Free and forced convection heat transfer in the thermal entry region for laminar flow inside a circular cylinder horizontally oriented

An experimental work is conducted on combined (free and forced) convection to study the local and average heat transfer for hydrodynamically fully developed and thermally developing laminar air flow in a horizontal circular cylinder. The experimental setup uses an aluminum cylinder as test section with 30 mm inside diameter and 900 mm heated length (L/D = 30) subjected to a constant wall heat flux boundary condition. The investigation covers the Reynolds number range from 400 to 1600 and the heat flux range from 60 W/m2 to 400 W/m2. The fully developed condition is achieved by using aluminum entrance section pipes (calming sections) having the same inside diameter as the test section pipe but with variable lengths. The entrance sections included two long calming sections, one with length of 180 cm (L/D = 60), and the other with length of 240 cm (L/D = 80), and two short calming sections with lengths of 60 cm (L/D = 20) and of 120 cm (L/D = 40). The surface temperature distribution along the cylinder surface and the local and average Nusselt number distributions with dimensionless axial distance Z+ were presented. For all entrance sections, an increase in the Nusselt number values was found as the heat flux increases. It was concluded that the free convection effects tended to decrease the heat transfer results at low Re number and to increase the heat transfer results for high Re number. The combined convection regime could be bounded by a suitable selection of Re number ranges and heat flux ranges. The obtained Richardson numbers (Ri) ranged approximately from 0.1171 to 7.125. The average Nusselt numbers were correlated with the Rayleigh numbers/Reynolds numbers.