Heat transfer intensification of agitated U-tube heat exchanger using twisted-tube and twisted-tape as passive techniques

摘要

In the current study, the thermal and hydraulic characteristics are experimentally investigated in an agitated heat exchanger which has an inner U-tube as the main flow path. This type of heat exchanger has been investigated for the straight flow path by few researchers, but not for the twisted configuration, particularly fitted with the twisted-tapes. Hence, the U-tube drowned in shell side liquid has two straight and twisted configurations filled with the twisted-tapes. In order to investigate the effects of the twist ratio, two different twisted-tapes are employed in turbulent flow with Reynolds number ranging from 4250 to 12750 by using water as working fluid. The results of straight-tube and twisted-tube alone are considered for comparisons. It is found that the heat transfer coefficient and pressure drop of the straight-tube and twisted-tube fitted with twisted-tapes are higher than those of the straight-tube and twisted-tube alone, and evidently, both the parameters increase with the decrease in the twist ratio. Further, the twisted-tube yields higher Nusselt number and friction factor than the straight-tube up to around 58% and 33%, respectively, while the Nusselt number and friction factor of the twisted-tube fitted with the twisted-tapes are, respectively, 33-168% and 46-82% higher than those of the straight-tube. Over the range studied, the maximum performance index of 2.28 is recorded with the use of the twisted-tape with lower twist ratio in the twisted-tube for the minimum Reynolds number where the Nusselt number and friction factor increase up to 168% and 61% of those in the straight-tube.