C. Gutfinger

Resuspension of particulates from surfaces to turbulent flows—Review and analysis

Abstract The paper reviews the state of the art of aerosol resuspension research. Five different theoretical models of particle reentrainment are described. Accordingly, the expressions for resuspension from a surface exposed to fluid flow are explained. The advantages and shortcomings of the models are compared. Experimental results from the literature are summarized and presented in the form of tables. Dimensional analysis is applied to the experimental results, introducing the wall shear velocity as a universal parameter which determines the flow character. The advantages and limitations of the existing models of aerosol resuspension are assessed by means of a comparison between theory and experiments, recast in terms of dimensionless groups. Critical analysis shows that, in general, presently available experimental data do not support the existing theoretical models. Models of adhesion of small particles to solid surfaces are also reviewed. The role of van der Waals and electrical interactions in formation of contact with a surface is analyzed, together with the influence of elastic and plastic deformations. The effects of surface roughness, particle type and system history are discussed. The work analyzes the application of boundary-layer turbulence, especially the presence of quasi-periodic repeating patterns of coherent motion, to resuspension. Various mechanisms for generating the hydrodynamic force in turbulent and shear flows at different Reynolds numbers are discussed. Dimensionless expressions for hydrodynamic and surface forces and moments are developed allowing comparison and evaluation of their relative importance. Possible mechanisms of resuspension are proposed.

A MODEL FOR TURBULENT DEPOSITION OF AEROSOLS

A model is developed for particle deposition to smooth surfaces in turbulent flow. The model is based on the calculation of particle trajectories in the wall region, using a detailed description of the flow in this region. The particle trajectories are derived from the equations of motion, including the lift force induced by the shear flow. This lift force, as is shown, is very important for particles with τ+ > 1, and clarifies the mechanism of deposition for these particles. The calculated particle flux compares favorably with the experimental data of various authors reported in the literature.

Adhesion moment model for estimating particle detachment from a surface

Abstract In this paper particle detachment from a surface by a hydrodynamic moment is analyzed. The detachment occurs when this moment exceeds the moment exerted on the particle by surface forces. An expression for the moment of surface forces is derived from the existing adhesion models. This moment is a product of the force acting on the particle and the variable contact radius, which decreases when the applied force increases. Accordingly, a condition for particle detachment from a smooth surface is obtained. In addition, particle detachment from a rough surface is considered. We show that a single asperity contact is similar to the contact of a particle with a smooth surface, but the detaching moment is reduced, because of the lower adhesion force and smaller contact radius. We also consider a particle in contact with two and three asperities, and obtain a condition for particle detachment from a rough surface. It is also shown that the hydrodynamic moment can cause particle detachment, while the hydrodynamic lift force is smaller than the adhesion force by several orders of magnitude. On the other hand, the lift force exceeds the weight of the particle. Hence, the detached particle is eventually removed from the surface by this force.

Theoretical and experimental investigation on granular bed dust filters

Abstract The separation efficiency of granular bed filters is high for small dust particles where diffusional deposition prevails, and coarse particles where inertial deposition is dominant. Between these two regions, there is a pronounced dip in the filtration efficiency curve. Prediction of filtration efficiency in the dip region is difficult due to interaction between inertia and diffusion. The theory presented in the present paper combines the effect of inertia, diffusion and interception on dust deposition, resulting in a single solution for the filtration efficiency accounting for all these effects. Experimental measurements are reported for a wide range of filtration parameters and dust particle characteristics. Agreement between theoretical and experimental values of the filtration efficiency was obtained for the case of an electrically neutral filter bed. It is shown that the presence of electrical charges enhances the filtration efficiency. This occurs particularly during the filtration process through fluidized beds comprized of dielectric materials.

Acoustic enhancement of heat transfer between two parallel plates

Abstract The paper considers a problem in which a steady-state sonic wave is propagated in the longitudinal direction in a fluid enclosed between two horizontal parallel plates which are kept at different temperatures. The distance between the plates is much smaller than the sound wavelength. Rayleighs vortical acoustic streaming that appears in the region between the plates as a result of the sound wave leads to forced heat convection. The effect of that forced convection on heat transferred between the plates is analyzed theoretically. An acoustic Peclet number, which represents the interaction between heat conduction and forced convection is introduced, and asymptotic relations expressing the mean Nusselt number in terms of this dimensionless group are derived. The results obtained demonstrate that acoustic streaming results in a marked enhancement of heat transfer between the plates.

Dust Deposition in Granular Bed Filters: Theories and Experiments

The paper reviews the status of granular bed dust filtration theory and its experimental verification. First, works on fluid flow around a single sphere and their relevance to filtration theory are reviewed and solutions of flows in packed and fludized beds of granules are presented. Solutions for creeping flows, potential flows, intermediate and high Reynolds number flows are discussed in relation to filtration theory. Then the various dust deposition mechanisms and their development are presented. Theories based on inertia, diffusion, and other mechanisms are reviewed. Some of the latest theories combining the different effects are described. Experimental data from the literature are presented and compared with various theoretical models.

Heat transfer to a draining film

Abstract Heat transfer to an attached falling film has been investigated. This problem is unique because both the film thickness (the hydrodynamics) and the temperature profile change with time and position. The original problem which has three independent variables, two in space and one in time, is reduced by a similarity transformation to a two dimensional parabolic problem. The equation was solved numerically with a digital computer for five cases of boundary conditions.

Particle behavior on surfaces subjected to external excitations

Abstract Oscillatory motion of a particle on a surface may be caused by mechanical vibrations of the surface, acoustic oscillations and shock waves in the surrounding fluid, and also by turbulent flow near the surface. Whatever the source of the excitation, it causes particle oscillations. The character of these oscillations depends on the direction and frequency of the external force, as well as on the stiffness of the bond. Several linear and nonlinear oscillation models are introduced and analyzed in order to show whether particle removal is possible for soft and hard particles on smooth and rough surfaces under various conditions. Application to existing methods of surface cleaning is discussed.

Mechanics of collisional motion of granular materials. Part 3. Self-similar shock wave propagation

(Received 2 November 1994 and in revised form 29 December 1995) Shock wave propagation arising from steady one-dimensional motion of a piston in a granular gas composed of inelastically colliding particles is treated theoretically. A selfsimilar long-time solution is obtained in the strong shock wave approximation for all values of the upstream gas volumetric concentration v,. Closed form expressions for the long-time shock wave speed and the granular pressure on the piston are obtained. These quantities are shown to be independent of the particle collisional properties, provided their impacts are accompanied by kinetic energy losses. The shock wave speed of such non-conservative gases is shown to be less than that for molecular gases by a factor of about 2. The effect of particle kinetic energy dissipation is to form a stagnant layer (solid block), on the surface of the moving piston, with density equal to the maximal packing density, vM. The thickness of this densely packed layer increases indefinitely with time. The layer is separated from the shock front by a fluidized region of agitated (chaotically moving) particles. The (long-time, constant) thickness of this layer, as well as the kinetic energy (granular temperature) distribution within it are calculated for various values of particle restitution and surface roughness coefficients. The asymptotic cases of dilute (v, 6 1) and dense (v, - vM) granular gases are treated analytically, using the corresponding expressions for the equilibrium radial distribution functions and the pertinent equations of state. The thickness of the fluidized region is shown to be independent of the piston velocity. The calculated results are discussed in relation to the problem of vibrofluidized granular layers, wherein shock and expansion waves were registered. The average granular kinetic energy in the fluidized region behind the shock front calculated here compared favourably with that measured and calculated (Goldshtein et al. 1995) for vibrofluidized layers of spherical granules.

Experimental Investigation of Particle Removal from Surfaces by Pulsed Air Jets

This work presents an experimental study of particle removal from surfaces by means of a pulsed air jet directed toward the particle-laden surface. During the experiments, solid particles were dispersed over the surface, forming a layer of particles that did not touch each other. Under these conditions, resuspension of an individual particle was independent of the number of particles and their location. We attempt to explain the observed phenomena by analogy to heat transfer enhancement by pulsed jets. It is expected that since pulsed jets are effective in surface cooling, their application to improved surface cleaning should be promising. For a pulsed jet, we investigated the effect of pulse frequency on particle removal. It was found that particle removal efficiency could be significantly affected by the frequency of the jet. In particular, for a fixed jet velocity, the efficiency increases with frequency, reaches a maximum, and then decreases.