Consider a $3-$m$-$high rectangular enclosure consisting of two surfaces separated by an air gap $(L)=0\mathrm{.}14-m$ at $1$ atm $.$ The surface temperatures across the air gap are $30C$ and $-10C.$ The properties of air at $T_f=\frac{T_S+T_{}}{2}=10C$ are $K=0\mathrm{.}02439\frac{W}{m}*K,v=1\mathrm{.}426{10}^{-5}\frac{m^2}{s},Pr=0\mathrm{.}7336,$ and $=\frac{1}{T_f}=3\mathrm{.}534{10}^{-3}{K}^{-1}.$

The Nusselt numbers for vertical orientation is