Heat transfer and pressure drop measurements for tubes fitted with twin and four twisted fins on rod

Abstract

Axial Nusselt number (Nu) distributions and tube averaged Fanning friction factors (f) for the tubular flows enhanced by twin or four twist-fin inserts with five twist ratios (y) of 2, 2.5, 3, 3.5 and ∞ (straight fins) were measured at Reynolds numbers (Re) of 750–70,000. For each enhanced tube with a twist-fin insert, the Nu and f augmentations from the plain-tube developed flow Nu∞ and f∞ references were compared with thermal performance factors (TPF) evaluated. Acting by the respective single- or twin-cellar axial swirls along the helical passages formulated by present twin or four twisted fins in the test tubes, the Nu/Nu∞ ratios at laminar and turbulent reference conditions are respectively raised to 1.87–4.98 and 1.3–1.95; or 3.93–9.03 and 1.37–2.83. Relative to the Nu and f results measured from the test tubes enhanced by the two co-twisted tape inserts, present four twist-fin inserts considerably amplify both laminar and turbulent HTE ratios at the expenses of f augmentations with the laminar and turbulent TPF values in the respective ranges of 1.45–2.84 and 0.56–0.98. For assisting the relevant applications using present sets of Nu and f data generated by this study, the heat transfer and pressure drop correlations for the tubes with twin or four twist-fin inserts are devised.