Typically, for free convection, the average Nusselt number is expressed as a function of the Rayleigh number and the Prandtl number, written as:
Otherwise, for forced convection, the Nusselt number is generally a function of the Reynolds number and the Prandtl number, or
Empirical correlations for a wide variety of geometries are available that express the Nusselt number in the aforementioned forms.