Thermal performances of tubular flows enhanced by twin and four spiky twisted fins on rod

An experimental study measuring the axial Nusselt number (Nu) distributions, averaged Fanning friction factors (f) and thermal performance factors (TPF) of the tubes fitted with swirl generator configured by twin or four spiky twist-fins along a rod was performed at Reynolds numbers (Re) of 750–70,000. A set of test results acquired from the swirl tubes with a spiky twin- or four-twist-fin insert was selected to illustrate the interdependent y and Re impacts on Nu, f and TPF by the aid of flow snapshots collecting from the smoke flow visualization tests. Comparative heat transfer enhancements (HTE), f augmentations and TPF properties attributed to previous smooth and present spiky twin- and four- twisted fins were analyzed. The turbulent and laminar Nusselt numbers detected from present swirl tube (Nu) were normalized by the referenced plain-tube turbulent and laminar Nusselt numbers (Nu_∞) as the Nu/Nu_∞ ratio to quantify the HTE impact. The laminar and turbulent Nu/Nu_∞ ratios of present swirl tubes in the developed flow region are respectively raised to 2.2–5.08 and 2.44–2.96 for the tubes with spiky twin-fins and 3.31–7.62 and 2.04–3.23 for the tubes with spiky four-fins. The HTE benefits with noticeable f augmentations have led the laminar and turbulent TPF to the respective ranges of 0.94–1.79 and 0.99–1.07 for spiky twin-fins and 0.98–1.82 and 0.82–0.94 for spiky four-fins. Two sets of empirical Nu and f correlations are devised to assist the relevant applications.