Falling particles wall effects

In 1851, Stokes derived Eq. (4.1) from the model of solid spherical particles falling independently through a homogeneous liquid without Brownian motion, slippage, and wall effects. Slippage is an inconstant rate of fall wall effects refer to axial orientation in the outermost planes of fluid in contact with a surface, and the differential velocity of flow in the outermost and innermost planes of a fluid in a confining tube ... 

The size of the particles is determined by the particular material selected and the vapor concentration used. In practice, limited variation in particle size can be achieved for a particular aerosol material because conditions for stable aerosol formation require a particular set of thermal and vapor concentration conditions. The monodispersity of the aerosol can be improved by revaporization and recondensation. In systems in which the condensation occurs in a container with a high ratio of volume to surface areas, relatively monodisperse particles can be obtained (frg 1.1). Otherwise, the particle size varies with the proximity to the wall. In cylindrical or tubular systems, such as in the condensation aerosol generator developed by Liu et al. [10] or the falling-film generator, the particle size that is produced varies radially (see Ref. [3]). A more monodisperse aerosol can be produced by extracting the central portion of the flow, which is less subject to wall effects. Liu et al. [10] found that the monodispersity improved from a ug value of 1.35 to 1.15 by using only the central 5% of the aerosol flow. A commercial version of a modified Sinclair-LaMer generator is available with particle size control suited for inhalation studies [11]. 

The hindering effect of the containing wall on the falling speed of the particles cannot be ignored either. For a spherical particle, the effect can be expressed by the Landenburg equation as... 

Wall effect correction factor for free-fall speed of a particle Acceleration due to gravity (9.78-9.81 m/s )... 

Elutriation differs from sedimentation in that fluid moves vertically upwards and thereby carries with it all particles whose settling velocity by gravity is less than the fluid velocity. In practice, complications are introduced by such factors as the non-uniformity of the fluid velocity across a section of an elutriating tube, the influence of the walls of the tube, and the effect of eddies in the flow. In consequence, any assumption that the separated particle size corresponds to the mean velocity of fluid flow is only approximately true it also requires an infinite time to effect complete separation. This method is predicated on the assumption that Stokes law relating the free-falling velocity of a spherical particle to its density and diameter, and to the density and viscosity of the medium is valid... 

In addition to the effect of size and shape of the particles upon the applicability of Stoke s law in particle size analysis there are certain experimental limitations that must be considered in the use of this principle. Since the rate of fall varies inversely with the viscosity of the medium, it is important to maintain a known constant temperature during the analysis. A constant temperature also helps to prevent convection currents which might arise as a result of difference in temperature near the walls of the vessel and within the suspension. Such currents acts as a hindrance to uniform settling of the particles. In addition convection currents may also be set up during stirring which is more difficult to eliminate than those arising out of temperature variation. 

Fresh whole milk is drawn into a vacuum pan and. a por its water removed. This condensation is halted while the x still in a fluid condition and before any of the milk albumen hs cooked on to the walls of the vacuum chamber. The milk i drawn from the vacuum pan and sprayed into a current, of fa The moisture in the milk is instantly absorbed by the air a particles of milk solids fall like snow. Upon examination, tb found to contain less than 2 per oent, and sometimes not mor one-half of one per cent of moisture. The hotter the air the mori the drying effect and the less, danger there is of injuring tin solids by heat. ... 

Hopkins and Epstein [1974] also noted that deposition of ferric oxide decreased as the heat flux was raised. At extremely high heat fluxes (up to 292 kW/m ) little deposit was observed on the stainless steel surface. The researchers suggested that the effects of thermophoresis could have held the particles off the surface. At high heat flux the wall temperature is correspondingly raised for given flow conditions and as wall temperature increases, the local fouling resistance falls. 

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