Laminar mixed convection heat transfer in externally finned pipes

The influence of aiding and opposing buoyancy forces on forced convection heal transfer in vertically oriented, externally finned pipes is studied numerically. A periodically varying external heat transfer coefficient is used to model the finned surface. Results are presented in terms of the streamwise variation of the fluid bulk temperature and tube-side Nusselt number, axially-averaged and periodically fully developed Nusselt number values, and axial velocity and temperature profiles. Average Nusselt number correlations for both constant and periodically varying Blot numbers are also presented. Buoyancy induced effects are observed to increase with higher levels of the external heat transfer coefficient (or fin effectiveness) and decrease with increasing inter-fin spacing. A constant spatially averaged Blot number approximation to the periodic variation is found to be satisfactory only in the periodically developed regions of the flow.