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An axial flow cyclone for removing nanoparticles was tested for collection efficiency. Data were validated numerically in vacuum conditions of several Torrs, with flow rates of 0.35-0.57 slpm. The experimental cutoff aerodynamic diameter of the cyclone ranged from 21.7 to 49.8 nm. A 3-D numerical simulation was conducted first to calculate detailed flow and pressure fields, then a Brownian Dynamics simulation was done to determine the collection efficiency of nanoparticles. Both centrifugal force and Brownian diffusion were taken into account. The simulated results for both pressure drop and cutoff aerodynamic diameter are in good agreement with the experimental data. In comparison, previous theories using simplified tangential flow field assumption are not able to predict collection efficiencies accurately. The numerical model developed in this study can facilitate cyclone design to classify valuable nanopowders below a certain diameter, or to remove toxic nanoparticles from the vacuum exhaust of process chambers commonly used in high-tech industries.

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Brownian dynamics for calculation of the single fiber deposition efficiency of submicron particles The motion of submicron particles involves the deterministic terms resulting from the aerodynamic convection and/or electrostatic attraction, and the stochastic term from the thermal displacement of particles. The Langevin equation describes such behavior. The Brownian dynamics algorithm was used for integration of the Langevin equation for the calculation of the single fiber deposition efficiency. Additionally the deterministic and stochastic of the particle motion were derived, using the Lagrangian and Eulerian approaches of particle movement and balance, for the calculation of the single fiber deposition efficiency due to both mechanisms separately. Combination of the obtained results allows us for calculation of the coupling effect of inertia and interception with the Brownian diffusion in a form of correlation. The results of calculation show that the omitting of the coupling effect of particular mechanism and using the simple additive rule for determination of the single fiber deposition efficiency introduces significant error, especially for particles with diameter below 300 nm.

The pattern of filling of the internal space in fibrous filters strongly influences the behavior of the filter at the stage of nonsteady-state deep bed filtration. The model of single-fiber loading with deposited particles, including the resuspension effect, is presented. Structures of deposited charged particles from the range of diameters 0.01-10 µm deposited on the electret fiber of diameter 30 µm were calculated. Results of calculations indicate that the distribution of deposits around the fiber depend on particle size. Fractal dimension and local porosity of deposits depend on the predominant mechanism of deposition. The incorporation of the resuspension effect into the deposition models shows significant differences of the geometry of dendrites in comparison with the case of model in which resuspension was neglected. INTRODUCTIONThe rational design of filtration process should be based on reliable predictions of the dependence on the effluent concentration and on the pressure drop variations with time for a given set of the operating conditions, i.e., particle concentration and size, filter packing density, size of filter element, gas velocity, etc. The pattern of filling of the internal space with the porous structure of fibrous filters strongly influences the behavior of the filter at the stage of nonsteady-state filtration. The initial deposition of particles on the clean collector (nucleation) and dendrite deposition in the next step of the process determine the properties of filters.The knowledge of a dynamics of dendrite growth can provide important information about the structure of clustering particles collected on a single fiber of the filter. It is especially impor-

SUMMARY The capture of aerosol particles during their production and purification of air at workplace and atmospheric environment require an efficient separation method of particulate matter from the carrier gas. Many papers have been published recently on the deposition of particles on fibrous collectors, often using the classical continuum approach to describe the process. However, the approach is not convenient for studying the influence of particle deposition on filter performance (filtration efficiency and pressure drop) when one has to introduce nonsteady‐state boundary conditions. For the purpose of this work, the lattice Boltzmann model describes fluid dynamics, while the solid particle motion is modeled by the Brownian dynamics. The aim of this study is to model the influence of single particle loading (amount and morphology – e.g., porosity and fractal dimension) on its performance. Copyright © 2014 John Wiley & Sons, Ltd.

Deep bed filtration is an effective method of submicron and micron particle removal from the fluid stream. There is an extensive body of literature regarding particle deposition in filters, often using the classical continuum approach. However, the approach is not convenient for studying the influence of particle deposition on filter performance (filtration efficiency, pressure drop) when nonsteady state boundary conditions have to be introduced. For the purposes of this work the latticeBoltzmann model describes fluid dynamics, while the solid particle motion is modeled by the Brownian dynamics. For aggregates the effect of their structure on displacement is taken into account. The possibility of particles rebound from the surface of collector or reentrainment of deposits to fluid stream is calculated by energy balanced oscillatory model derived from adhesion theory. The results show the evolution of filtration efficiency and pressure drop of filters with different internal structure described by the size of pores. The size of resuspended aggregates and volume distribution of deposits in filter were also analyzed. The model enables prediction of dynamic filter behavior. It can be a very useful tool for designing filter structures which optimize maximum lifetime with the acceptable values of filtration efficiency and pressure drop.