Heat transfer in a channel with a straight portion followed by a portion with mild curvature is studied at Dean numbers from 300 to 750. The channel aspect ratio is 4C, radius ratio is 0.979, and the ratio of shear layer thickness to channel inner radius is 0.011. Important Nusselt number increases occur due to a variety of laminar-to-turbulent transitional phenomena which occur mostly in the upstream straight portion of the channel but then continue to cause important variations in the downstream curved portion. Nusselt numbers from concave and convex surfaces with these transitional phenomena show the effects of curvature, streamwise development, Dean number, buoyancy and flow tripping. (C) 1997 Elsevier Science Ltd.