Yoshio Otani

Effect of Charging State of Particles on Electret Filtration

Collection performance of an electret filter with rectangular fibers was studied experimentally for cases in which electrostatic effect and Brownian diffusion are predominant by using particles from 0.02 to 0.4 μm in diameter and at different charging states. A single fiber collection efficiency ηED was found to be expressible as a function of dimensionless parameters of Peclet number Pe, and Coulombic and induced force parameters, K c and K In as, here, A, B, C, and D are the numerical constants depending upon the charging density of electret fiber. Indices of each dimensionless parameter determined through the experiment coincided with the previous theory. A maximum penetration of particles appeared in the transition region of predominant collection mechanisms, i.e., between Brownian diffusion and induced force effect, ranging smaller than 0.1 μm in diameter for uncharged particles, and between Coulombic and induced force effects, ranging larger than 0.1 μm in diameter, for charged particles. Semiempiri...

Removal of fine particles from smooth flat surfaces by consecutive pulse air jets

In order to develop an effective dry surface cleaning method, removal of fine particles by pulse air jets was experimentally investigated. A dimensionless resuspension parameter, F*, which is the ratio of drag force on particles to van der Waals adhesion force, was introduced to correlate the removal efficiency. Resuspension experiments were carried out with monodisperse PSL particles and wax particles with diameter between 0.25 and 1.1 μm on silicon wafer and glass plate. As a result, it was found that deposition process of particles on the surface (gravitational settling and impaction at a relatively low impaction velocity) has little effect on the removal efficiency and that consecutive pulse air jet is effective in the removal of fine particles. Further, F* is the key parameter in determining the removal efficiency. The prediction method for the removal efficiency by pulse air jets with F* is proposed.

Adsorption of mercury vapor on particles.

The adsorption of mercury vapor on particles was studied by using soot particles generated by incineration of sewage sludge (EP-ash) and activated carbon particles. Through the experiments, it was found that, at 298 K, the EP-ash has a fairly high adsorption capacity for mercury vapor in the order of 10/sup -6/g/g, which is between that of the ordinary soils and that of activated carbon particles. Furthermore, it was found that physical adsorption of mercury vapor on the studied particles at high temperature is described by Dubinins equation. On the basis of the equation, it was shown that EP-ash physically adsorbs very little mercury at high temperature, and therefore, most mercury in the EP-ash is chemically adsorbed or contained in a form of mercury compounds. Nevertheless, the total amount of mercury contained in the particles is very little compared to the total mercury in the exhaust gases so that most mercury behaves as a vapor in the presence of particulate matter. 8 references, 4 figures, 3 tables.

Determination of deposition velocity onto a wafer for particles in the size range between 0.03 and 0.8 μm

Abstract An experimental technique for measuring deposition velocity onto a wafer for particles with a diameter smaller than 0.8 μm by means of fluorometry was developed. Employing this technique, deposition velocities were measured for particles with a diameter between 0.03 and 0.8 μm in bipolar charge equilibrium and with one single charge at a flow velocity from 0.02 to 0.5 m s −1 for horizontal and vertical wafers in vertical downward airflow. It was found that the experimental data agree well with an equation by Liu and Ahn [Liu, B.Y.H. and Ahn, K.H. (1987) Aerosol Sci. Technol. 6, 215–224] for particles in charge equilibrium with diameters between 0.03 and 0.8 μm. When both the particles and the wafer were charged, the experimental deposition velocities agreed with the Liu and Ahn equation when adding the Coulomb drift velocity term.

Charging of particles in unipolar coronas irradiated by in-situ soft X-rays: enhancement of capture efficiency of ultrafine particles

The charging of*ne particles in unipolar coronas irradiated by sof t X-rays (3 :5–9:5 keV ;� =0 :13–0:41 nm) was studied. Voltage–current characteristics were used to examine the corona inception voltage in presence and absence ofX-ray irradiation and to estimate the ion concentrations. The capture characteristics ofsynthesized particles: iron oxide, sodium chloride, silica and titanium dioxide, were established in coronas, with and without X-ray irradiation. Enhanced charging ofultra*ne particles in coronas was observed in conjunction with soft X-ray irradiation. A positive corona with X-ray irradiation resulted in the highest charging e)ciencies, followed by a negative corona with X-ray, X-ray only, negative corona only and *nally positive corona only. Particle charging theories available in the literature were used to qualitatively explain the observed trends. ? 2002 Elsevier Science Ltd. All rights reserved.

Experimental study of aerosol filtration by the granular bed over a wide range of Reynolds numbers

The collection performance of granular bed filters consisting of uniform spheres with diameters of 0.5–2.0 mm was experimentally studied by using monodisperse aerosol particles ranging from 0.02 to 2 μm in diameter at superficial velocity from 0.4 to 120 cm/s. Based on the experimental data, prediction equations of collection efficiency due to individual mechanical collection mechanisms were obtained, elucidating the influence of the Reynolds number on the particle collection. Furthermore, by assuming the additivity of the individual mechanical collection efficiencies, a prediction equation applicable to the wide range of filtration conditions is proposed.

COLLECTION MECHANISMS OF ELECTRET FILTER

Abstract Collection efficiency of a single electret fiber, which carries permanent positive and negative charges, was studied. Theoretically, it was obtained by solving the equation of particle motion taking account of the induced and Coulombic forces simultaneously. When either induced or Coulombic forces dominates particle collection, the collection efficiency was found to be proportional to 2/5 power of induced force parameter KIn, and 3/4 power of Coulombic force parameter KC, respectively. However, when both forces are effective simultaneously, the efficiency was not expressed by the simple superposition of both effects because of negative interaction between both forces. Experimentally, collection efficiency of a single electret fiber was measured by using monodisperse sodium chloride particles ranging from 0.01 to 0.4 μm in diameter for filtration velocities from 5 to 200 cm/s, under different charging state of particles, i.e., uncharged, singly or doubly charged and charged in Boltzmann equilibriu...

Growth and deposition of saline droplets covered with a monolayer of surfactant

Experiments and theoretical calculations were carried out to investigate the effects of a surfactant monolayer on the growth and deposition of saline droplets in a horizontal elutriator. Cetyl alcohol was used as the surfactant in the study. In the calculations, the condensation coefficient of water vapor on the surface of a droplet covered with a surfactant monolayer was determined from the specific evaporation resistance data. It was found, in the presence of a surfactant monolayer, the growth of saline droplets is retarded and the deposition efficiency in the elutriator is reduced. The deposition pattern in the elutriator is mainly determined by two time scales: the growth retardation time and the time for gravitational settling. When these two time scales are comparable, the distribution of particle deposits in the elutriator is markedly nonuniform.

Filtration of Multi-Walled Carbon Nanotube Aerosol by Fibrous Filters

Filtration efficiency of multi-walled carbon nanotube (MWCNT) aerosol by fibrous filter was evaluated experimentally. Mono-mobility test aerosols with electrical mobility diameter of 100, 200, and 300 nm were generated by the atomization of MWCNT aqueous suspension followed by mobility classification with a differential mobility analyzer (DMA). By analyzing the shape of classified aerosol particles under a scanning electron microscope, it was found that the DMA-classified 300 nm particles were fibrous in shape and had uniform diameter of about 60 nm and length of 2.1 micrometer. On the other hand, 100 nm and 200 nm particles contained a fairly large amount of multiply charged fibrous particles with a larger diameter. These test aerosols were challenged to a medium performance fibrous filter at various filtration velocities. As a result, fibrous particles were captured by fibrous filter at a higher collection efficiency than the spherical particles with the same mobility. By analyzing the single fiber capturing efficiency, interception incorporating the rotation of fibrous particles is found to be the dominant capturing mechanism for the fibrous particles in the studied size range.

Generation of nanometer size particles and their removal from air

Abstract Monodisperse nanometer size silver particles as small as 1 nm were generated by a new differential mobility analyzer and an IR ray furnace at a concentration higher than 10 3 cm −3 . The test nanometer size particles were introduced into stainless steel wire screens and circular aluminum tubes to investigate the particle rebound on solid surfaces. As a result, it was found that the particle rebound may increase the particles penetration through circular tubes when the particles are smaller than 2 nm.