Thermal performances of enhanced smooth and spiky twisted tapes for laminar and turbulent tubular flows

This experimental study investigates the heat transfer properties over developing and developed flow regimes, the pressure drop coefficients and the thermal performance factors (TPF) of tubular flows with the continuous and spiky twist tapes enhanced by perforated, jagged and notched winglets. The axial distributions of Nusselt number (Nu) and the mean Fanning friction factors (f) of the tubular flows at Reynolds numbers (Re) ranging from 500 to 40000 are comparatively examined for five different types of twisted tapes with three twist ratios (y) of 1.875, 2.186 and 2.815 for each type of twisted tapes. Through this comparative study, the favorable types of twisted tapes which generate the higher degrees of HTE impacts over the developing and developed flow regimes are respectively identified. These newly devised twist tapes enrich the varieties of passive heat transfer enhancement (HTE) devices, especially for retrofit applications. A set of selective Nu and f results illustrates the thermal characteristics of the enhanced tubular flows by these twisted tapes. The HTE and TPF properties for all the present types of twisted tapes are subsequently compared with those reported for other types of twisted tapes in the literature. Among these comparative groups, the present V-notched spiky twisted tape generally offers the highest HTE impacts with favorable TPF performances. Empirical correlations that evaluate the averaged Nu over the developing and developed flow regimes; as well as and tube-wise averaged f for the enhanced tubular flows fitted with all the present types of twisted tapes are generated.