes equations with the Lagrange based particle tracking model, where the one-way coupling with fluid phase was considered. The derived algorithm was used to test a possible design of high gradient magnetic separation in a narrow channel by computing particles trajectories in channel flow under the influence of hydrodynamic and magnetic forces. Magnetic field gradient was obtained by magnetization wires placed outside of the channel. Simulations with varying external magnetic field and flow rate were preformed in order to asses the collection efficiency of the proposed device. We found that the collection efficiency decreases linearly with increasing flow rate. Also, the collection efficiency was found to increase with magnetic field strength only up a saturation point. Furthermore, we found that high collection efficiently is not feasible at high flow velocity and/or at weak magnetic field. Recommendation for optimal choice of external magnetic field and flow rate is discussed.     

Interactive simulation and visualization of massless, massed, and evaporating particles

Most software packages available for particle tracing focus on visualizing steady or unsteady vector fields by using massless particle trajectories. For many applications, however, the use of massed and evaporating particles would provide a model of physical processes that could be used in product testing or design. In this article we describe the TrackPack toolkit, which provides an integrated interface for computing massless, massed, and evaporating particle trajectories in steady flow. In all cases, we assume a noncoupled model and compute particle trajectories through an existing vector field by numerically integrating with forward Euler, fourth-order Runge-Kutta, or an analytic streamline calculation. The TrackPack software effort was motivated by an industrial application to model pollution control systems in industrial boilers. We briefly describe the project and the visualization environment, and we demonstrate the necessity for massed, evaporating models in the application.     

Analysis of triboelectric charging of particles due to aerodynamic dispersion by a pulse of pressurised air jet

Triboelectric charging of powders causes nuisance and electrostatic discharge hazards. It is highly desirable to develop a simple method for assessing the triboelectric charging tendency of powders using a very small quantity. We explore the use of aerodynamic dispersion by a pulse of pressurised air using the disperser of Morphologi G3 as a novel application. In this device particles are dispersed by injection of a pulse of pressurised air, the dispersed particles are then analysed for size and shape analysis. The high transient air velocity inside the disperser causes collisions of sample particles with the walls, resulting in dispersion, but at the same time it could cause triboelectric charging of the particles. In this study, we analyse this process by evaluating the influence of the transient turbulent pulsed-air flow on particle impact on the walls and the resulting charge transfer. Computational Fluid Dynamics is used to calculate particle trajectory and impact velocity as a function of the inlet air pressure and particle size. Particle tracking is done using the Lagrangian approach and transient conditions. The charge transfer to particles is predicted as a function of impact velocity and number of collisions based on a charge transfer model established previously for several model particle materials. Particles experience around ten collisions at different velocities as they are dispersed and thereby acquire charges, the value of which approaches the equilibrium charge level. The number of collisions is found to be rather insensitive to particle size and pressure pulse, except for fine particles, smaller than about 30 µm. As the particle size is increased, the impact velocity decreases, but the average charge transfer per particle increases, both very rapidly. Aerodynamic dispersion by a gas pressure pulse provides an easy and quick assessment of triboelectric charging tendency of powders.     

Powder characteristics and coating conditions of fresh and reused polyester resins for electrostatic powder coating: powder recycling and loss prevention

Powder characteristics and coating conditions are significant factors in electrostatic powder coating. In this work, powder characteristics of the reused polyester resin or recycled powder particles in terms of shape, size, particle size distribution, moisture content, density, flowability, fluidity and chargeability were compared with those of fresh resin or as-received powder to consider powder recycling. The coating conditions for a metal lantern electrostatic powder coating were studied with fresh polyester resin on a work-piece of 70 × 150 mm using a manual corona spray gun by varying the spray gun voltage, the distance between the spray gun and work-piece, and the gun output. For full utilization of powder particles, the effects of different weight ratios of fresh and reused polyester resins on film thickness were observed using the obtained coating conditions accordingly. The suitable weight ratio of fresh and reused polyester resins was 3:1. In addition, to prevent loss of powder particles in automatic electrostatic powder coating due to over-spraying, the relations between the displacements of the spray guns and conveyor distance were simulated by Microsoft Excel XP on a model metal lantern of 600 × 1216 × 120 mm. No overlapping of the displacements of the spray guns must be considered.