LAMINAR FULLY DEVELOPED INCOMPRESSIBLE FLOW WITH MIXED CONVECTION IN INCLINED TUBES

The effects of tube inclination and Grashof number on the fully developed hydrodynamic and thermal fields are investigated numerically for laminar ascending flow of air and water in uniformly heated circular tubes. The effects of the buoyancy induced secondary flow on the hydrodynamic and thermal fields are complex and strongly dependent on the Grashof number, the Prandtl number and the tube inclination. The influence of these parameters on the intensity of the secondary flow, on the distortion of the axial velocity profile and of the temperature field from the corresponding distributions for pure forced flow, as well as on the circumferential variation of the local shear stress and of the local Nusselt number are analysed. The average shear stress is higher than for pure forced flow and it increases with both the tube inclination and with the Grashof number. The average Nusselt number is higher than for pure forced flow and increases with the Grashof number. For a given fluid and Grashof number there exists an optimum tube inclination which maximizes the average Nusselt number. Correlations for the average Nusselt number in terms of Gr and Pr are presented for four different tube inclinations.