  Tracer dispersion is studied experimentally in periodic or disordered arrays
of beads in a capillary tube. Dispersion is measured from light absorption
variations near the outlet following a steplike injection of dye at the inlet.
Visualizations using dye and pure glycerol are also performed in similar
geometries. Taylor dispersion is dominant both in an empty tube and for a
periodic array of beads: the dispersivity $l\_d$ increases with the P\'eclet
number $Pe$ respectively as $Pe$ and $Pe^{0.82}$ and is larger by a factor of 8
in the second case. In a disordered packing of smaller beads (1/3 of the tube
diameter) geometrical dispersion associated to the disorder of the flow field
is dominant with a constant value of $l\_d$ reached at high P\'eclet numbers.
The minimum dispersivity is slightly higher than in homogeneous nonconsolidated
packings of small grains, likely due heterogeneities resulting from wall
effects. In a disordered packing with the same beads as in the periodic
configuration, $l\_d$ is up to 20 times lower than in the latter and varies as
$Pe^\alpha$ with $\alpha = 0.5$ or $= 0.69$ (depending on the fluid viscosity).
A simple model accounting for this latter result is suggested.

Fluid Dynamics