Thermal performances of turbulent tubular flows enhanced by ribbed and grooved wire coils

Abstract Thermal performances of the tubular flows enhanced by the grooved or/and ribbed square wire coils are experimentally examined by disclosing the characteristics of Heat Transfer Enhancement (HTE), pressure drop augmentation and transmitted efficiencies for heat convection. The axial Nusselt number (Nu) distributions, mean Fanning friction factors (f) and thermal performance factors (TPF) in the tubes fitted with each type of present wire coils with five pitch ratios (P/d) of 0.5, 1, 1.5, 2 and 2.5 are measured at Reynolds numbers (Re) between 10,000 and 40,000. The unsteady separated flows tripped by ribs and the jet-flows bursting through the grooves trigger the complex and cyclic vortical interactions with the axial swirl to boost HTE properties from the smooth-coil tube references with various degrees of f augmentations. While the present set of grooved or/and ribbed wire coils enrich the varieties of passive insert HTE technologies, especially for retrofit applications, the favorable families of enhanced wire coils for acquiring HTE and/or TPF benefits among present six types of enhanced square wire coils by 90° ribs (R-90), 90° grooves (G-90), 45° ribs (R-90), 45° grooves (G45), 90° ribs and 90° grooves (RG-90) and 45° rib and 45° groove (RG-45) are disclosed. Relative to the heat transfer references measured from the smooth-coil tubes (Nu₀) for present R-90, G-90, R-45, G-45, RG-90 and RG-45 tubes, the ranges of Nu^DE/Nu₀ over developing flow region and Nu^FD/Nu₀ for this family of enhanced coil wires over developed flow region are raised to the respective ranges of 0.97-1.49 and 0.99-1.42 at P/d = 0.5; 0.98-1.44 and 1.09-1.43 at P/d = 1; 1-1.5 and 1.04-1.29 at P/d = 1.5; 0.99-1.39 and 1.04-1.38 at P/d = 2 and 0.99-1.43 and 0.96-1.29 at P/d = 2.5 with 10,000 ≤ Re ≤ 40,000. While the TPF values for the smooth-coil tubes turn below than unity at Re > 20,000, the TPF values for R-90, G-45 and RG-45 tube at present test conditions are all above than unity, indicating the improved HTE properties with justified efficiencies for heat transmission. Two set of Nu and f correlations for turbulent tubular flows enhanced by each type of present wire coils are generated to assist the relevant applications.