In heat transfer at a boundary (surface) within a fluid, the dimensionless Nusselt number is the ratio of convective to conductive heat transfer across (normal to) the boundary.

A Nusselt number close to one, namely convection and conduction of similar magnitude, is characteristic of laminar flow. A larger Nusselt number corresponds to more active convection, with turbulent flow typically in the range of 100 to 1000.

Symbol
$\mathrm{Nu}_{\mathrm{L}}$
Unit
-
Formulae
$C_{\mathrm{Nu}} \mathrm{Ra}_{\mathrm{L}}^{\frac{1}{m_{\mathrm{Nu}}}}$
Related
$C_{\mathrm{Nu}}$
$m_{\mathrm{Nu}}$
$\mathrm{Ra}_{\mathrm{L}}$
Used in
$h_{\mathrm{tr}}$