On the Correct Nusselt Numbers for Laminar Fully Developed Flow in a Plane Channel with Isothermal Walls

In all common heat transfer books, the Nusselt number (Nu) given for laminar fully developed incompressible flow in a plane channel with isothermal walls is Nu=7.54. This result is obtained through the neglect of viscous dissipation. However, we show that when the viscous dissipation is very small but not identically zero (which is a physical impossibility), the Nusselt number should be either Nu=35/2 for a liquid or Nu=0 for an ideal gas. Furthermore, the exact corresponding temperature distributions and mean temperatures for arbitrary viscous dissipation are given.