The formula is based on Kuehn and Goldstein 'Correlating equations for natural convection heat transfer between horizontal circular cylinders', 1976 who presented a correlation for natural convection heat transfer from a horizontal cylinder which is valid at any Rayleigh and Prandtl number.

Symbol
$\mathrm{Nu}_{\mathrm{encl}}$
Unit
-
Formulae
$- \frac{2}{\ln{\left (- \frac{2}{\left(0.000338510012403076 G_{\mathrm{encl}}^{15} \mathrm{Ra}_{\mathrm{encl}}^{3.75} + 1.54070215745864 \cdot 10^{-14} \mathrm{Ra}_{\mathrm{encl}}^{3.75}\right)^{0.0667}} + 1 \right )}}$
Related
$G_{\mathrm{encl}}$
$\mathrm{Ra}_{\mathrm{encl}}$