A 30 cm x 30 cm horizontal duct made of sheet metal carries warm air. The duct is maintained at 650C and is 10 m long. Heat loss from duct to the ambient air takes place due to free convection only. If the ambient air is at 250C, calculate the heat loss from the duct.
Use the following free convection correlations. Heat transfer from the upper surface of a horizontal plate:
$\overline{Nu}$ = 0.54 (RaL)1/4
Heat transfer from the lower surface of a horizontal plate :
$\overline{Nu}$ = 0.27 (RaL) ¼
Heat transfer from vertical plate :
$\overline{Nu}$ = 0.59 (RaL)1/4
The properties of air at 450 are
Pr = 0.695, u = 1.85 x 10–5 m2/sec,
k = 0.028 w/mk. 20