Heat transfer enhancement of TiO2-water nanofluid flow in dimpled tube with twisted tape insert

Abstract

This paper deals with the thermo-hydraulic behaviors of dimpled tubes mounted with twisted tape inserts using TiO2-water nanofluids as the test fluids. The possible heat transfer mechanisms were discussed. Experiments were conducted using (1) the dimpled tubes with dimple angles (θ) of 0, 15, 30 and 45°, (2) the tapes having twist ratios (y/W) of 3.0, 4.0 and 5.0, and (3) TiO2-water nanofluids with ϕ = 0.05, 0.1 and 0.15 vol.%. The experimental results revealed that the dimpled tubes with twisted tapes yielded higher heat transfer rates than the dimpled tube alone. The results also indicated the strong influence of dimple angle, twist ratio and TiO2-water nanofluid concentration on the thermo-hydraulic performance. Among the tested dimpled tubes, the one with a dimple angle of 45° yielded the highest heat transfer enhancement. Heat transfer (Nu) increased with decreasing twist ratio (y/W) and increasing nanofluid concentration. Over the investigated range, the highest thermo-hydraulic performance of 1.258 was achieved by using the nanofluid with ϕ = 0.15 vol.% in the dimple angled having the dimple angle of 45°, inserted with twisted tape possessing the twist ratio of 3.0. In the present work, the Wilson plot method was employed to develop the Nusselt number correlation for the flow of TiO2-water nanofluid through the dimpled tubes mounted with twisted tape inserts. The predicted heat transfer rate, friction factor and thermo-hydraulic performance were found in good agreement with the experimental results.