Generally, the Nusselt number is ratio of convection to conduction Times the hydraulic diameter. If you use the fluent, the software calculated automatically.

The Nusselt number between two flat plates with a laminar depends on the distance L between the position considered (distance to the flow inlet). For very large distances Nu=1 . This is for (U h/a)(h/L)1, one can use a boundary layer model. We used this model to describe the viscous flow throug a slit between the mouthpiwce of a musical instrument and the reed ( J. Van Zon et al. 1990 see also Andrey Ricardo da Silva Jasa more recent paper). An analytical approximation can be obtained by using the Von Karnan boundary layer apriximation. This is not valid for very short distances L

Nusselt number basically calculated by using the formula (Nu=((hLc)/k), Where h- convective heat transfer coefficient, Lc is characteristic length. In your case, the distance between parallel plates is Lc. and Finally k is thermal conductivity of the material. If the flow is laminar, conductive heat transfer dominates. In case of Turbulent flow the convective term dominated in the nature of Nusselt number calculation.