Experimental investigation of heat transfer and pressure drop in a circular tube with multiple inserts

The heat transfer ability of a convectional heat exchanger needs to be improved in order to transport a desired rate of energy at the reduced size and cost. Among the different means to increase the heat transfer coefficient, use of inserts emerged as a promising technique to bring out the enhancement in heat transfer at the cost of bearable increase in frictional losses. The present work aims to investigate the heat transfer and fluid flow characteristics of a circular tube fitted with multiple inserts arranged in co-swirl and counter-swirl orientations. The experimental data have been collected for single, twin and four twisted tape inserts having twist ratios of 2.5, 3 and 3.5 while the Reynolds number is varied in the range of 4000 to 14000. A significant rise in the heat transfer has been registered by the use of multiple twisted tapes having twist ratio of 2.5 with the concurrent rise in friction losses with respect to the smooth tube. The maximum enhancement in the heat transfer and friction are found to be 2.42 and 6.96 times that of smooth tube while the maximum value of the thermo-hydraulic performance factor is found to be 1.26 for a set of four counter-swirl twisted tapes with the twist ratio of 2.5. The correlations of Nusselt number and friction factor as function of Reynolds number and twist ratio are developed for different twisted tape configurations. (C) 2016 Elsevier Ltd. All rights reserved.