Unified View of Diffusion and Dispersion

Diffusion and dispersion are basically convective transport mechanisms. The method chosen for representing the convection in a mass balance equation is the determining factor in deciding whether diffusion or dispersion must be included in the conservation equation for a given flow. This principle is illustrated for molecular diffusion, turbulent diffusion, and longitudinal dispersion. In a given flow, the relative importance of diffusion or disperion in different transport problems depends on the steepness of the concentration gradient. It is proposed that the term diffusion be reserved for transport that is associated primarily with time-averaged velocity fluctuations and that dispersion be used for transport associated primarily with the spatial average of velocity variations.