6.7 Consider both hydrodynamically and thermally fully developed steady laminar flow of a constant-property fluid through a circular pipe with a constant heat-flux condi- tion maintained at the pipe wall. Neglecting axial conduction of heat and assuming that the velocity profile can be approximated to be uniform across the entire flow area of the pipe (i.e., slug flow), obtain an expression for the Nusselt number. Compare the result with Equation 6.68 and explain the reason for the difference on a physical basis. Hence, the temperature distribution is given by (x,r)=2 00 n=1 1 Jo(nr) (a/um)x no J(10) (6.88)