The unidirectional mass transfer model defines a positive mass flow rate per unit volume from phase to phase :

(14–577)

where

(14–578)

and is a constant rate of particle shrinking or swelling, such as the rate of burning of a liquid droplet (units of 1/ time unit).

If phase is a mixture material and a mass transfer mechanism is defined for species of phase , then

(14–579)

where is the mass fraction of species in phase .