Schmidt Number, Sc, is a dimensionless parameter representing the ratio of diffusion of momentum to the diffusion of mass in a fluid. It is defined as
where ν is kinematic viscosity and δ diffusivity.
Schmidt number is the mass transfer equivalent of Prandtl Number. For gases, Sc and Pr have similar values (≈0.7) and this is used as the basis for simple heat and mass transfer analogies (see Analogy Between Heat, Mass and Momemtum Transfer).
Hewitt, G. F, Shires, G. L. and Bott, T. R. (1994) Process Heat Transfer, CRC Press.