In this paper, a numerical study is presented of the three dimensional laminar steady and transient flows, with heat transfer, of a Newtonian incompressible fluid through a circular-sectioned 90° bend. The effects of the Dean number and the bend curvature ratio on the dynamic and thermal behaviours are explored. The different steady flow regimes obtained emphasize the strong influence of these parameters on the flow structure which may be characterized by one, two or three pairs of counter-rotating vortices and on the heat transfer between the fluid and the curved duct wall. In particular, the effects on the displacement of the secondary vortices with respect to the primary vortex pair, from the outer to the inner wall, are highlighted. It is found that an increase in the curvature ratio results in strikingly different qualitative behaviours of the skin friction coefficient and the Nusselt number along the duct wall. In the initial transient period of the flow, particular attention is paid to the birth, growth, possible oscillation and stabilization of the secondary vortices observed. It is hoped that the findings reported in this work would motivate future investigations on the control of secondary flows in curved ducts such as their suppression to reduce head losses, or intensification to enhance heat transfer. Time evolution of the friction coefficient and the Nusselt number on the curved duct surface are also investigated.

3D flow, numerical heat transfer bend, growth of vortical structures, transient.