When the Nu is equal to 1, it means that all the heat transfer is made by the diffusion.
 The diffusion term in fluids is equivalent to the conduction term with solids and can easily be calculated with the Fourrier law:
where Phi is the surfacic heat transfer and lambda the thermal conductivity.
The flux is reduced in our case to:


As soon as there is a movement of the fluid particles at the macroscopic level, it creates a heat flux due to convection. As a consequence, Nu become greater than one.
 We determine the
value of the total heat flux using one property of Fluent.
Report => Fluxes  
A window appears and we can choose to compute the Total heat transfer
rate at a Wall, and the result is given in W.
Using both calculation formerly defined, we can
plot the value of the Nusselt number depending on the Rayleigh number (equivalent
to the difference of temperature) for several configuration.
We see on the graph above
that the critical Rayleigh number is approximately 2000. This is the result
given by Fluent. The theoretical critical Ra is 1708.
The difference can be explained by the sensibility
of the results to several conditions, such as the initial conditions. In
our case, the initial conditions are a constant temperature in the domain,
equal to the average between the walls temperatures, and a velocity equal
to zero.
Furthermore, there is an approximation inherent to the solving. We can have an example of it when we compare the heat fluxes on each walls. They are supposed to be equal, as there is a symmetric condition on the vertical boundaries, but the computation given by Fluent shows some differences.