Particle gravitational sedimentation

Sample particle trajectories for 0.5-pm particles in a three-generation alveolar structure are shown in Fig. 59. Here, the color corresponds to the particle residence time. For 0.5-p,m particles, it is seen that the Brownian motion and gravity do not have noticeable effects on the particle trajectories, and these particles follow the airflow in the alveolar duct. Chang and Ahmadi" showed that for smaller particle, the Brownian motion effect becomes important, while for larger particle gravitational sedimentation effect is significant. 

Gravitational Sedimentation Methods In gravitational sedimentation methods, particle size is determined from settling velocity... 

Filtration frequently is accompanied by hindered or free gravitational sedimentation of solid particles. The directions of action of gravity force and filtrate motion may be cocurrent, countercurrent or cross current, depending on the orientation of the... 

These collectors are used primarily for large particles (> 2.5 /im), that is, those in the coarse particle range. They include collection by gravitational sedimentation (e.g., dustfall jars) as well as by centrifugal... 

Gravitational sedimentation only collects the large particles that settle out of the atmosphere fairly quickly. This dustfall generally consists of particles that are relatively large and, as such, are not particularly relevant to the focus of this book. Thus dustfall collectors will not be discussed further. 

Bomb products, rocket reentry bumup products, meteoritic dust, and other trace constituents will be redistributed rapidly within the mesosphere by large scale circulation, by eddy diffusion and, for particles of significant size, by gravitational sedimentation as well (1). Murgatroyd and Singleton (32) discuss the circulation of the mesosphere and indicate a meridional pattern, with ascent over the summer pole, descent over the winter pole, and a well defined flow from the summer hemisphere to the winter hemisphere above 50 km., with speeds of the order of meters sec."1 horizontally and cm. sec."1 vertically. 

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. 

Gravitational sedimentation causes a change in the particle size distribution anywhere in and below the cloud compared with the size distribution at stabilization time. Thus, to reconstruct the size distribution at stabilization time, corrections must be applied to the size distributions measured in the samples. These corrections were calculated by assuming Stokesian settling modified by a drag slip correction. It was assumed further that at stabilization time the cloud was axially symmetric and consisted of spherical particles. Wind and diffusion effects were neglected. 

Filtration is a physical separation whereby particles are removed from the fluid and retained by the filters. Three basic collection mechanisms involving fibers are inertial impaction, interception, and diffusion. In collection by inertial impaction, the particles with large inertia deviate from the gas streamlines around the fiber collector and collide with the fiber collector. In collection by interception, the particles with small inertia nearly follow the streamline around the fiber collector and are partially or completely immersed in the boundary layer region. Subsequently, the particle velocity decreases and the particles graze the barrier and stop on the surface of the collector. Collection by diffusion is very important for fine particles. In this collection mechanism, particles with a zig-zag Brownian motion in the immediate vicinity of the collector are collected on the surface of the collector. The efficiency of collection by diffusion increases with decreasing size of particles and suspension flow rate. There are also several other collection mechanisms such as gravitational sedimentation, induced electrostatic precipitation, and van der Waals deposition their contributions in filtration may also be important in some processes. 

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. 

Gravitational sedimentation is the predominant deposition mechanism in the last five to six generations of airways (e.g., the bronchioles and alveolar region), where the air velocity is very low [23]. It allows deposition of particles into the... 

Consider the flux of particles crossing a horizontal surface A, where the concentration of the particles is C and the concentration gradient is dc/dx. The flux due to gravitational sedimentation on this plane is C dx/dt and the flux due to diffusion is D dC/dx. Because these two fluxes are equal at equilibriiun, we have the foUowing equation ... 

The equations to quantify this effect were developed by Mason and Weaver in 1924 [2]. For monosize distributions Brownian motion leads to broadened measured distributions [3] but the effect is reduced as the width of the distribution increases [4,5]. Chung and Hogg [6] carried out theoretical and experimental studies of clay particles using centrifugal and gravitational sedimentation. Agreement between theory and practice was not too good. 

For an estimate of the lower size limit, the displacement of the smallest particle by Brownian diffusion should be at least ten times smaller than its settling distance [10]. Other criteria could be selected since the error is both a function of the size and the spread of the distribution. It is reasonable however, for the sake of simplicity, to stipulate that if more than 10% of the distribution is smaller than the lower size limit, gravitational sedimentation should not be used. 

Gravitational sedimentation methods of particle size determination... 

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