Starting from the appropriate momentum and energy differential equations, evaluate the Nusselt number for both surfaces (Nu1 and Nu2) of a parallel-planes duct in which there is fully developed laminar flow (both velocity and temperature developed) and in which there is heating from both surfaces but the heat flux (energy/area.time) from the bottom surface is twice the heat flux from the top surface. The heat-transfer rate per unit of duct length is constant. Neglect body/gravitational forces, dissipation and axial conduction.


Nu= hDh / k, Dh = 4 rh, rh = cross-sectional (Ac) / Perimeter, q = h (Tw-Tm)