Find the expression for velocity profile, the mass flow and heat transfer rates of air due to natural convection in a vertical channel of circular and square cross-section for the fully developed flow. The walls are kept at temperature Tw= 30 C and the channel has the height of H=3 m.. The walls are separated by a distance of t=10 cm in the square channel case or the channel has diameter of d=10 cm in the circular-cross section case. The air enters the duct between walls at T =10 C.