Angled ribs were employed to induce a secondary flow as well as to cause the flow separation and reattachment. They were mounted on the inner surface of a tube with an angle between the plane of each rib and the mean flow direction of 75degrees, 60degrees and 45degrees for the angled ribs and 90degrees for the transverse ribs. Heat transfer experiment, flow visualization experiment and numerical simulation were conducted in order to clarify the flow structure and its effects on the circumferential heat transfer distribution. The present study explicitly demonstrates the heat transfer superiority of angled ribs over conventional transverse ribs due to a development of the rib-induced secondary flow in a form of a pair of vortices, which convey the colder and higher momentum fluid from the core region towards the heated wall and augment heat transfer especially at the bottom region of the tube. (C) 2003 Elsevier Ltd. All rights reserved.