2.2. Phenomena and Mechanism of Hydrodynamic Dispersion
23
FIGURE 2.7. The longitudinal and
transversal dispersion in hydrodynamic dispersion phenomenon.
concentration areas to form a uniform concentration distribution. Even in
a stationary fluid, the concentration gradient will still cause the tracer to
spread out to a larger area.
When a fluid flows in a porous medium, both mechanical dispersion and
molecular diffusion are in action. The total effect of them forms the hydrodynamic dispersion. The mechanical dispersion makes the tracer particles
move along the channels, while the molecular diffusion causes the tracer
concentration to become homogeneous, not only within a channel, but also
among channels. Mechanical dispersion plays a major role in the total dispersion. However, when the flow velocity is extremely low, molecular diffusion may be co me more prominent. Obviously, both mechanical dispersion
and molecular diffusion will force the tracer to move longitudinally and
transversally with respect to the mean flow direction. The former is called
longitudinal dispersion and the latter transversal dispersion. Figure 2.7 gives
an illustration of hydrodynamic dispersion.
2.2.3 Mass Transport in Porous Media
There are many factors which cause changes of solute concentration in porous media. The primary factors are listed below:
1. M acroscopic advection. The solute flows on a macroscopic scale with its
carrier, the solvent, from one place to another.
2. Hydrodynamic dispersion. As mentioned above, the combined effects of
mechanical dispersion and molecular diffusion make the solute spread to
an even larger area than simple advection.
3. Sources and sinks of the solute. The solute may enter or leave the porous
area through sources or sinks. Discrete and distributed pollutant sources,
weHs, discharge ditches and recharge weHs are examples of solute sources
and sinks.
4. Adsorption and ion exchange. Adsorption and ion exchange occur at the
interface between the solid and liquid phases. The solute in the liquid may
23
FIGURE 2.7. The longitudinal and
transversal dispersion in hydrodynamic dispersion phenomenon.
concentration areas to form a uniform concentration distribution. Even in
a stationary fluid, the concentration gradient will still cause the tracer to
spread out to a larger area.
When a fluid flows in a porous medium, both mechanical dispersion and
molecular diffusion are in action. The total effect of them forms the hydrodynamic dispersion. The mechanical dispersion makes the tracer particles
move along the channels, while the molecular diffusion causes the tracer
concentration to become homogeneous, not only within a channel, but also
among channels. Mechanical dispersion plays a major role in the total dispersion. However, when the flow velocity is extremely low, molecular diffusion may be co me more prominent. Obviously, both mechanical dispersion
and molecular diffusion will force the tracer to move longitudinally and
transversally with respect to the mean flow direction. The former is called
longitudinal dispersion and the latter transversal dispersion. Figure 2.7 gives
an illustration of hydrodynamic dispersion.
2.2.3 Mass Transport in Porous Media
There are many factors which cause changes of solute concentration in porous media. The primary factors are listed below:
1. M acroscopic advection. The solute flows on a macroscopic scale with its
carrier, the solvent, from one place to another.
2. Hydrodynamic dispersion. As mentioned above, the combined effects of
mechanical dispersion and molecular diffusion make the solute spread to
an even larger area than simple advection.
3. Sources and sinks of the solute. The solute may enter or leave the porous
area through sources or sinks. Discrete and distributed pollutant sources,
weHs, discharge ditches and recharge weHs are examples of solute sources
and sinks.
4. Adsorption and ion exchange. Adsorption and ion exchange occur at the
interface between the solid and liquid phases. The solute in the liquid may