Queston: Starting from the appropriate momentum and energy differential equations, evaluate the Nusselt number for both surfaces 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 from one surface is twice the flux from the other surface: and again, when the heat flux from one surface is five times the flux from the other surface. The heat-transfer rate per unit of duct length is constant. Check your results with those given in the text (Table).