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

• Nu and f data for tubes with twin or four twist-fin inserts are measured at 750 < Re < 70,000.
• Flow structures along with Nu and f performances are examined.
• Nu and f between present HTE inserts and multiple twist-tape inserts are compared.
• Comparative HTE benefits and thermal performances for twin or four twist-fins are performed.
• Nu and f correlations are devised.

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.