Convective diffusion in laminar flows which develop from rest and in the velocity entrance region of tubes, is studied. Criteria for the validity of the simple dispersion model are established by comparison of analytical results with numerical experiments.
It is found that the extent of dispersion is less in developing velocity fields than in those which are fully developed. This occurs because dispersion is enhanced by differences in the velocity of the fluid particles on a plane perpendicular to the main direction of flow. Such differences are greatest when the flow is fully developed.