Heat transfer characteristics in a tube fitted with helical screw-tape with/without core-rod inserts

Effects of insertion of a helical screw-tape with or without core-rod in a concentric double tube heat exchanger on heat transfer and flow friction characteristics are experimentally investigated. The heat exchanger has the outer and the inner tube diameters of 50 turn (D.) and 25 nun (D) where the cold and the hot waters used as the test fluids are in shell and tube sides, respectively. The stainless steel helical screw-tape has the geometrical dimensions of width (W) 17 turn with the clearance to the tube wall (D-W)/ 2=4 mm. Thus, the insertion of the screw-tape in the tube is considered as a loose-fit. In the experiment, the loose-fit helical tape with or without core-rod, is inserted in the inner tube of the heat exchanger and the hot water enters the tube based on its Reynolds number in a range of 2000 to 12,000. The experimental results show that the increases in average Nusselt number of using the loose-fit, helical tape with and without core-rod are found to be 230% and 340%, respectively, over the corresponding plain tube. It is worth noting that for the loose-fit, helical tape without core-rod, the friction factor is around 50% less than that for the one with core-rod while the Nusselt number is about 50% higher. Furthermore, the enhancement efficiency of the helical screw-tapes varies between 1.00 and 1.17, 1.98 and 2.14, for the tapes with and without core-rod, respectively. (c) 2006 Elsevier Ltd. All rights reserved.