Combined natural and forced convection heat transfer for assisting thermally developing flow in a uniformly heated vertical circular cylinder

Combined convection heat transfer in a vertical circular cylinder has been experimentally studied for assisting, thermally developing and thermally fully developed laminar air flows under constant wall heat flux boundary conditions for Reynolds number range from 400 to 1600, and the heat flux is varied from 60 Wm− 2 to 400 Wm− 2. This paper has examined the effect of the cylinder inclination angle on the mixed convection heat transfer process. The experimental setup consists of aluminum cylinder as test section with 30 mm inside diameter and 900 mm heated length (L / D = 30). The hydrodynamically developed condition has been achieved by using aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths. The entrance sections included two long calming sections, one with length of 1800 mm (L / D = 60), another one with length of 2400 mm (L / D = 80) and two short calming sections with lengths of 600 mm (L / D = 20), 1200 mm (L / D = 40). The results present the surface temperature distribution along the cylinder length, the local and average Nusselt number distribution with the dimensionless axial distance Z+. The results have clearly shown that the surface temperature values decrease as the cylinder inclination angle moves from θ = 90° vertical cylinder to θ = 0° horizontal cylinder. The results have demonstrated that an increase in the Nusselt number values as the heat flux increases and as the angle of cylinder inclination moves from θ = 90° vertical cylinder to θ = 0° horizontal cylinder. The mixed convection regime has been bounded by the convenient selection of Re number range and the heat flux range, so that the obtained Richardson numbers (Ri  ) varied approximately from 0.1 to 10. The average heat transfer results have been correlated with an empirical correlation by dimensionless groups as LogNu― against LogRa―/Re―, and compared with available literature and with laminar forced convection and showed satisfactory agreement.