(written by: Willy Yanto Wijaya)
Nusselt number is defined as:
where h = convective heat transfer coefficient (W/m2K)
lamda = thermal conductivity of fluid (W/mK)
L = characteristic length (m)
which can be viewed as the ratio of convective to conductive heat transfer across the boundary (surface). It is, as a matter of fact, can be derived easily from the Newton’s Law of Cooling (convective term) and the heat conduction term (at the same condition as the heat convection). Therefore, it becomes obvious that:
Nusselt number is a dimensionless number. The characteristic length (L) is determined by the direction of the growth (thickness) of the boundary layer.
Then, what kind of physical meaning does this Nusselt number imply? If Nu=1, this means that the convection and conduction terms have relatively similar magnitude and thus is characterized by the laminar flow. On the other hand, large Nu implies that the convective term is dominant, which typically characterized by turbulent flows (usually Nu value in the range of 100-1000).
Therefore, by understanding the Nusselt number of a flow system, we can infer the dominance between convection and conduction heat transfer terms, and thus enabling us to design better and more efficient thermal engineering systems, especially in the convective heat transfer field.