= Nusselt number
{c}
{title2=$\mathrm{Nu}$}
{wiki}
The <Nusselt number> compares heat exchange associated with flow with a reference conductive exchange across a chosen length. If a surface has convective heat-transfer coefficient $h_c$, fluid <thermal conductivity> $k$ and characteristic length $L$, then
$$
\mathrm{Nu}=\frac{h_cL}{k}=\frac{qL}{k\Delta T}.
$$
Here $q=h_c\Delta T$ is the relevant normal <heat flux>, and $\Delta T$ must use the temperature difference specified for that geometry, such as surface-to-bulk fluid <temperature>.
A thin <thermal boundary layer> gives a large surface <temperature gradient> and can produce a large <Nusselt number>: flow continually brings fluid that can accept or supply <heat>. In a reference planar layer with purely conductive transport and thickness $L$, the same definition gives one. The numerical value depends on the chosen geometry, length and thermal boundary conditions, so values quoted with different conventions cannot be compared without conversion. A measured surface <power> and temperature difference can determine $h_c$ and hence $\mathrm{Nu}$.
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