An experimental study is carried out to investigate heat transfer and pressure drop characteristics of a decaying swirl flow in a horizontal pipe. The decaying swirl flow is produced by the insertion of vortex generators with propeller-type geometry, a kind of passive heat enhancement tools. Three different positions of the vortex generator in the axial direction are examined: at the inlet (x = 0), x = L/4 and x = L/2. For each position, effects of the vane angle about the core of the insert device and the number of the vanes attached circumferentially to the device on heat transfer and pressure drop are studied. Experiments are conducted at Reynolds numbers ranging from 5000 to 30,000. At first, experimental data obtained for the smooth pipe are compared with those available in the literature. Depending on the Reynolds number, the position of the vortex generator, the angle and the number of the vanes, the Nusselt numbers were found to show increases ranging from 18.1% to 163%. (c) 2007 Elsevier Inc. All rights reserved.