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Dimensionless quantity characterising the ratio of thermal convection to thermal conduction in fluids across (normal to) the boundary with solids:

where  $//<![CDATA[ \begin{array}{l}U\end{array} //]]>$ is the convective heat transfer coefficient of the flow,  $//<![CDATA[ \begin{array}{l}L\end{array} //]]>$ is the characteristic length,  $//<![CDATA[ \begin{array}{l}\lambda\end{array} //]]>$ is the thermal conductivity of the fluid.

Stagnant Fluid

For  Stagnant Fluid the Nusselt number is a constant number (OEIS sequence A282581):

Natural Convection

In Natural Fluid Convection becomes dependant on Rayleigh number  $//<![CDATA[ \begin{array}{l}\rm Ra\end{array} //]]>$ and Prandtl number  $//<![CDATA[ \begin{array}{l}\rm Pr\end{array} //]]>$:  $//<![CDATA[ \begin{array}{l}\mbox{Nu} = f (\mbox{Ra}, \mbox{Pr})\end{array} //]]>$.

Forced Convection

In Forced Fluid Convection the Nusselt number becomes dependant on Reynolds number  $//<![CDATA[ \begin{array}{l}\rm Re\end{array} //]]>$ and Prandtl number  $//<![CDATA[ \begin{array}{l}\rm Pr\end{array} //]]>$:  $//<![CDATA[ \begin{array}{l}\mbox{Nu} = f (\mbox{Re}, \mbox{Pr})\end{array} //]]>$.

See also

Physics / Thermodynamics / Heat Transfer

[ Heat Transfer Coefficient (HTC) ] [ Heat Transfer Coefficient @model ]

[ Dimensionless Heat Transfer Numbers ]

References