Interception Parameter Research Articles

INERTIAL DEPOSITION OF SUBMICRON AEROSOLS IN MODEL FIBROUS FILTERS COMPOSED OF ULTRAFINE FIBERS

The influence of the inertia of submicron particles on their deposition from a Stokes flow in model fine-fiber filters is considered at low Reynolds numbers. The fiber collection efficiencies are calculated by the limiting trajectory method for a cell model of the filter and for a row of parallel fibers oriented perpendicularly to the direction of the gas flow for the ranges of interception parameter R = 0.01–1, Stokes number Stk = 0–20, and Knudsen number Kn = 0–1. The simulation results are consistent with the experimental data.

A new expression for inertial particle collection efficiency by nanofibers with slip effect

The inertial particle filtration process of nanofibers in slip/transitional flow regime is studied by numerical methods. Effects of fiber Knudson number (Knf), fiber packing density (C), interception parameter (R), and particle Stokes number (St) on particle filtration performance are investigated. Results indicate that particle trajectories for slip flow are much closer to the fiber surface when compared to those with non-slip flow condition, which leads to a higher particle collection efficiency under slip flow condition than the case of no-slip flow. Particle collection efficiency improves as slip effect increases, especially for small and weak inertial particles, whereas, it is weakly related to slip effect for particles with large size and high-inertia. Particle deposition distribution on fiber surface is found to be negligibly influenced by slip effect. Based on the simulation results, a reliable prediction expression for particle collection efficiency of nanofibers due to the combination effects of inertial impaction and interception is developed in the range of 0.05 ≤ Knf ≤ 2, 0.01 ≤ C ≤ 0.1, 0.01 ≤ R ≤ 0.5, 0.1 ≤ St ≤ 10.

Simulation of aerosol fibrous filters at Reynolds numbers of the order of unity

The deposition of aerosol particles from a flow in a model filter composed of parallel rows of parallel fibers oriented normal to the flow direction has been studied at Reynolds numbers of the order of unity, Re ∼ 1, which are inherent in analytical filtration. The Oseen flow field in a row has been determined by the method of fundamental solutions. It has been shown that the drag forces of the fibers to the flow and the efficiencies of inertial particle collection on the fibers calculated based on the obtained field insubstantially differ from those calculated by the Navier-Stokes equations. The higher the filter packing density, the larger the Re numbers at which the drag forces begin to differ. In the case of inertial particle deposition, the difference between the collection efficiencies increases with the interception parameter. For rows with a/h > 0.2 (a is the fiber radius, and 2h is the interfiber distance in a row), including rows with a nonuniform arrangement of fibers, the calculations performed by both the linearized and complete Navier-Stokes equations yield almost equal results up to Re ∼ 1.

Silver Nanowire Penetration Through Screen Filter

Understanding the filtration characteristics of fibrous particles is important since those particles have caused health and environmental concerns. Due to the straight morphology of metal nanowires, unlike carbon nanotube (CNT) particles nanowires can be considered as appropriate test material to evaluate existing filtration theory for cylindrical particles. We measured the penetration of silver nanowires in the size range of dm = 200 to 400 nm through screen mesh filter. By using Li et al. (2012)'s theory, we determined the orientation status of silver nanowires inside differential mobility analyzer (DMA) and calculated the dynamic shape factor of nanowires. Theoretical penetration was obtained by using single fiber theory with modified interception parameter including orientation angle between a filter wire and a particle. The orientation angle obtained by fitting experimental data into single fiber theory for the 1 layer of screen mesh filter is found to be close to 40° indicating random orientation of...

Theoretical study for the most penetrating particle size of dust-loaded fiber filters

This study obtained the most penetrating particle size of dust-loaded fiber filters for polydispersed aerosol. The enhancement factor (λ) introduced by Kanaoka et al. [4] was generalized for different interception parameter and by using the simplified generalized formula, the most penetrating particle size and minimum collection efficiency for dust loaded filters was estimated. The most penetrating particle size decreases as flow velocity increases, which means that the most penetrating particle size shifts to a smaller size as flow velocity increases. This tendency is stronger for filters with dust-loaded particles than for clean filters. In addition, the results showed that the minimum collection efficiency increases as dust loading increases. The increased enhancement factor increases the interception parameter and reduces the most penetrating particle size.Further, this study calculated the most penetrating particle size of dust-loaded fiber filters by using the generalized enhancement factor obtained under the assumption of the linear approximation of the coefficient regarding the interception parameter.

Moment approach to the polydispersed particle size distribution of dust‐loaded fibers due to convective diffusion

AbstractIn this study, convective diffusional collection efficiency of polydispersed particles on a dust‐loaded fiber filter is estimated. The change in the polydispersed particle size distribution on dust‐loaded fibers due to convective diffusion was studied. A log‐normal size distribution was assumed and the moment approach was applied. Consequently, the effect of dust loading on polydispersed particle size distribution could be obtained.In the convective diffusional region, the geometric mean diameter increases and the geometric standard deviation decreases as filtration times goes. The collection efficiency increases further for a large polydispersed interception parameter for clean filters and this tendency is enhanced when dust loading is present. The geometric mean diameter increases further for a large polydispersed interception parameter. This study presents simplified methods for examining filter performance under dust‐loaded conditions. The obtained results enhance our understanding of the change in the collection efficiency of polydispersed particle when particles are deposited on a fiber. © 2011 American Institute of Chemical Engineers Environ Prog, 2011

Structure and density of deposits formed on filter fibers by inertial particle deposition and bounce

The morphology and packing density of particle deposits formed by accumulation on thin steel fibers suspended in an aerosol stream were studied by confocal microscopy. Measurements were made with electrically neutral polystyrene spheres ( d P=1.3, 2.0, 2.6 and 5.2 μm) as a function of flow velocity ( v=0.7–5 m/s) and fiber diameters ( d F=8 and 30 μm). Deposition under these conditions was dominated by inertia (Stokes number St=0.3–9), interception (interception parameter R=0.04–0.35) and particle bounce, with a negligible contribution from diffusion. The experiments show a systematic transition of deposit morphologies with a newly introduced particle bounce parameter β∼St/ R, where St and R are based on the diameter d F of the bare fiber. Compact, forward facing deposit structures dominate in case of significant particle bounce (i.e. for β> β⁎, where β* represents the critical conditions for the onset of bounce on the bare fiber). Dendritic structures with pronounced sideways branching are formed at β< β*. R is of relatively little influence as an independent parameter, probably because interception occurs mostly on preexisting deposit structures with dimensions in the order of d P. The mean porosity ε of the deposit structures was determined on the basis of contour measurements by confocal microscopy, in combination with data on the accumulated particle volume per unit fiber length (known accurately from a previous paper by Kasper, Schollmeier, Meyer, and Hoferer (2009). Once noticeable deposits had formed, ε was found to attain stable values between 0.80 at d P=1.3 μm and 0.55 at d p=5.2 μm.

Aerosol Deposition on Electroformed Wire Screens

Insect screens, which are usually an integral component of an air sampling inlet, can cause inadvertent deposition of larger aerosol particles. Numerical and experimental studies were performed to characterize aerosol deposition on commercially available electroformed wire screens for aerodynamic particle diameters between 3 and 20 μ m, Stokes numbers between 0.49 and 20, wire widths between 35 and 160 μ m, and screen open area fractions of 0.56 to 0.90. With increasing values of Stokes numbers, the actual collection efficiency increases gradually to a maximum value that is asymptotic to the fraction of open area. Deposition is characterized in terms of a standardized screen efficiency, which is the actual efficiency divided by the areal solidarity (1–fraction of open area). A correlation equation has been developed for the electroformed mesh screens, which relates the standardized efficiency to the fraction of open area, the Stokes number, the interception parameter, and the Reynolds number based on wire...

Deposition efficiency of fractal-like aggregates in fibrous filters calculated using Brownian dynamics method

Nonspherical particles, such as fractal-like aggregates emitted by diesel engines, are commonly met in the ambient air. Some of them are believed to be carcinogenic to humans, thus their efficient removal is of crucial practical importance. A fibrous filter is the device commonly used for aerosol purification but the literature lacks experimental data concerning aggregates filtration. Effect of aggregates' parameters (fractal dimension, primary particle radius) as well as fiber diameter and air velocity on the filtration efficiency is investigated theoretically using the modified Brownian dynamics method. Three different expressions for the friction coefficient evaluation for the aggregates were examined. The results obtained indicate that structure of an aggregate, filter structure and process conditions strongly influence the aggregates deposition efficiency, which significantly differs from the values determined for mass-equivalent spherical particles. The results determined using the Brownian dynamics approach were compared with the values calculated using classical single fiber theory and noticeable discrepancy was observed for the most penetrating particles, while both approaches agree for the limiting cases of small or large particles. Peclet number based on the mobility radius and the interception parameter based on the outer radius are the proper criteria to describe diffusional and deterministic deposition of aggregates.

The Deposition of Aerosol Submicron Particles on Ultrafine Fiber Filters

The diffusion deposition of submicron aerosol particles of a finite size on a model filter composed of parallel ultrafine fibers with a radius comparable with the mean free path of air molecules was considered. The diffusion capture coefficient with allowance made for particle interception ηDRis found by the numerical solution of the elliptic equation of steady-state convective diffusion in the wide ranges of interception parameter R, Peclet (Pe) and Knudsen (Kn) numbers at small Reynolds numbers. It was shown that, at small Kn numbers, the ηDR value exceeds the sum of capture coefficients due to specific deposition mechanisms, interception and diffusion, ηΣ = ηR + ηD, whereas, at Kn > 1, ηDR ≤ ηΣ. Within the range of intermediate Pe, Kn, and R numbers, the radius of the most penetrating particles is higher than the fiber radius.

Estimation of collection efficiency enhancement factor for an electret fiber with dust load

In a previous study to investigate how the morphology of particles accumulating on an electret fiber evolves and affects the collection efficiency of the filter at dust-loaded condition, a three-dimensional stochastic model is utilized to simulate the deposition and agglomeration of particles on a cylindrical electret fiber via two different electrical effects, namely, induced force (for uncharged particles) and Coulombic force (for charged particles). In the present study, the additional effect of Brownian diffusion is incorporated in the model and the morphology of particle agglomerates obtained in the simulated results is found to agree well with experimental observations obtained by Hiragi and Kanaoka et al. for both uncharged and charged particles. In addition, the ratio of dust-loaded collection efficiency, η, to the clean-fiber collection efficiency, η 0, can still be approximated as linear function in the case of weak electrical effects. However, when the electrical parameters are large, the normalized collection efficiency has to be represented by two linear relations, i.e., at low dust load and high dust load. Estimates of the initial collection efficiency and efficiency enhancement factor are given graphically and tabulated as function of Peclet number, the interception parameter and the two electrical parameters.

Estimation of fibrous aerosol deposition in upper bronchi based on experimental data with model bifurcation.

Lung deposition of fibrous aerosol was studied by using a model of the lung bifurcation with dimensions based on the symmetric model A of Weibel. The fibrous aerosols were introduced to the model under steady inspiratory flow conditions. Glass fibers depositing in the daughter tubes and these escaping from them were observed under a scanning electron microscope (SEM). The deposited fractions were calculated for each length and diameter using a bivariate lognormal distribution. In this study the authors propose empirical equations to estimate total deposited fractions of fibrous aerosol at upper bronchi. The dimensionless relationship among deposited fractions Fd, Stokes number St for randomly-oriented fibers and interception parameter I were used to estimate the deposited fraction at arbitrary position in the upper bronchi. The calculation procedure for Fd(St,I) consists of two steps: the determination of the four deposited fractions, Fdn for each In (n=1-4) followed by interpolation or extrapolation from Fdn to find the deposited fraction Fd at a given I. With this procedure, it is possible to determine the deposited fraction Fd in the range of 2 x 10(-3)<St<5 x 10(-2) and 5 x 10(-4)<I<5 x 10(-2). Penetration from the trachea to the 10th generation of lung bifurcation was calculated for several cases of fiber diameter, length and flow rate. In the case corresponding to an inspiration of 500 cm3 air at a constant rate for 2 s, more than half of the fibers with a length of 100 microm and diameter of 3 microm are deposited at upper bronchi but more than 90% of fibers of the same length and 1 microm in diameter pass through the region.

The Effect of van der Waals' Forces on Aerosol Filtration with Fibrous Filters

The collection of aerosol particles with a fibrous filter under the effect of van der Waals' forces was considered with allowance for an electromagnetic retardation and a curvature of a fiber surface. To estimate van der Waals' forces, the method of pair summation of power potentials was used. The capture coefficient of inertialess particles of definite size was calculated. It was shown that as the interception parameter and the flow velocity decrease, the account for van der Waals' forces sharply increases the capture coefficient.

The Deposition of Particles from an Air Flow on a Single Cylindrical Fiber in a Uniform Electrical Field

The deposition of particles from an air flow onto a single cylindrical fiber perpendicular to the flow in a uniform electrical field is theoretically investigated. Both the electrostatic deposition and the direct interception are taken into account to calculate the deposition efficiency in potential and viscous flows. The results are compared to the Zebel model in which only the electrostatic deposition is considered. The effects of the electrostatic parameter, interception parameter, and velocity field as well as the dielectric constant of the cylinder on the deposition efficiency are discussed.

Particle Deposition in a Two-Dimensional Slot from a Transverse Stream

ABSTRACT This paper addresses the problem of coarse mode particle deposition in a two-dimensional slot from a transverse flow. A dimensional analysis enables us to identify five independent dimensionless groups: the leak Reynolds number, Re, the dimensionless velocity gradient of the approaching flow, α, the Stokes number, Stk, the slot aspect-ratio, e/h, and the interception parameter, dp/h.. A sticky aerosol (MMD ≊3–7 μm; GSD ≊ 2–3) was injected into a 15 cm diameter duct and blown out through a rectangular opening (3 × 40 mm; e = 0.6 mm) drilled in a direction perpendicular to the flow. A video camera was set up to continuously monitor the leak behavior in time. The image then was digitized, enabling us to assess the slot-width as a function of time. It appeared that the slot could be sealed in less than 20 minutes. These results were compared with a simplified model based on a scale analysis of the deposition velocity on the slot thickness. Agreement between predicted and measured leak-widths is very ...

ニードルフェルトフィルターの集塵特性

The present paper deals with the filtration performance of needle felt filters prepared from various kinds of materials. Single fiber collection efficiency was correlated experimentally with Reynolds number and the direct interception parameter, taking into account both volume fraction and needle punching density. Although the difference in the collection efficiency stemming from electrostatic charges on the fiber surface was observed, its dependence on the interception parameter showed the same tendency for all the materials.It was verified that the drag coefficient of felt filters varied inversely with Reynolds number and depended strongly on the volume fraction. Also an empirical expression of the drag coefficient applicable was obtained in a wide range of the volume fraction.

Efficiency of Fibrous Filters with Rectangular Fibers

A theoretical study of filtration characteristics of rectangular electret fibers arranged in a staggered array perpendicular to the flow direction was conducted assuming the Brownian diffusion and interception as the dominant particle capture mechanisms. The single-electret fiber efficiency due to pure diffusion was found to be proportional to -2/3, 0.278, and 1/2 powers of the Peclet number (based on the fiber hydraulic diameter), filter solid-volume fraction, and fiber aspect ratio, respectively. For pure interception, the single fiber efficiency was determined to be proportional to 1.55 and 0.54 powers of the interception parameter and filter solid-volume fraction, respectively. This is Particle Technology Laboratory Publication No. 741.

繊維状粒子のふるい用金網の通過率と繊維長さ 繊維状粒子の平均長さの新しい計測法

Wire mesh screens were used as a length classifier of fibrous aerosol. The penetration rates of fibrous aerosol were simulated by means of the Monte Carlo method. The penetration rates depend only on the interception parameter, that is, the ratio of fiber length to the wire distance of each screen. Experimental results of penetration rates of glass fiber aerosol showed good agreement with the simulated results.Length distributions of outlet fibrous aerosol through the wire mesh screens were also calculated using the penetration rate and length distributions of inlet fibrous aerosol. Longer fibers were eliminated by the wire mesh screens, and the geometric standard deviation of length distribution decreased after passing through the screen.Finally, a new method for measuring the median fiber length was presented. If lognormal distribution is assumed for fiber length and the total penetration rates of the fibrous aerosol are measured for a few kinds of wire mesh screens, one can graphically obtain the median fiber length of fibrous aerosol using Fig. 5.

Convective diffusional collection of polydisperse aerosols on a dust loaded fiber

A three-dimensional method for the stochastic simulation of dendritic growth of polydisperse aerosol particles on a fiber was proposed and its capability was demonstrated for the case of convective diffusion ( Pe = 200-25000, R = 0.1,0.2, σ g = 1.1, 1.2, 1.4). Based on the simulation result, time dependency of the morphology of particle dendrites and the collection efficiency were investigated. Particle dendrites on a fiber simulated by the proposed method grew up three-dimensionally and also randomly, and were similar to those in actual filters. The average distribution of the deposited particles on a fiber was largely dependent on the Peclet number, Pe, and interception parameter, R, but not so much on the deposited mass of particles nor on the polydispersity of aerosol particles. Furthermore, the maximum deposition of particles appeared at a certain angle from the front stagnation point and shifted to a larger angle as Pe increased but did not change so much even if the mass of deposited particles increased. A single fiber collection efficiency at zero dust load agreed well with the analytical solution, and that of a dust loaded fiber increases almost linearly with the loaded mass of deposited particles in a unit filter volume, m, regardless of filtration conditions. The coefficient in the linear function or the collection efficiency raising factor Δ increased as Pe increased and R decreased. However, values of Δ did not show a clear tendency by the change of geometric standard deviation of particle size σ g. Most of the conclusions obtained in this study were almost the same as those obtained for monodisperese aerosols although the measure of polydispersity of aerosol particles, the logarithmic standard deviation σ g, varied from 1 to 1, 4. Therefore, it could be concluded that in the studied range of σ g the polydispersity of aerosol particles does not affect the average performances of the dust loaded filter.

An Analysis of Vegetation Interception Data Pertaining to Close-in Weapons Test Fallout

This paper reports an analysis of literature data pertaining to the fraction of particles resulting from regional weapons fallout intercepted by vegetation. A simple model for the interception parameter, alpha, was developed that predicted an increasing interception fraction with increasing distance from detonation site. The predicted values of alpha close to the detonation site are much smaller than most accepted values that pertain to aerosols and gases. The apparent dependence of the interception parameter on distance is believed to partially reflect the changes of the particle-size distribution of the fallout cloud with the time aloft and the ability of vegetation surfaces to better retain particles of small size. The validity of a set of predicted values was tested using an assessment model and literature data of radioiodine concentrations in milk following a nuclear test.