Convection dispersion

Edwards, DA, Charge Transport Through a Spatially Periodic Porous Medium Electrokinetic and Convective Dispersion Phenomena, Philosophical Transactions of the Royal Society of London A 353, 205, 1995. 

Hynek, S. J., 1969, Forced Convection Dispersed Flow Film Boiling, Ph.D. thesis, Massachusetts Institute of Technology, Cambridge, MA. (4)... 

We can illustrate the salient features of convective dispersal by choosing a simple velocity distribution in a rectangular convection cell (0associated with the onset of Benard instability in the conditions of Boussinesq approximations (e.g., Turcotte and Schubert, 1982). Let us make the calculation for the so-called free-slip conditions, which permit free movement along the boundaries, both vertical and horizontal, such as a convection cell which would be limited by no rigid boundary. From Turcotte and Schubert (1982), we take the velocity field to be... 

Figure 8.6 Convective dispersal of a passive tracer located at t=0 inside the circle in the upper right corner by the velocity field described by equations (8.3.11).

However, the two mechanisms interact and molecular diffusion can reduce the effects of convective dispersion. This can be explained by the fact that with streamline flow in a tube molecular diffusion will tend to smooth out the concentration profile arising from the velocity distribution over the cross-section. Similarly radial dispersion can give rise to lower values of longitudinal dispersion than predicted by equation 4.39. As a result the curves of Peclet versus Reynolds number tend to pass through a maximum as shown in Figure 4.6. 

As such, suspended 3,3 -dichlorobenzidine is subject to atmospheric convection, dispersion, gravitational settling, and wash-out by rain. 

Axial and radial dispersion coefficients are equal at low Reynolds numbers because the dispersion is due to the molecular diffusion and the axial and radial structures of the bed are similar (Gunn, 1968). However, at high Reynolds numbers, the convective dispersion dominates and the values are different because the axial dispersion is primarily caused by differences in the fluid velocity in the flow channels, whereas the radial dispersion is primarily caused by deviations in the flow path caused by the particles. 

Mechanistic Multiphase Model for Reactions and Transport of Phosphorus Applied to Soils. Mansell et al. (1977a) presented a mechanistic model for describing transformations and transport of applied phosphorus during water flow through soils. Phosphorus transformations were governed by reaction kinetics, whereas the convective-dispersive theory for mass transport was used to describe P transport in soil. Six of the kinetic reactions—adsorption, desorption, mobilization, immobilization, precipitation, and dissolution—were considered to control phosphorus transformations between solution, adsorbed, immobilized (chemisorbed), and precipitated phases. This mechanistic multistep model is shown in Fig. 9.2. 

Cameron, D. R., and Klute, A. (1977). Convective-dispersive solute transport with a combined equilibrium and kinetic adsorption model. Water Res. 13, 183-188. 

The use of convection-dispersion models in oral drug absorption was first proposed in the early 1980s [177, 178]. The small intestine is considered a 1-... 

For absorbable substances, a first-order absorption term can be coupled with the convection-dispersion (6.15) to model both the fluid flow and the absorption process ... 

Recently, a novel convection-dispersion model for the study of drug absorption in the gastrointestinal tract, incorporating spatial heterogeneity, was presented [182]. The intestinal lumen is modeled as a tube (Figure 6.7), where the concentration of the drug is described by a system of convection-dispersion partial differential equations. The model considers ... 

Niro, R., Byers, J., Fournier, R., and Bachmann, K., Application of a convective-dispersion model to predict in vivo hepatic clearance from in vitro measurements utilizing cryopreserved human hepatocytes, Current Drug Metabolism, Vol. 4, No. 5, 2003, pp. 357-369. 

Edwards, D., A general theory of the macrotransport of nondepositing particles in the lung by convective dispersion, Journal of Aerosol Science, Vol. 25, No. 3, 1994, pp. 543-565. 

Fig. 25 Convection, dispersion / diffusion, retardation and degradation of a species (single peak input) versus time along a flow path...

Solute movement through soil is a complex process. It depends on convective-dispersive properties as influenced by pore size, shape, continuity, and a number of physicochemical reactions such as sorption-desorption, diffusion, exclusion, stagnant and/or double-layer water, interlayer water, activation energies, kinetics, equilibrium constants, and dissolution-precipitation. Miscible displacement is one of the best approaches for determining the factors in a given soil responsible for the transport behavior of any given solute. 

In an effort to describe effluent results obtained from the different soil layers, we utilized various versions of the multireaction model described above. In principle, we based our efforts on the assumption of the miscible displacement approach that describes retention reactions of solutes during transport in porous media (Selim, 1992). Several simplifying assumptions were necessary in order to describe the S04 experimental data based on these models. Briefly, we tested the capability of the convection-dispersion (CD) equation to describe the mobility of applied sulfates in individual soil layers where steady-state conditions were assumed. 

The convective-dispersive transport equation for reactive solutes in porous media... 

At high velocity and with gases of low diffusivity, convective dispersion in the bed will be the dominating factor for particularly with relatively large catalyst particles. 

The convection-dispersion equation can be written as (see for example Bear [ 1] or Nielsen [2] )... 

Zijl, W., 1989. Three-dimensional flow systems analysis II, Steady and unsteady nested flow systems and their relation to field-scale convective-dispersive transport of chemical constituents. TNO Institute of Applied Geoscience, Delft, The Netherlands, Report no. OS 90-14... 

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