Heat transfer enhancement by spirally coiled spring inserts with and without segmental solid cords

Thermal performance improvements for tubular flows using spirally coiled spring (SCS) inserts to disturb boundary layers by tripping near-wall separated vortices are proposed. Combined with the segmental solid cords to induce axial swirls, a new convenient passive heat transfer enhancement (HTE) method is formulated. The interval of segmental solid cords is fixed at one tube diameter (d); while the pitch ratios (P/d) of the SCSs are 0, 1, 1.5, 2, 2.5 and ∞. The axial Nusselt numbers (Nu), mean Fanning friction coefficients (f) and thermal performance factors (TPF) at Reynolds numbers (Re) between 750 and 70,000 are measured for each test tube. Acting by the interactive near-wall vortices and the axial swirls in the tubes with the SCSs and segmental solid cords, both HTE effectiveness and f augmentation are boosted from those developed in the tubes with SCSs alone. Relative to the plain tube heat transfer references over the developing (Nu‾₀^DE) and developed (Nu‾₀^FD) flow regions, the ratios of Nu‾^DE/Nu‾₀^DE and Nu‾^FD/Nu‾₀^FD are elevated to 1.81–9.04 and 1.01–8.21 at 750 ≤ Re ≤ 2000; and 1.11–2.41 and 1.02–2.33 at 10,000 ≤ Re ≤ 70,000 by the SCSs of 0 ≤ P/d ≤ ∞ without cord. For present SCSs of 0 ≤ P/d ≤ ∞ with segmental cords, the ratios of Nu‾^DE/Nu‾₀^DE and Nu‾^FD/Nu‾₀^FD are raised to 1.81–10.74 and 1.01–9.07 at 750 ≤ Re ≤ 2000, and 1.11–3.15 and 1.02–2.66 at 10,000 ≤ Re ≤ 70,000, respectively. With present geometrical and flow conditions tested, the TPF values for the tubes enhanced by the SCSs without and with segmental solid cords are in the respective ranges of 5.07–0.61 and 3.75–0.64. For each type of enhanced tubular flows, the Nu and f correlations are devised to assist the relevant applications.