The Lewis Number (Le) is defined as the ratio of the Schmidt Number (Sc) and the Prandtl Number (Pr). The Lewis Number is also the ratio of thermal diffusivity and molecular diffusivity as is found from the definitions of Schmidt and Prandtl Number, as follows:
(1)
where κ = (λ/ρcp) is the Thermal Diffusivity. The Lewis Number is important in determining the relationship between mass and heat transfer coefficients (the Lewis Relationship).