 Nusselt number

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 $L$, thenHere $q=h_c\Delta T$ 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 $\mathrm{Nu}$.