Heat transfer augmentation in a double-pipe heat exchanger with dimpled twisted tape inserts: an experimental study

The effect of the geometrical structure of dimpled twisted tape inserts in the inner pipe of a double-pipe heat exchanger (HX) on the Nusselt number (Nu), friction factor (f), and thermal performance factor (eta) was experimentally investigated. Various geometries of twisted tape (TT) including typical, continuous, continuous winglet, and discontinuous louvered twisted tape geometries in both dimpled and non-dimpled forms were considered. The Reynolds number in the annular space was selected to be constant at 11000 and a range of Reynolds number between 5000 and 9500 was considered for cold flow in the inner pipe. The results indicated that all these inserts significantly augmented the Nu and friction factor compared to the plain pipe, but under the same conditions, the dimpled twisted tapes exhibited a higher Nu, friction factor, and thermal performance in comparison with similar non-dimpled twisted tapes. The discontinuous dimpled louvered twisted tape achieved the maximum Nu and thermal performance as compared with the other twisted tape inserts. The maximum obtained thermal performance factor of the heat exchanger is 1.24 at Re = 5300 for the dimpled louvered twisted tape.