Thermal characteristics of developing flow in an annular pipe

The paper investigates the behaviour of turbulent flow through a domain exhibiting an abrupt transition in cross-sectional geometry from a circular to an annular pipe using Large Eddy Simulation (LES). Three Reynolds numbers, based on the hydraulic diameter of the annular section, were selected, namely 8 900, 16 200 and 40 500, with an inner-to-outer diameter ratio of D inner / D outer = 0.5 and a Prandtl number of 4.327 (liquid water). Equal heat fluxes were applied to both the inner and outer surfaces of the annular section, ensuring a flux ratio q * = q ″ inner / q ″ outer = 1 . The results reveal significant flow laminarisation along the outer wall at the entrance of the annular section, driven by strong flow acceleration from the vena contracta, followed by its deceleration. In the locally retarded flow region near the outer wall, laminar and turbulent fluxes are both relevant. Farther downstream, as the flow redevelops a fully turbulent regime, turbulent heat fluxes become the primary driver. By contrast, the separated flow region adjacent to the inner wall displays turbulent shear stresses nearly an order of magnitude higher than those in fully developed annular flows. Consequently, turbulent fluxes are most significant at the inner wall. The heat transfer results obtained in this work allow assessment of Nusselt number correlations in developing annular flow, showing that the Gnielinski (2015, Heat Transfer Engineering , 36 (9), pp. 787–789 ) and Taler and Taler (2017, E3S Web of Conferences , 13, p. 02008), when combined with the Hausen distance correction, yield a good level of accuracy at the inner wall (10–30% relative difference).