Influence of the disorder on solute dispersion in a flow channel

Solute 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 Peclet 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 Peclet numbers. The minimum dispersivity is slightly higher than in homogeneous nonconsolidated packings of small grains, likely due to wall e. ffects. In a weakly disordered packing with the same beads as in the periodic con.figuration, ld is up to 20 times lower than in the latter and varies as Pe. with. = 0.5 or = 0.69 ( depending on the fluid viscosity). A simple model accounting for this latter result is suggested.