longitudinal dispersion in nonuniform isotropic porous media

126
medium type. This deviation of the experimental breakthrough curve
from the theoretical curve is called "tailingll, since the "tail il of the
breakthrough curve requires more time to complete breakthrough (C/C O =
1) than would be expected theoretically. One hypothesis, to be
discussed in Chapter 5, is that the tailing behavior is a result of
heterogeneous features in the sand packs.
Since the experimental breakthrough curves tend to deviate from
the theoretical curve at high values of CIC O ' the only way we may use
equation (4.5) to calculate dispersion coefficients is to match the
theory to the leading portion of the experimental breakthrough curves.
As a consistent and practical technique for analyzing the breakthrough
curves, we will use CIC O between 1% and SO% to evaluate D L • The
theoretical curves are seen in Figures 4.14 and 4.15 to fit within 5%
the leading portion of the breakthrough curves (up to CIG O = O.S).
Figures 4.16 through 4.21 show breakthrough curves in the uniform
and nonuniform medium for various Peclet numbers and probe positions.
Note that the fit is slightly improved in the tail when "the Peclet
number is lower. Comparing Figure 4.15 with Figures 4.20 and 4.21, we
see more pronounced tailing as the breakthrough curve is displaced
further along the column. Further discussion of the tailing phenomenon
and its inteq>retaLion are given in Chapter 5.
The variation of DL/D with the molecular Peclet number for both
uniform and nonuniform media is shown in Figure 4.22 and Table 4.5.
The s2paration of the two data sets on Figure 4.22 is dependent on the