Gravitational particle transport

Figure 3 Schematic filter characteristic of the human respiratory tract for aerosol particles. Three domains can be recognized the domain of deposition decreasing with particle size is solely due to diffusional particle transport, the domain of minimum deposition is due to simultaneous diffusional and gravitational particle transport, and the domain of deposition increasing with particle size due to gravitational and inertial particle transport.

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. 

Particles from a dispersion can be convected to the inner or outer surface of a porous substrate in contact with the dispersion due to fluid flow through the porous support. Also body forces due to centrifugal or electric fields can, in principle, be used to assist the particle transport towards the substrate. When the support is not permeable for the particles in the dispersion, the particle transport results in a more or less dense particle compact. The gravitational force on the particles can also contribute to the particle packing process when the gravitational force is in the same direction as the fluid flow. 

The force fields of most interest i n particle transport are gravitational, electrical, and thermal. with the last field produced by temperature gradients in the gas. If a balance exists locally in the gas between the force field and the drag on the particle, the two can be equated to give... 

Figure 11.1 Processes taking place in an elemental volume included in the general dynamic equation. Gas flows produce particle transport across the element bound-aric.s. In addition lo gravitation, olherforce lields that drive Iluacs not shown] aie electrical potential and temperature gradients.

The sedimentary environment of the Argentine Basin is largely controlled by powerful current systems and intense gravitational mass transport (Ledbetter and Klaus 1987). Sinking particles, or particles already deposited, are subjected to a lateral drift over wide passages, or become resuspended (compare with Section 12.3.2). This... 

This concept is confined to particles transported by gravitational sedimentation without interference of diffusion and, consequently, to particles larger than 1 pm in aerodynamic diameter. For smaller particles an aerodynamic diameter is not defined. Therefore, the abdssa in Fig. 7 (bottom) has to have a 1-pm origin. 

When dealing with the sedimentation of colloidal particles, it is principally necessary to regard the Brownian motion of the particles, which results in diffusive particle transport and, thus, acts against the migration in the gravitational or centrifugal field. The relevance of the Brownian motion can be roughly estimated by means of a Peclet-number ... 

Mechanisms of Mechanical Particle Transport. When particles do not follow, but diverge from, airflow streamlines and thereby come in contact with airspace surfaces, particle deposition occurs. This diverging from airflow streamlines and particle trajectories is mainly due to mechanisms of mechanical particle transport inertial, gravitational, and diffusional particle transport (Fig. 3). The... 

With a typical ablated particle size of about 1 -pm diameter, the efficiency of transport of the ablated material is normally about 50% most of the lost material is deposited on contact with cold surfaces or by gravitational deposition. From a practical viewpoint, this deposition may require frequent cleaning of the ablation cell, transfer lines, and plasma torch. 

Transport Disengaging Height. When the drag and buoyancy forces exerted by the gas on a particle exceed the gravitational and interparticle forces at the surface of the bed, particles ate thrown into the freeboard. The ejected particles can be coarser and more numerous than the saturation carrying capacity of the gas, and some coarse particles and clusters of fines particles fall back into the bed. Some particles also coUect near the wall and fall back into the fluidized bed. 

Environmental Fate. It can be concluded from the transport characteristics that surface water sediment will be the repository for atmospheric and aquatic thorium. Normally, thorium compounds will not transport long distances in soil. They will persist in sediment and soil. There is a lack of data on the fate and transport of thorium and its compounds in air. Data regarding measured particulate size and deposition velocity (that determines gravitational settling rates), and knowledge of the chemical forms and the lifetime of the particles in air would be useful. 

Nickel releases to the atmosphere are mainly in the form of aerosols that cover a broad spectrum of sizes. Particulates from power plants tend to be associated with smaller particles than those from smelters (Cahill 1989 Schroeder et al. 1987). Atmospheric aerosols are removed by gravitational settling and dry and wet deposition. Submicron particles may have atmospheric half-lives as long as 30 days (Schroeder et al. 1987). Monitoring data confrrm that nickel can be transported far from its source (Pacyna and Ottar 1985). Nickel concentrations in air particulate matter in remote, rural, and U.S. urban areas are 0.01-60, 0.6-78, and 1-328 ng/m, respectively (Schroeder et al. 1987). 

It should be kept in mind that these calculated rates of diffusion and gravitational settling are only applicable to still air. In fact, in the atmosphere the air is rarely still and is usually undergoing some degree of turbulent motion. In this case, the transport of particles becomes more complex and faster due to the velocity gradients and contorted patterns of air flow however, a discussion of this is outside the scope of this book. 

In natural systems there are two types of transport phenomena (1) transport by random motion, and (2) transport by directed motion. Both types occur at a wide range of scales from molecular to global distances, from microseconds to geological times. Well-known examples of these types are molecular diffusion (random transport) and advection in water currents (directed transport). There are many other manifestations such as dispersion as a random process (see Chapters 24 and 25) or settling of suspended particles due to gravitation as a directed transport. For simplicity we will subdivide such transport processes into those we will call diffusive for ones caused by random motions and those called advective for ones resulting from directed motions. 

As Martell has pointed out (30), in the region of the stratospheric large particle layer near 18-20 km. altitude, radioactive aerosol particles become attached to natural sulfate particles in the size range of about 0.1-0.4 jumeter radius. Subsequent upward transport of the radioactive aerosols is opposed by gravitational sedimentation. This combination of processes affords an explanation for the observed accumulation of 210Pb near 20 km. in the tropical stratosphere (2). At higher latitudes where slow mean motions are directed poleward and downward, no such accumulation is possible. 

Tin may be transported in the atmosphere by the release of particulate matter derived from the combustion of fossil fuels and solid wastes. The vapor pressure of elemental tin is negligible (Cooper and Stranks 1966). Tin in aerosol samples that existed in particulate-carbon masses was removed from the atmosphere predominantly by gravitational settling (Byrd and Andreae 1986). The half- life of airborne particles is usually on the order of days, depending on the size of the particle and atmospheric conditions (Nriagu 1979). Removal by washout mechanisms (such as rain) is thought to be unimportant. 

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