Nusselt number, object to duct

Symbol
$\mathrm{Nu}_{od}$
Formulae
$0.188\left(\mathrm{Gr}_{od} \mathrm{Pr}_{gas}\right)^{0.322} \left(\frac{L_d}{\delta_d}\right)^{-0.238} \left(\frac{D_{di}}{D_o}\right)^{0.442}$