The mathematical models describing the transfer processes are based on the system of the Navier-Stokes equations for momentum transfer and similar equations for energy and mass transfer. These equations are conservation laws expressed in terms of the velocity components, temperature, and concentration. For incompressible fluid flows with constant properties, these expressions are given by equations 1.4–1.8 (without dissipation function S) from Chapter 1 supplemented by equation for mass transfer similar to energy equation, where C is dimension-less concentration and ρD_m is the coefficient similar to μ.