Peclet number, Pe, is a dimensionless group representing the ratio of heat transfer by motion of a fluid to heat transfer by thermal conduction.
where u is fluid velocity, L is a characteristic dimension, and k is thermal diffusivity of the fluid. Re is Reynolds Number and Pr is Prandtl Number.
Heat transported by the fluid per unit area is proportional to uρcp where ρ is density and cp is specific heat capacity, while heat conducted per unit area is proportional to λ/L where λ is thermal conductivity. Hence
Peclet number for mass transfer has the corresponding form
where δ is diffusivity and Sc is Schmidt Number.