Chikao Kanaoka

Simulation of the growing process of a particle dendrite and evaluation of a single fiber collection efficiency with dust load

Abstract The growing process of particle dendrites on a fiber and the time dependency of a single fiber collection efficiency under dust loaded conditions were studied. Monte Carlo simulation technique was used to express the growing processes of particle dendrites on a fiber in Kuwabaras cell. Fairly good agreement in shape of dendrites was obtained with experimental observations. Single fiber collection efficiency was estimated from the simulation results and it was further correlated with the mass of deposited particles in unit filter volume. Through these calculations for various combinations of Stokes number and interception parameter, the ratio of a single fiber collection efficiency to that of a clean fiber was approximated to a linear function of deposited mass in unit filter volume. The coefficient of the linear function, which was named collection efficiency raising factor, decreases as Stokes number and interception parameter increase.

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...

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.

Stochastic simulation of the agglomerative deposition process of aerosol particles on an electret fiber

To design a high-performance air filter with a longer service life, we need to predict how the morphology of particle accumulates on a constituent fiber changes and affects the collection efficiency and pressure drop of the filter under dust-loaded condition. This paper describes a practical three-dimensional simulation method for predicting the agglomerative deposition process of fine aerosol particles on an electret fiber. The simulated results are shown to agree quite well with the experimental observations for both uncharged and charged particles. For the former, only the gradient force, and for the latter, the coulombic force needs to be considered as long as an oncoming particle has not come in close proximity to any previously deposited particles. In contrast, once the oncoming particle enters a region of close proximity to a deposited particle at the tip of a dendrite or chain-like agglomerate, it suffices to consider only the high-gradient or particle string formative force in the present stochastic model.

Effect of filter permeability on the release of captured dust from a rigid ceramic filter surface

Abstract Accumulation and release mechanisms of captured dust from a ceramic filter element were tested using five elements made of different materials and structures. Increase of the pressure drop due to the accumulation of dust was continuously monitored until it reached a predetermined value. Captured dust was then removed from the element surface by injecting a high-pressure pulse cleaning air. The behavior of the dust detachment was monitored by several means, such as pressure change at various locations along the element, movement of released dust and their high-speed video images. As a result, pressure drop of dust on the element was found to increase quickly at the beginning because of the formation of a dense dust layer on the filter surface and then a slowdown in the increase in rate due to the formation of a coarse layer. A similar trend was observed for all tested elements; however, quantitatively, it strongly depended upon the filter material and structure. To reach a predetermined pressure drop, the filtration time of each filter is different and increases with filter permeability. Furthermore, the calculated dust porosity at a large dust-load condition also indicated that it increases as filter permeability increases. These obtained results indicate the dependence of filter permeability on the structure of the accumulated dust layer. In the cleaning process, it is found that pressure traces after the injection of cleaning air also have different behaviors, depending on the filter permeability. In case of a high-permeability filter, the cleaning process starts almost immediately after injection of cleaning air. For a low-permeability filter, cleaning air is stored before the cleaning process occurred. Based on the experimental observation, the detachment behavior of captured dust from a filter surface is related to the filtration condition and filter properties.

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...

Experimental observation of collection efficiency of a dust-loaded fiber

Abstract Time dependency of collection efficiency of a dust-loaded filter has been experimentally studied to confirm the theoretical predictions of the previous papers. The experiment was carried out in the region of inertial and interceptional collection. The experimental collection efficiency of a dust loaded fiber normalized by the corresponding efficiency of a clean fiber was found to be expressed as a linear function of mass load of captured particles in a unit filter volume: η αm η α0 = 1 + λ m . The coefficient of the linear function, the collection efficiency raising factor λ, was found to decrease from 10 to 0.1 m3/kg as Stokes number increases from 0.05 to 1. However, λ was insensitive to the interception parameter. The experimental values of λ were in qualitative agreement with a previous simulation.

Pressure drop of air filter with dust load

Abstract A model to predict the pressure drop of dust loaded filter based on drag theory was proposed. The model was derived from direct observation of growth processes of particle agglomerates on a cylindrical fiber of a model filter. Then, effective fiber diameter and drag coefficient at dust loaded condition were correlated with filtration condition and accumulated volume, VC, of particles on a fiber. It was observed that outward growth rate of agglomerates depends upon the collection mechanism and is fastest for the case of diffusion, moderate for interception and slowest for inertial capture. The rate of increase of the effective fiber diameter as a function of VC was found to fall into three stages regardless of collection mechanism, namely ‘no growth’ at very low VC, ‘rapid growth’ (effective fiber diameter is a linear function of VC), and ‘damped growth’ (effective fiber diameter is proportional to VC1/2) for large VC. The drag coefficient of a dust loaded fiber was also correlated with VC and found to be large when filtration velocity was slow. The model proposed in this paper was validated through the comparison of the predicted and experimental pressure drop and was found to predict very accurately the pressure drop of a dust loaded model filter and fairly well that of a real filter. The model is applicable to the prediction of the pressure drop of a dust loaded filter at any filtration condition if effective fiber diameter and drag coefficient of a dust loaded fiber are given as a function of accumulated volume of captured particles. Accuracy of the prediction is improved if filter structure is well defined.