Nusselt number gas-enclosure

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
$\mathrm{Nu}_{encl}$
Formulae
$- \frac{2}{\ln{\left(1 - \frac{3.40806269406455}{\left(G_{encl}^{15} \mathrm{Ra}_{encl}^{3.75} + 4.55142270836015 \cdot 10^{-11} \mathrm{Ra}_{encl}^{3.75}\right)^{0.0667}} \right)}}$
$\mathrm{Ra}_{encl}$