Effects of Axial Conduction and Convective Boundary Conditions in Slug Flow inside a Circular Tube

An exact solution is presented for the temperature distribution and the local Nusselt number in slug flow of a low Pe´cle´t number fluid inside an infinitely long tube by including in the analysis the axial conduction in the fluid and convection from the walls for both the upstream and downstream regions. It is shown that the Pe´cle´t number and the relative magnitudes of the upstream and downstream Biot numbers significantly effect the local Nusselt number and the wall and mean fluid temperatures in the thermal entrance region. A particularly interesting result of the present investigation is the crossover of the wall and the mean fluid temperatures in such a manner as to give rise to a negative Nusselt number in the thermal entry region. Depending on the Pe´cle´t number and the relative magnitudes of the upstream and downstream Biot numbers, the negative Nusselt number can occur in the upstream or downstream region.