Gravitation settling of particles

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

Gravitational settling of particle in fluid Flow of viscoelastic fluid Pressure and momentum in fluid Unsteady state heat transfer/mass transfer Fluid flow with free surface Gravitational settling of particle in fluid Heat transfer by natural convection Heat transfer to fluid in tube Flow of fluid exhibiting yield stress... 

Gravitational settling of particles in liquids is an age-old process which can be used for a variety of purposes. For example, it is used for the classification of solids, washing, particle size measurement or mass transfer, and in solvent extraction. The majority of applications of gravity sedimentation, however, are in solid-liquid separation duty. The object here is to remove the solids from the liquid either because the solids and/or the liquid are valuable or because the two phases have to be separated before disposal. 

Gravitational settling of particles is enhanced by increased particle size, which occurs spontaneously by coagulation (see Chapter 6, Figure 6.11). Thus, over time, the size of particles increases and the number of particles decreases in a mass of air that contains particles. Brownian motion of particles less than about 0.1 mi in size is primarily responsible for their contact, enabling coagulation to occur. Particles greater than about 0.3 pm in radius do not diffuse appreciably and serve primarily as receptors of smaller particles. 

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... 

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

Other factors also come into play in laboratory systems. For example, McMurry and Rader (1985) have shown that particle deposition at the walls of Teflon smog chambers is controlled by Brownian and turbulent diffusion for particles with Dp 0.05 yxm and by gravitational settling for particles with Dp > 1.0 yxm. However, in the 0.05- to 1.0-yxm range, the deposition is controlled by electrostatic effects Teflon tends to... 

Consider an axially symmetric cloud of spherical particles in the atmosphere. If wind and diffusion effects are neglected, the only change will be caused by gravitational settling of individual particles. Denote by f(a,x,t) the number density of particles with diameter a at position z and time t. The continuity equation is... 

A chemical may be removed from the atmosphere either by physical processes or by chemical transformation. If a chemical exists in the atmosphere in the form of sufficiently large particles or liquid droplets, physical deposition of the chemical to soil, vegetation, or water bodies may occur by gravitational settling. Smaller particles with negligible settling velocities also may be deposited to surfaces by impaction or diffusion. Chemicals in gaseous form may either sorb onto surfaces directly or sorb onto airborne particles that are subsequently removed. 

Maxey, M. R. 1987 The gravitational settling of aerosol particles in homogeneous turbulence and random flow fields. Journal of Fluid Mechanics 174,441 65. 

Example 8.3 Gravitational Settling and the Vertical Distribution of Aerosol Particles Let us consider the simultaneous Brownian diffusion and gravitational settling of aerosols above a surface atz = 0.Atr=0,a uniform concentration Nq — 1000 cm of particles is assumed to exist for 2 > 0 and at all times the concentration of particles right at the surface is zero as a result of their removal at the surface. 

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