Study of a Nanoparticle Charger Containing Multiple Discharging Wires in a Tube

Abstract

A unipolar charger containing multiple discharging wires in a tube (inner diameter: 50 mm) was developed and tested in order to increase the aerosol flow rate and the charging efficiency of nanoparticles. Four gold wires of 25 µm in diameter and 15 mm in length were used as the discharging electrodes to generate positive ions (N_i) from 2.72 × 10⁸ ions/cc to 3.87 × 10⁹ ions/cc in concentration at the discharging voltage of + 4.0 ～ + 10 KV. Monodisperse NaCl particles of 10 ～ 50 nm in diameter were used to test the charging efficiency and the particle loss of charged particles with different aerosol flow rates, corona voltages and sheath flow rates. The sheath air near the tube wall was found to increase the extrinsic charging efficiency, and the highest efficiency was obtained at + 6.0 KV discharging voltage, 10 L/min aerosol flow rate and 9 L/min sheath flow rate. The extrinsic charging efficiency increased from 10.6% to 74.2% when the particle diameter was increased from 10 to 50 nm. The TDMA (tandem differential mobility analyzer) method was used to determine the charge distribution and the mean charge per particle and it was found that the Fuchs charging theory corrected for the extrinsic charging efficiency matched with the experimental data very well.     

Improvement of the Nanoparticle Charging Efficiency of a Single-Wire Corona Unipolar Charger by Using Radial Sheath Airflow: Numerical Study

A single-wire corona unipolar charger with radial sheath air was proposed to enhance the nanoparticle charging efficiency. The charger consists of an insulated Teflon tube (inner diameter = 6.35 mm) with a 6 mm-long grounded porous metal tube placed at its center from which radial sheath air is introduced, and a discharge gold wire of 50 μm in the outer diameter and 6 mm in the effective length. The performance of the charger was evaluated and optimized numerically. The effect of the position of the sheath air opening on reducing charged particle loss was found to be important and two designs were studied. In design 1, both ends of the 6 mm wide sheath air opening are aligned with the ends of the 6 mm-long discharge wire, while in design 2 the sheath air opening is shifted 2 mm toward the left of the leading edge of the wire. At the same operating condition, design 2 was found to have less electrostatic loss than design 1 because of its smaller deposition region for charged particles. Compared to two unipolar chargers with the highest extrinsic charging efficiency for particles smaller than 10 nm in diameter, design 2 operated at the applied voltage of +3.5 kV, aerosol flow rate of 0.5 L/min, and sheath airflow rate of 0.7 L/min has a comparable extrinsic charging efficiency of 17.2%–70.5% based on particle number for particles ranging from 2.5 to 10 nm in diameter.

Copyright 2013 American Association for Aerosol Research     

Unipolar Charging of Fine and Ultra-Fine Particles Using Carbon Fiber Ionizers

A simple and novel unipolar charger using carbon fiber ionizers was developed to effectively charge fine and ultra-fine aerosol particles without the generation of ozone. The particle penetration in the charger was investigated for non-charged, neutralized, and singly charged particles in the size range of 20–200 nm. Particle loss and the intrinsic, exit and extrinsic charging efficiencies of fine and ultra-fine particles were also investigated for non-charged particles at different applied voltages to the charger. Particle penetrations in the charger were nearly 100% for particles larger than 20 nm, irrespective of the initial particle charging state. Particle losses in the charger could be decreased by decreasing the applied voltage to the charger from 4.0 kV to 2.3 kV. The intrinsic charging efficiencies were proportionally increased with the applied voltage, whereas the exit charging efficiencies were almost independent of the applied voltage. Therefore, the extrinsic charging efficiency of the charger becomes higher for the lower applied voltage (2.3 kV), at which about 60% of 20 nm particles were charged. Little (less than 4 ppb) to no ozone was generated under all operation conditions. It can be concluded that the newly developed unipolar charger using carbon fiber ionizers can charge fine and ultra-fine particles at least as effectively as currently available unipolar chargers, but with the major advantage of negligible ozone generation, a highly desirable feature if the charged particles are to be used for chemical or biological analysis.     

Experimental study of a new corona-based unipolar aerosol charger

A corona-based unipolar aerosol charger has been constructed, and its performance has been systematically evaluated. The prototype consists of completely separated corona ionization and charging chambers. With this configuration the electrostatic loss of charged particles is eliminated, and particle loss by diffusion and the space charge effect is minimized by the angular injection of the ionizer flow and the rapid exit of charged particles. The charger performance was optimized by varying different operational parameters, i.e., total and ionizer flowrates, and ion concentration. It was found that operation with one corona ionizer gave higher extrinsic charging efficiency than operation with two ionizers. The corona-discharge current has negligible effect on the charging performance. Operating the charger at a total flowrate of 5 lpm, with 1.0 lpm flow in each of the two ionizers, gave the highest extrinsic charging efficiency. Further, the performance of prototype charger was not compromised even at a total flowrate of 10 lpm. The charger provides higher extrinsic charging efficiency than other corona-based unipolar chargers. Extrinsic charge distributions for particles of different sizes were at last measured by the tandem-DMA technique.     

Multiple charging of ultrafine particles in a corona charger

The charge distribution on ultrafine aerosol particles in the size range below 35 nm has been measured using a corona-based unipolar charger, in which ion generation and aerosol charging take place simultaneously in the region around a sharp-point discharge electrode. The mean number of charges per particle predicted by Fuchs’ diffusion charging theory is in relatively good agreement with the experimental results, and this implies that diffusion charging is the predominant mechanism in spite that the electric field in the charging region is very high. Since a steady state is unattainable in unipolar charging, the charge distribution depends on the geometry and operating conditions of each particular charger. When the present device operates under the conditions (nt-product) which yield the maximum charging efficiency, double charge appears on particles with diameter as small as 15 nm. At larger values of nt, 32 nm particles can acquire up to five charges. The critical particle diameter above which multiple charging occurs is about four times smaller than for bipolar radioactive chargers. In order to use corona charging in aerosol particle size measurement by electrical methods, the required mobility data inversion is thus straightforward for particle diameter below about 15 nm, but becomes quite complex for larger sizes.     

Charging of Ultra-Fine Aerosol Particles by an Ozone-Free Indirect UV Photo-Charger

Particle charging by indirect photoemission may be an alternative to charging methods based on corona discharge or on bipolar charging by radioactive ion sources. An indirect charger using a low energy UV radiation is introduced and characterized in detail. Depending on the carrier gas, photoelectrons or ions formed by electron-attachment charge the particles by diffusion charging. The achieved charging efficiency is in the range of 20–70% for particle sizes of 20 to 100 nm. It can easily be modulated by a small voltage applied to the photoemitter. To achieve stable electron emission, glassy carbon with its very inert surface is used as photoemitter. Particle losses are very small. The charge distribution has been measured by a tandem DMA setup. The experimental results are compared with the theory of unipolar diffusion charging based on the Fuchs’ combination probability of ions with the particles. The charger characterization has been performed by carbon particles in a size range of 20–100 nm, produced by a spark discharge generator.

Copyright 2013 American Association for Aerosol Research     

Design and development of a self-contained personal electrostatic bioaerosol sampler (PEBS) with a wire-to-wire charger

Here, we present a concept of a personal electrostatic bioaerosol sampler (PEBS), which is an open channel collector consisting of a novel wire-to-wire particle charger and a collection section housing a double-sided and removable metal collection plate and two quarter-cylinder ground electrodes. The charger consists of a tungsten wire (25.4 mm long and 0.076 mm in diameter) connected to high voltage and positioned in the center of the charging section (a cylinder 50.8 mm long and 25.4 mm in diameter); a ring of stainless steel wire 0.381 mm in diameter surrounds the hot electrode at its midpoint and is grounded. The newly designed wire-to-wire charger produces lower ozone concentrations compared to traditional wire-to-plate or wire-to-cylinder charger designs. The particles captured on the collection plate are easily eluted using water or other fluids. The sampler was iteratively optimized for optimum charging and collection voltages, and collection electrode geometry. When tested with polystyrene latex particles ranging from 0.026 µm to 3.1 µm in diameter and 10 L/min collection flow rate, the sampler's collection efficiency was approximately 70%–80% at charging and collection voltages of +5.5 kV and ?7 kV, respectively. The PEBS showed this collection efficiency at sampling times ranging from 10 min to 4 h. Preliminary tests with Bacillus atrophaeus bacterial cells and fungal spores of Penicillium chrysogenum showed similar collection efficiency. The use of a unique wire-to-wire charger resulted in ozone production below 10 ppb. Due to low ozone emissions, this sampler will allow maintaining desirable physiological characteristics of the collected bioaerosols, leading to a more accurate sample analysis.

© 2017 American Association for Aerosol Research     

Unipolar Field and Diffusion Charging in the Transition Regime—Part II: Charging Experiments

Unipolar charging of narrowly distributed 30–100 nm DEHS aerosols in air is investigated, in order to determine the influence of the external electric field (E ₀ ≤ 5 kV/cm) and high charging intensities (n·t ≤ 5 · 10 ¹⁴ s/m ³ ) on the charging efficiency. The results are compared with a combined diffusion and field charging model based on the limiting-sphere concept described in Part I.

The experiments were carried out in a wire corona charger under conditions of complete radial turbulent mixing, which makes the determination of charging history straightforward and very accurate. The state of mixing was verified on the basis of the Deutsch model, by separate measurements of particle losses.

For positive charging, the agreement between measured and predicted mean charge was generally better than 5% for particles larger than 45 nm, which typically carried more than 4 unit charges; for 30 nm particles and relatively low charge levels the uncertainties in the model lead to deviations up to 30%.

In case of negative charging, the observed charge levels progressively exceeded those predicted on the basis of mean ion mobilities by factors up to 2 as the charging intensity increased, and there was evidence of additional charging by free electrons.     

Filtration Characteristics of a Miniature Electrostatic Precipitator

This study investigates the filtration characteristics of a miniature dual saw-like electrodes electrostatic precipitator (ESP). Parameters such as particle size, rate of airflow through the ESP, voltage of charge electrode, and discharge polarity were considered to study their influence on aerosol penetration through the ESP. Polydisperse and monodisperse particles with sizes ranging from 30 nm to 10 w m were used as the challenge aerosols. Experimental results indicated that the aerosol penetration through the ESP decreased (from 96% to 15% for 0.3 w m) as the voltage of the discharge electrode increased (from + 4 kV to +8 kV) at a flow rate of 30 L/min. At a fixed electrode voltage (+8 kV), aerosol penetration increased from 15% to 69% for 0.3 w m particles as the flow rate increased from 30 to 120 L/min. The most penetrating particle size was in the range of 0.25 w m to 0.5 w m depending on the discharge voltage and the flow rate. In general, the most penetrating particle size of the ESP decreased with decreasing discharge voltage or with increasing flow rate. At the same voltage level but opposite polarity, the aerosol penetration through the ESP with negative corona was lower than that with positive corona. The difference in aerosol penetration was a factor of about 2 between the negative and positive coronas for 0.3 w m particles, and this difference was found to be independent of discharge voltage. Regarding energy conservation, use of a negative-polarity ESP was more economical if the same efficiency was required. However, the ozone generated by the ESP with negative polarity was about five times greater than that generated with positive polarity. Therefore when using an ESP as an indoor air cleaner, the search for an optimum balance between ozone production and aerosol collection efficiency should be considered.     

Modeling and Validation of Nanoparticle Charging Efficiency of a Single-Wire Corona Unipolar Charger

A single-wire corona unipolar aerosol charger with a sheath air to avoid particle loss was designed and experimental charging efficiencies were obtained at a fixed aerosol flow rate of 1 L/min using monodisperse silver nanoparticles of 2.5 to 20 nm in diameter. The charger has a cylindrical casing of 30 mm in inner diameter in which a gold wire of 50 μm in diameter and 2 mm in length is used as the discharge electrode. A two-dimensional (2-D) numerical model was developed to predict nanoparticle charging efficiency in the unipolar charger. Laminar flow field was solved by using the Semi-Implicit Method for Pressure Linked Equations (SIMPLER method), while electric potential and ion concentration fields were solved on the basis of Poisson and convection–diffusion equations, respectively. The charged particle concentration fields and charging efficiencies were then calculated on the basis of the convection–diffusion equation in which ion–particle combination coefficient was calculated by Fuchs diffusion charging theory (Fuchs, N. A. (1963 Fuchs, N. A. 1963. On the Stationary Charge Distribution on Aerosol Particles in a Bipolar Ionic Atmosphere. Geophys. Pura. Appl., 56: 185–193. [Crossref] , [Google Scholar]). On the Stationary Charge Distribution on Aerosol Particles in a Bipolar Ionic Atmosphere. Geophys. Pura. Appl., 56:185–193). Good agreement between predicted and experimental extrinsic charging efficiencies was obtained. Numerical results showed the advantage of using sheath air to minimize charged particle loss and indicated the location where major charged particle loss occurred. It is expected that the present model can be used to facilitate the design of more efficient corona-wire unipolar charger in the future.     

Ozone-free post-DBD aerosol bipolar diffusion charger: Evaluation as neutralizer for SMPS size distribution measurements

A postplasma neutralizer for submicron particles size measurements by mobility analysis has been evaluated. Bipolar ion currents have been measured downstream a dielectric barrier discharge (DBD) to estimate the ion fluxes at the inlet of charging volume and the n_i·τ product that define the theoretical maximal concentration that can be neutralized. Charge distributions were measured versus DBD voltage, aerosol diameter and concentration for monodisperse aerosols. It is confirmed that the charge distribution of particles depends on the ratio of initial positive and negative ion currents controlled by the DBD voltage leading to a tuneable mean charge of aerosol in this post-DBD bipolar charger. As expected from Gunn's law, the mean charge and the variance are proportional to particle diameter above 50 nm and independent of the aerosol concentration. The size distributions measured with ⁸⁵Kr and post-DBD neutralizer present the same modal diameters and a maximal overestimation of the total concentration of 10%, for aerosol from 15 to 730 nm with concentrations up to 6 × 10¹² m^?3. This post-DBD bipolar charger can be used for submicron aerosol neutralization and thus for scanning mobility particle sizer size distribution measurements in air as well as in nitrogen to suppress ozone downstream DBD.

Copyright © 2017 American Association for Aerosol Research     

Triboelectrostatic separation of covering plastics in chopped waste electric wire

To develop a triboelectrostatic separation technique for covering plastics in waste electric wire, process variables such as charger material, relative humidity, air velocity, electrode potential, and splitter position have been studied. Polypropylene was found to be the most effective material for a tribo‐charger in the separation of cross‐linked polyethylene (XLPE) and polyvinyl chloride (PVC). The charge density of XLPE and PVC was increased with deceasing relative humidity and increasing air velocity in the tribo‐charger. The charge density of a 1:1 mixture of XLPE and PVC was determined to be higher than that of individual XLPE and PVC. A dominant charging mechanism in the tribo‐charger with a 1:1 mixture of XLPE and PVC seems to be the charging of a combination of particle‐particle and particle‐charger surface. However, in case of individual plastics, the charging of particle‐particle occurred. Over 98.05% PVC recovery with 99.50% grade was successfully obtained under the conditions of a splitter position of +2 cm from the center, 25 kV electrode potential and over 13 nC/g PVC charge density for the optimum triboelectrostatic separation. POLYM. ENG. SCI., 47:1975–1982, 2007. © 2007 Society of Plastics Engineers     

Charge distributions of aerosol dioctyl sebacate particles charged in a dielectric barrier discharger

The collection efficiencies of submicron aerosol particles using a two-stage, dielectric barrier discharge (DBD) type electrostatic precipitator have been reported previously [Byeon et al. (2006). Collection of submicron particles by an electrostatic precipitator using a dielectric barrier discharge. Journal of Aerosol Science, 37, 1618–1628]. In this paper, the charge distributions of aerosol dioctyl sebacate (DOS) particles, which had a mobility equivalent diameter of 118, 175, and 241 nm and were charged in a DBD charger, were examined using a tandem differential mobility analyzer (TDMA) system at applied voltages of 9–11 kV and frequencies of 60–120 Hz. The mean number of elementary charges for positively or negatively charged particles increased slightly with increasing applied voltage or frequency. However, the number of elementary charges increased significantly with increasing particle size. At any applied voltage and frequency, the charge distributions of these particles of these sizes indicated asymmetric bipolar charging. The positive-to-negative charge ratios were 10.4, 4.7, and 3.0 for particle sizes of 118, 175, and 241 nm, respectively, at a DBD voltage and frequency was 9 kV and 60 Hz, respectively. Fluorometric analysis showed that average positive-to-negative charge ratios were 11.5, 4.9, and 3.7 for particle sizes of 118, 175, and 241 nm, which agrees well with the TDMA results. Further fluorometric analyses with larger particles (514 and 710 nm) and higher frequencies (1 and 2 kHz) showed that the positive-to-negative charge ratio reached almost unity with increasing particle size or frequency.     

Charge neutralization of submicron aerosols using surface-discharge microplasma

In this study we investigated the charging characteristics of a novel aerosol neutralizer (Surface-discharge Microplasma Aerosol Charger; SMAC) based on the dielectric barrier discharging. The surface discharge was induced by supplying positive and negative DC pulses with a pair of micro-structured electrodes. We confirmed the occurrence of the surface discharge by measuring the microdischarge current, and evaluated the charging performance of the SMAC as a particle neutralizer by measuring the penetration efficiency, neutralizing probability, and charge distribution for particles in the size range of 10–200 nm. The SMAC was found to obtain a particle penetration exceeding 90% for the whole particle size range. The neutral fraction obtained by the SMAC showed good agreement with a bipolar diffusion charging theory and the fraction obtained by an ²⁴¹Am radioactive source when the SMAC was optimized for aerosol neutralization with the offset voltage control. The charge distributions of negatively and positively charged particles by the SMAC and the ²⁴¹Am neutralizer were in good agreement also. The charge balance of positive and negative particles obtained by the SMAC was effectively controlled by adjusting the offset voltage on each electrode. This is the first study to demonstrate the successful use of dielectric barrier surface discharge to bring particles of 10–200 nm to an equilibrium charging state in a controllable manner.     

Detection near 1-nm with a laminar-flow,water-based condensation particle counter

Presented is a laminar-flow, water-based condensation particle counter capable of particle detection near 1 nm. This instrument employs a three-stage, laminar-flow growth tube with a “moderator” stage that reduces the temperature and water content of the output flow without reducing the peak supersaturation, and makes feasible operation at the large temperature differences necessary for achieving high supersaturations. The instrument has an aerosol flow of 0.3 L/min, and does not use a filtered sheath flow. It is referred to as a “versatile” water condensation particle counter, or vWCPC, as operating temperatures can be adjusted in accordance with the cut-point desired. When operated with wall temperatures of ～2°C, >90°C, and ～22°C for the three stages, respectively, the vWCPC detects particles generated from a heated nichrome wire with a 50% efficiency cut-point near 1.6 nm mobility diameter. At these operating temperatures, it also detects 10–20% of large molecular ions formed from passing filtered ambient air through a bipolar ion source. Decreasing the temperature difference between the first two stages, with the first and second stages operated at 10 and 90°C, respectively, essentially eliminates the response to charger ions, and raises the 50% efficiency cut-point for the nichrome wire particles to 1.9 nm mobility diameter. The time response, as measured by rapid removal of an inlet filter, yields a characteristic time constant of 195 ms.

Copyright © 2017 American Association for Aerosol Research     

Effect of Soot Particles on Corona Discharge

A study has been made on the corona charging of soot particles in diesel engine exhaust for the purpose of collection and removal of these particles. Continuous operation was achieved by a specially designed corona charger in which the insulator surfaces were kept clean by a protective film of cleaning gas. This measure prevents shorting of high voltage caused by the soot deposited on the surface of the insulators. The system makes possible a basic study of the effect of soot particles on the corona discharge, the space charge effect, and the electrical conductivity of the gas-particle mixture through experimental measurements and numerical computation.     

Performance Evaluation of Electrostatically Augmented Air Filters Coupled with a Corona Precharger

An experimental study of electrostatically augmented air (EAA) filters coupled with a corona precharger has been conducted using Arizona road dusts and tobacco smoke. The measurements of filter efficiency and pressure drop across the EAA filter have been made using an ASHRAE 52.1-1992 filter test system and an opacity meter to measure the particle concentration upstream and downstream of the test filter. The two-stage EAA filter unit consists of the positive corona precharger upstream of a filter, to precharge particles with the electrical strength of 4.7 kV/cm, and an electrified filter collector, which has folded media with meshy metal separators, in the upstream and downstream side gaps. DC voltage of +1,000 V (1.4 kV/cm) is applied between the upstream and downstream separators to produce an electric field between the separators and media as well as across the media in a polarity so that most of the precharged particulates are collected on the upstream filter collector. A conventional filter was measured and had 70.0% efficiency with dusts of 1.96 w m in mass median diameter and 2.5 m/s face velocity, while the EAA filter had 92.9% efficiency. An electrical effect on the EAA filter was evaluated to both improve the filter efficiency and reduce the pressure drop across the filter. Also, the performance evaluation of the EAA filter using an air handling chamber system in occupied space was investigated with tobacco smoke particles.     

An Improved Method for Charging Submicron and Nano Particles with Uniform Charging Performance

An improved method for charging submicron and nano silver particles with uniform charging performance was developed. Monodisperse silver particles were grown into microdroplets through condensation. The aerodynamic diameter and GSD of the condensed droplets were the same regardless of their original diameter. The diameter of the droplets increased from 1.7μm to 2.5 μm as the temperature of the saturator increased from 45°C to 55°C. They were charged by an indirect corona-based charger, in which the ion-generation zone is followed by a particle-charging zone through which the condensed droplets pass. The charges of the droplets were controlled by varying the droplet size, ion concentration, and strength of electric field in the charger. The solvent of the charged droplets was evaporated in an evaporator. The size distribution of the evaporated particles was measured by SMPS spectrometer and compared with their original size distribution. The particles after evaporation were slightly larger than their original particles, due to recondensation. The total charge and number concentration of the particles were measured by aerosol electrometer and CPC, to calculate the average charge. Their electrical mobility distribution was measured by SMPS spectrometer without a neutralizer, to calculate the charge distribution and average charge of the evaporated particles. The results showed the average charges of the particles were similar, regardless of initial diameter and manner of calculation. The charge distributions of the evaporated particles were identical, except for 16.9 nm particles. Ion evaporation phenomenon of particles smaller than 40 nm in diameter was not detected.     

双区静电除尘模拟设计中的颗粒荷电模型选用

大气污染治理水平的提高对静电除尘器的净化效率提出更高要求。利用数值模拟设计静电除尘器有助于优化结构和提高性能,而模拟过程中选用的颗粒荷电模型很大程度决定了模拟结果的准确性。通过建立双区静电除尘器电场、流场和颗粒运动模型,计算两种荷电模型下双区静电除尘器内部颗粒运动轨迹,分析荷电模型对荷电区和收尘区内颗粒轨迹的影响。通过两种荷电模型下0.4 μm颗粒和7.5 μm颗粒去除率模拟值与实测值的对比,发现荷电模型导致的模拟值差异随颗粒粒径的减小而增大。定电量模型适用于电场荷电为主的大粒径颗粒,对扩散荷电的忽略使得该模型下小粒径颗粒的模拟去除率远小于实测值。综合考虑电场荷电量和扩散荷电量随时间变化的模型可以很好地反映小粒径颗粒的荷电特性,更适用于对小粒径颗粒荷电行为特征的数值模拟。