LAMINAR FULLY-DEVELOPED MIXED CONVECTION IN INCLINED TUBES UNIFORMLY HEATED ON THEIR OUTER SURFACE

The problem of conjugate heat transfer involving mired convection laminar ascending flow of water in inclined circular tubes uniformly heated on their outer surface has been studied numerically using a unified formulation for the solid and fluid domains. The highly coupled governing equations were discretized using the control volume approach, and solved according to the SIMPLER algorithm. Results have clearly demonstrated that the conduction within the tube wall has an important influence on both the hydrodynamic and thermal fields. High wall thermal conductivity or large thickness reduces the temperature stratification within the fluid and intensifies the secondary motion, consisting of two symmetrical vortices. The effects of wall conduction are particularly significant for horizontal tubes for which the average Nusselt number is bounded by two limits corresponding to the cases of infinite wall thermal conductivity and zero wad thermal conductivity. For Gr = 2 x 10(5) these limits are 10.42 and 9.03, respectively. These effects are negligible for tubes inclined at 30 degrees and for Grashof number below 3 x 10(4).