Annular ducts: Nusselt number correlations for laminar flows of liquids with temperature dependent properties

In this work, new correlations are proposed for the mean Nusselt number in the entrance region of concentric annular ducts with uniform heat flux boundary conditions specified at the inner wall. These correlations are obtained on the basis of the numerical results of a previous parametric investigation on the effects of temperature dependent viscosity and thermal conductivity in simultaneously developing laminar flows of liquids in straight ducts of constant cross-sections. A superposition method is applied for the estimation of the Nusselt number by considering separately the effects of temperature dependent viscosity and thermal conductivity.