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The aim of this study is to investigate the enhancement of thermal performance characteristics in a plain tube fitted with helical screw louvered rod inserts using water and carbon nanotube (CNT)/water nanofluids of 0.1%, 0.2% and 0.5% volume concentration under turbulent flow condition. In the experiments, the swirling flow was introduced by using helical screw louvered rod inserts arrangements (forward and backward) inside the inner test tube with different twist ratios, Y = 1.78, 2.44 and 3.0. The experimental results revealed that the increase in heat transfer rate of the helical screw louvered rod insert was found to be strongly influenced by turbulence or vortex motion. The use of helical louvered rod inserts in plain tube causes intensification in heat transfer compared to plain tube for a given Reynolds number, while the Friction Factor was higher which increases with decrease in twist ratio. The maximum thermal performance factor value of 1.23 was found with the use of CNT/water nanofluid of 0.5% volume concentration with twist ratio of 1.78 in backward arrangement. Thermal performance analysis based on the constant pumping power criteria showed that helical louvered rod inserts with backward arrangement led to better thermal performance than that with forward arrangement. [ABSTRACT FROM AUTHOR]