Heat transfer and pressure drop characteristics of a decaying swirl flow in a horizontal pipe are investigated experimentally. The decaying swirl flow is produced by the insertion of vortex generators with propeller-type geometry, a kind of passive heat enhancement tools. Two different cases are comparatively examined: one-propeller case and two-propeller case. In the one-propeller case, the first propeller is placed at the entrance of the flow. In the two-propeller case, the second is placed at a specific distance where the swirl effect generated by the first propeller is decayed. The focus of the study is concentrated on comparatively examining the usage of one or two propeller-type swirl generators on the friction factor and heat transfer. For both cases, the effects of the joint angle about the core of the insert device and the number of the joint vanes attached circumferentially to the device on heat transfer and pressure drop are also investigated. Experiments are conducted at Reynolds numbers ranging from 5000 to 30,000. For validation, experimental data obtained for the smooth pipe are compared with those available in the literature. (C) 2014 Elsevier Ltd. All rights reserved.