Settling, gravitational

One of the important properties of particles that contributes to both the observed size distribution and the number concentration of aerosols in the atmosphere is the motion they undergo when suspended in air. This includes gravitational settling and Brownian diffusion. 

In the free troposphere, particles are subjected to gravitational forces. They also can be subjected to electrical forces in nature as well as in the course of detection and measurement. When such forces are applied, the particle moves relative to the gas and hence is subjected to a resistance force. Stokes law gives the force (FR) acting on smooth spherical particles due to the laminar flow of air over them  

One can apply Stokes law to atmospheric particles to calculate how fast they will settle out of the air when subjected to gravity alone. Thus the terminal settling velocity occurs when the frictional and gravitational forces are balanced, that is, 

The settling velocity thus increases with the density of the particle and with the square of its diameter. In developing Eq. (O), the buoyancy effect of air, which tends to lower the effective particle density, has been ignored since it is much smaller than the particle density it can be included if desired by replacing p by (pp PairX where pp is the particle density and p.,h. is the air density (1.2 X 10 3 g cm 3 at 20°C and 1 atm pressure). 

The expression given in Eq. (O) for the terminal settling velocity only applies to particles with diameters 1.5 p,m because its derivation is based on the assumption that the relative speed of the air at the surface of the particle is zero. However, as the particle becomes smaller, the air molecules appear less as a continuous fluid and more as discrete molecules separated by space through which the particles can slip. The net effect is that the particles can move faster than predicted by Eq. (O) due to this slipping between gas molecules. To correct for this effect, a correction factor must be applied to the resistance force predicted by Stokes law, Eq. (M). The correction factor, C, is a number greater than 1. Thus Eq. (M) is modified to 

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Between V 9 and the concentration of solids gradually becomes more uniform in the vertical direction. This transition has been modeled by several authors as a concentration gradient where turbulent diffusion balances gravitational settling. See, for example, Karabelas (AJChE]., 23, 426 34 [1977]). 

As indicated in previous chapters, the atmosphere serves as the medium through which air pollutants are transported and dispersed. While being transported, the pollutants may undergo chemical reactions and, in addition to removal by chemical transformations, may be removed by physical processes such as gravitational settling, impaction, and wet removal. 

Other lesser mechanisms that result in aerosol removal by filters are (1) gravitational settling due to the difference in mass of the aerosol and the carrying gas, (2) thermal precipitation due to the temperature gradient between a hot gas stream and the cooler filter medium which causes the particles to be bombarded more vigorously by the gas molecules on the side away from the filter element, and (3) Brownian deposition as the particles are bombarded with gas molecules that may cause enough movement to permit the aerosol to come in contact with the filter element. Browruan motion may also cause some of the particles to miss the filter element because they are moved away from it as they pass by. For practical purposes, only the three mechanisms shown in Fig. 29-1 are normally considered for removal of aerosols from a gas stream. 

There are a large number of processes in the chemical industries that handle a variety of suspensions of solid particles in liquids. The application of filtration techniques for the separation of these heterogeneous systems is sometimes very costly. If, however, the discrete phase of the suspension largely contains settleable particles, the separation can be effected by the operation of sedimentation. The process of sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening,... 

The most favorable filtration operation with cake formation is process whereby no clogging of the filter medium occurs. Such a process is observed at sufficiently high concentrations of solid particles in suspension. From a practical standpoint this concentration may conditionally be assumed to be in excess of 1% by volume. Filtration is frequently accompanied by hindered or free gravitational settling of solid particles. The relative directions of action between gravity force and filtrate... 

Sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening, i.e.. 

The ratio of settling velocity in a centrifuge to that of pure gravitational settling is ... 

The rest of the less volatile fission products along with constituents of zircalloy, stainless steel, and the control rods are assumed to be in condensed form as inert aerosols that are treated together in TRAPMELT as "other aerosols." The aerosols are modeled as agglomerating and depositing on surfaces by several mechanisms (e.g., gravitational settling). 

In the case of gravitational settling, the unit design depends on the method of solid removal after settling. The methods in use are... 

S. Murakami, S. Kato, S. Nagano, Y. Tanaka. Diffusion characteristics of airborne particles with gravitational settling in a convective-dominant indoor flow field. ASHRAE Transactions. 98(1), 1992, 82-97. 

Dry The interception and retention by surfaces of gases or particulate matter by diffusion, gravitational settling, or thermal forces. 

Gravitational settling The fallout of particulate matter from a gas stream due to the gravity forces being predominant over the flow velocity forces... 

For particles smaller than O.lp the random Brownian motion is greater than the motion due to gravitational settling. Therefore the above relations based on Stokes Law will not hold. 

A r Archimedes Ga pips - P)g <3 "t P- Gravitational settling of particle in fluid... 

F5 Gravitational settling of large (aeolian) particles to land 12 18... 

Americium released to the atmosphere will be associated with particulate matter and will be deposited on land or surface water by dry deposition or wet deposition (Essien et al. 1985). Dry deposition results from gravitational settling and impaction on surfaces, and wet deposition returns americium to earth in precipitation. Radionuclides resulting from atmospheric weapons tests are often injected into the lower stratosphere, while other atmospheric releases are into the troposphere. The residence time of particles in the atmosphere will depend on the altitude, latitude, season, and hemisphere because of atmospheric... 

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