Unlike conduction heat transfer, however, the diffusion of
a species always involves the movement of molecules or atoms from one location to
another. In many cases, this molecular scale motion results in bulk motion. In this section
we define the total or absolute flux of a species, which includes both diffusive and advective
components.
The absolute mass (or molar) flux of a species is defined as the total flux relative to a
fixed coordinate system. To obtain an expression for the absolute mass flux, consider
species A in a binary mixture of A and B. The absolute mass flux is related to the
species absolute velocity vA by