Laminar heat transfer inside concentric annular duct in entrance region and effects of variable fluid properties

Laminar flow and heat transfer characteristics of concentric annular duct in the entrance region were numerically investigated by solving two-dimensional steady-state incompressible Navier-Stokes equations. SIMPLER algorithm was used for computation under two boundary conditions. Variations of local Nusselt numbers along axial distance vs Prandtl numbers and radius ratios, as well as the variations due to the effects of variable fluid properties on flow and heat transfer were presented. The correlations of average Nusselt number with Reynolds number, Prandtl number and l/D were also obtained for different radius ratios. It was also found that the annular duct could be used to enhance heat transfer, especially at lower radius ratio. In addition, the higher heat transfer coefficients in the entrance region might also attribute to the higher radius velocity component in this region.