Convectiondiffusion equation
The convectiondiffusion equation can be written on a mass or mole basis, and it describes density or concentration field of a component in a multicomponent flowing fluid.
Mass basis
If is the total mass flux of species in a multicomponent mixture, the convectiondiffusion equation is

(1) 
where
 is the local change in the density of species A.
 is the difference in the mass influx and outflux of species A.
 is the rate of generation or consumption per unit volume of species due to a homogenous reaction.
For a fluid of constant mass density and with constant diffusivities, Eq. (1) can be expressed as
(2) 
This can also be expressed in the compact, material derivative form:
(3) 
Mole basis
If is the total mole flux of species in a multicomponent mixture, the convectiondiffusion equation is

(4) 
where
 is the local change in the concentration of species A.
 is the difference in the molar influx and outflux of species A.
 is the molar rate of generation or consumption per unit volume of species due to a homogenous reaction.
For a fluid of constant concentration and with constant diffusivities, Eq. (4) can be expressed as
(5) 