Air flows through a long 0.3 m square duct maintains the outer duct surface temperature at 10°C. If the duct is uninsulated and exposed to air at 35°C, what is the heat gain per unit length of the...

Air flows through a long 0.3 m square duct maintains the outer duct surface temperature at 10°C. If the duct is uninsulated and exposed to air at 35°C, what is the heat gain per unit length of the duct ?

How is the friction factor related to pressure drop and pumping power ?