This empirical correlation is valid over a wide range of Reynolds and Prandtl numbers and is recommended for external flow over cylinders.

\begin{equation}\label{eq:insulated-pipe-nusselt-forced} \mathrm{Nu}_{forced} = 0.3 + \frac{0.62\,\mathrm{Re}^{1/2}\,\mathrm{Pr}^{1/3}} {\left[1+\left(\frac{0.4}{\mathrm{Pr}}\right)^{2/3}\right]^{1/4}} \left[1+\left(\frac{\mathrm{Re}}{282000}\right)^{5/8}\right]^{4/5} \end{equation}

Symbols

\(\mathrm{Nu}_{forced}\)	Forced-convection Nusselt number \([\unit{ \unitless}]\)
\(\mathrm{Re}\)	Reynolds number \([\unit{ \unitless}]\)
\(\mathrm{Pr}\)	Prandtl number of air at the film temperature \([\unit{ \unitless}]\)