I read, that for a box with length L, with heated bottom and cooled top and boundary layers of thickness $\delta$, the Nusselt number Nu can be approximated as $$ \text{Nu}= \begin{cases} 1,& \text{if } \text{Ra} \ll \text{Ra}_\text{crit}\\ \frac{L}{2 \delta}, & \text{if Ra} \gg \text{Ra}_\text{crit}\\ \end{cases} $$
where Ra is the Rayleigh number. The first case is obvious, but how can one derive the second case?
