Aerosol diffusion

Turbulent agglomeration. Far turbulent agglomeration two cases should be considered. First, if the inertia of the aerosol particles is approximately the same as that of the medium, the particles will move about with the same velocities as associated air parcels and can be characterized by a turbulence or eddy diffusion coefficient DT. This coefficient can have a value 104 to 106 times greater than aerosol diffusion coefficients. Turbulent agglomeration processes can be treated in a manner similar to conventional coagulation except that the larger diffusion coefficients are used. 

Table 2.2 Cunningham Factor cq [10] and the Dependence of Aerosol Diffusivity on Temperature and Pressure...

Thus the energy of attraction becomes infinite as the particle approaches a flat surface. For this reason, it is usually assumed that a surface acts as a p>erfect sink in the theory of aerosol diffusion that is, when a particle penetrates to a distance one radiu.s from the surface, the particles adhere. This holds best for submicron particles moving at thermal velocities. Rebound occurs for larger particles moving at high velocities (Chapter 4). This analysis does not lake into account the effects of surface roughness of the scale of the particle size or of layers or patches of adsorbed gases or liquids. Such factors may be important in practical applications. 

If we relate the Brownian diffusivity D to the mean square displacements given by (9.66), then (9.67) can provide a convenient framework for describing aerosol diffusion. To do so, let us repeat the experiment above, namely, let us follow the Brownian diffusion of N0 particles placed at t = 0 at the y — z plane. To simplify our discussion we assume that N does not depend on y or z. Multiplying (9.67) by x2 and integrating the resulting... 

FIGURE 9.8 Aerosol diffusion coefficients in air at 20°C as a function of diameter. 

Thus, the energy becomes infinite for z = 0. Hence, the surface acts as a perfect sink for aerosol diffusion. The range of operation of the van der Waals force may be estimated by comparing the thermal energy with . Values of Hamaker constant A are in the range of 10 to 10 J. Thus,... 

The other mechanism appears in scrubbers. When water vapor diffuses from a gas stream to a cold surface and condenses, there is a net hydrodynamic flow of the noncondensable gas directed toward the surface. This flow, termed the Stefan flow, carries aerosol particles to the condensing surface (Goldsmith and May, in Davies, Aero.sol Science, Academic, New York, 1966) and can substantially improve the performance of a scrubber. However, there is a corresponding Stefan flow directed away from a surface at which water is evaporating, and this will tend to repel aerosol particles from the surface. 

Recent applications of e-beam and HF-plasma SNMS have been published in the following areas aerosol particles [3.77], X-ray mirrors [3.78, 3.79], ceramics and hard coatings [3.80-3.84], glasses [3.85], interface reactions [3.86], ion implantations [3.87], molecular beam epitaxy (MBE) layers [3.88], multilayer systems [3.89], ohmic contacts [3.90], organic additives [3.91], perovskite-type and superconducting layers [3.92], steel [3.93, 3.94], surface deposition [3.95], sub-surface diffusion [3.96], sensors [3.97-3.99], soil [3.100], and thermal barrier coatings [3.101]. 

GASFLOW models geometrically complex containments, buildings, and ventilation systems with multiple compartments and internal structures. It calculates gas and aerosol behavior of low-speed buoyancy driven flows, diffusion-dominated flows, and turbulent flows dunng deflagrations. It models condensation in the bulk fluid regions heat transfer to wall and internal stmetures by convection, radiation, and condensation chemical kinetics of combustion of hydrogen or hydrocarbon.s fluid turbulence and the transport, deposition, and entrainment of discrete particles. 

Airstream neutralization of acid aerosols by NH3 present in the airway-lumen reduces the health risk associated with acid particles by reducing the acid concentration prior to particle deposition.- In addition, the liquid lining of the respiratory tract probably acts as a chemical buffer," further reducing the health hazard posed by inspired acid particles. Principal factors controlling airstream neutralization of acid aerosols, which is considered to be a diffusion-limited process, are particle surface area, and particle... 

Pui, D. Y. H. Experimental Study of Diffusion Charging of Aerosols. Thesis, Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota (1976). 

D Mass diffusivity Used in mass transfer applications involving aerosols. 

Diffusion The mixing of substances by molecular motion to equalize a concentration gradient. Applicable to gases, fine aerosols and vapors. (See Brownian diffusion.)... 

Diisopropyl methylphosphonate is slightly soluble in water (0.1—0.3 g/L at 25°) and has been demonstrated in laboratory studies to quickly diffuse between the surface microlayer into the water column after deposition as aerosols on fresh water (Van Voris et al. 1987). The solubility of diisopropyl methylphosphonate was 80 g/L (8%) and remained in solution even when the temperature was lowered to freezing (Bucci et al. 1997). In addition, there was no significant loss of diisopropyl methylphosphonate from the water column to the atmosphere, in either the presence or absence of a light wind over the water surface. Human exposure resulting from the vaporization of diisopropyl methylphosphonate from surface water is considered insignificant (EPA 1989). 

Particulate diffusion does not play a significant role in the deposition of pharmaceutical aerosols. However, it is worth noting the mechanism by which diffusion of particles occurs in the lungs. The principle of Brownian motion is responsible for particle deposition under the influence of impaction with gas molecules in the airways. The amplitude of particle displacement is given by the following equation ... 

Davies, C.N., Diffusion and Sedimentation of Aerosol Particles from Poiseuille Flow in Pipes, Aerosol Sci. 4 317-328 (1973). 

Ingham, D.B., Diffusion of Aerosols from a Stream Flowing Through a Cylindrical Tube, Aerosol Sci. 6 125-132 (1975). 

Knutson, E.O., A.C. George, L. Hinchliffe, and R. Sextro, Single Screen and Screen Diffusion Battery Method for Measuring Radon Progeny Size Distributions, 1-500 nm, presented to the 1985 Annual Meeting of the American Association for Aerosol Research,... 

The two fundamental theories for calculating the attachment coefficient, 3, are the diffusion theory for large particles and the kinetic theory for small particles. The diffusion theory predicts an attachment coefficient proportional to the diameter of the aerosol particle whereas the kinetic theory predicts an attachment coefficient proportional to the aerosol surface area. The theory... 

The original theory of diffusional coagulation of spherical aerosol particles was developed by von Smoluchowski (1916,1917). The underlying hypothesis in this theory is that every aerosol particle acts as a sink for the diffusing species. The concentration of the diffusing species at the surface of the aerosol particle is assumed to be zero. At some distance away, the concentration is the bulk concentration. 

From equation (1) it can be seen that application of the diffusion theory leads to the conclusion that the rate of attachment of radon progeny atoms to aerosol particles is directly proportional to the diameter of the aerosol particles. 

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