To reduce heat losses, a horizontal rectangular duct that is $W=1\mathrm{.}0m$ wide and $H=0\mathrm{.}3m$ high is encased in a metal radiation

shield. The duct wall and shield are separated by an air gap of thickness $t=0\mathrm{.}06m.$

For a duct wall temperature of $T_d=40°C$ and a shield temperature of $T_{sh}=20°C,$ determine the rate of convection heat loss per

unit length from the duct, in W$/$m$.$

$q_{\text{c}\text{o}\text{n}\text{v}}^{\text{'}}=\frac{W}{m}$