Peclet number, Pe, is a dimensionless group representing the ratio of heat transfer by motion of a fluid to heat transfer by thermal conduction.
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.