Nusselt number

A dimension-less parameter characterizing the intensity of convective heat exchange between the surface of a body and the flow of a gas (fluid): $\mathrm{Nu}=\alpha l/\lambda$, where $\alpha=Q/S\Delta T$ is the coefficient of heat exchange, $Q$ is the amount of heat radiated (or received) by the surface of the body in unit time, $\Delta T>0$ is the difference between the temperature of the surface of the body and that of the gas (fluid) outside the boundary layer, $S$ is the area of the surface, $l$ is the characteristic length, and $\lambda$ is the coefficient of heat conductivity of the gas (fluid).

It is named after W. Nusselt.

References

[a1]	E.U. Condon, "Heat transfer" E.U. Condon (ed.) H. Odishaw (ed.), Handbook of Physics, McGraw-Hill (1967) pp. §5.5.7–5.5.8
[a2]	E.R.G. Eckert, "Introduction to the transfer of heat and mass", McGraw-Hill (1950)