Aerosol inertial deposition

The basic operations in dust collection by any device are (1) separation of the gas-borne particles from the gas stream by deposition on a collecting surface (2) retention of the deposit on the surface and (3) removal of the deposit from the surface for recovery or disposal. The separation step requires (1) application of a force that produces a differential motion of a particle relative to the gas and (2) a gas retention time sufficient for the particle to migrate to the coUecting surface. The principal mechanisms of aerosol deposition that are apphed in dust collectors are (1) gravitational deposition, (2) flow-line interception, (3) inertial deposition, (4) diffusional deposition, and (5) electrostatic deposition. Thermal deposition is only a minor factor in practical dust-collectiou equipment because the thermophoretic force is small. Table 17-2 lists these six mechanisms and presents the characteristic... 

Another conditioning method, adaptable to scrubber systems, consists of inducing condensation of water vapor on the aerosol particles as nuclei, increasing the size of the particles and making them more susceptible to collec tion by inertial deposition. 

Aerosol particles deposit in the lung by three principal mechanisms inertial impaction gravitational sedimentation and Brownian diffusion. Particles with a larger MMAD are deposited by the first two mechanisms, while smaller particles access the peripheral region of the lung by diffusion. 

There are five basic physical mechanisms that cause the deposition of aerosols. Inertial Impaction... 

Very often inertial deposition in impactors is used to characterize the aerodynamic behavior of aerosol particles. However, much larger inertial forces are applied for particle deposition in impactors than are available for particle deposition in the human respiratory tract. The particle size obtained by this technique is the inertial diameter. This diameter is defined in the same way as the aerodynamic diameter but based on inertial rather than gravitational particle transport. When a particle is not only inertially but also gravitationally transported its inertial diameter is identical with its aerodynamic diameter. 

Deposition mechanisms are principles by which particles can be deposited onto airway walls. For inhaled aerosol particles only two major mechanisms are important inertial deposition and sedimentation (see definitions). In the literature also diffusion, interception and electrostatic precipitation are sometimes mentioned as deposition mechanisms, but these mechanisms, if occurring at aU, are of lower relevance. 

One of the two dominant deposition mechanisms for aerosol particles in the respiratory tract (see definitions) is inertial deposition or impaction. Inertial deposition is based on the particle s inertia or momentum, which is the product of particle mass (m) and velocity (U). Inertial deposition occurs particularly in the upper respiratory tract where air velocity is high and the largest aerosol particles are still airborne. 

The aerosol model In VICTORIA accounts for the following basic mechanisms (1) condensation or evaporation from aerosol particle surfaces (2) deposition onto structural surfaces (3) agglomeration of aerosol particles (4) and transport of aerosols from one cell to another by advection. The deposition mechanisms modeled are gravitational settling, laminar or turbulent deposition. Brownian motion, thermophoresis, diffusiophoresis, and inertial deposition in curved channels (bends). Agglomeration mechanisms include Brownian motion, relative gravitational motion. Interactions In a shear field, and inertia in a turbulent field. 

Deposition by sedimentation and impaction is a function of the inertial aerodynamic size characteristics of the aerosol particles. Deposition by diffusion is a function of the diffusional properties of the aerosol. Deposition by interception occurs when one of the edges of a particle touches the surface of the respiratory tract. Interception is an especially important determinant of deposition of fibers. Deposition of particles in the respiratoiy tract by electrostatic precipitation... 

In addition to the deposition mechanisms themselves, methods for preliminary conditioning of aerosols may be used to increase the effectiveness of the deposition mechanisms subsequently apphed. One such conditioning method consists of imposing on the gas nigh-intensity acoustic vibrations to cause collisions and flocculation of the aerosol particles, producing large particles that can be separated by simple inertial devices such as cyclones. This process, termed sonic (or acoustic) agglomeration, has attained only hmited commercial acceptance. 

Impaction is caused by the inertial mass of the traveling aerosol particles that forces them to move in a straight-line direction even when the flow of the inhaled air transporting them is bent around a curvature. Hence the particles tend to deposit on obstacles placed in the path of their travel. The inertial mass depends on particle size, density, and velocity. The stopping distance S of a particle having mass mP and initial velocity v0iP is defined according to... 

When aerosols are in a flow configuration, diffusion by Brownian motion can take place, causing deposition to surfaces, independent of inertial forces. The rate of deposition depends on the flow rate, the particle diffusivity, the gradient in particle concentration, and the geometry of the collecting obstacle. The diffusion processes are the key to the effectiveness of gas filters, as we shall see later. 

When particles experience a mean curvilinear motion and also have Brownian agitation, they are deposited on obstacles by both mechanisms. For very small particles of radii less than 0.1 /xm, Brownian motion dominates particle collection on surfaces. For larger particles, inertial forces dominate. An example of the difference in collection efficiency for spherical collectors of different size is shown in Fig. 3 for different particle diameters and aerosol flow velocity. 

Aerosol particles used for inhalation deposit within the lower respiratory tract mainly by inertial impaction, sedimentation, and diffusion. Loose fractal aerosols were found to settle slower and therefore had more time to increase gravitational coagulation with other floes leading to much more rapid particle growth. This will increase the chance of the aerosol floes settling on the airway walls before reaching the end of the airways. 

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