In this experimental study, hydrothermal characteristics of a U-tube equipped with twisted-tape inserts are investigated experimentally. The twisting effects of side wall are also investigated for the U-tube as compared to the smooth case. Two twist ratios of 2 and 6 are considered for the twisted-tape and tested in both the smooth and the twisted U-tube at the turbulent flow regime (3843 < Re < 11,436). In order to provide a condition close to uniform wall temperature, the test sections are immersed in the isothermal external fluid. The obtained results show an ascending trend for both the Nusselt number and the friction factor as the twist ratio is decreased. It is found that decreasing the twist ratio decreases the entropy generated by heat transfer and increases the entropy generated by friction loss. In the twisted U-tube fitted with the twisted-tape, the highest augmentations of 122.4% and 78.4% are recorded for Nusselt number and friction factor as compared to the smooth Utube. The non-dimensional entropy generated by heat transfer governs the magnitude order of the non-dimensional total entropy generation. The entropy generation values of the enhanced models range between 0.63 and 1.09.

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