The result is known as the analogy between heat and mass
transfer and is applicable to both laminar and turbulent flows.
The relation is also valid for free convection. The ratio α⁄Dab
is defined as the Le number, which describes simultaneous
heat and mass transfer by convention. Kusuda (1965) recommends
that c = 1/3 so that the Le number is raised to the 2/3
power for forced-convection flow. Moist air properties yield
an Le number of about 0.85, and the right-hand side of Equation
3 would therefore be about 0.9, close to unity.