Experimental investigation on heat transfer enhancement in a circular tube with equilateral triangle cross sectioned coiled-wire inserts

The thermo-hydraulic performance of coiled-wire inserted in a tube was experimentally investigated. The wire inserts have equilateral triangular cross-sections; they were coiled in a specific way to ensure that a vertex of the triangle was oriented to face the incoming air flow. The coiled-wires were installed with a 1 mm separation from the inner tube wall so that the heat transfer enhancement due to the viscous sublayer disturbance could be investigated. In total, six different coiled-wire configurations (three different pitch-to-diameter ratios: P/D =1, P/D = 2 and P/D = 3 and two different ratios of triangle side length to tube diameter: e/D = 0.0714 and e/D = 0.0892) were chosen for the experiment. To determine the effect of triangle position on thermal performance, the results of present study were compared with a previous study results. The maximum thermal performance is approximately 1.67 times that for a smooth tube was observed for a wire with ratios of e/D = 0.0892, P/D =1 and 11-18% more heat transfer enhancement was achieved than the previous work results. In conclusion, to enhance heat transfer, a coiled-wire that a vertex of the triangle is oriented to face the flow can be widely applied wherever heat exchangers with tubes are used. (C) 2017 Elsevier Ltd. All rights reserved.