The experimental investigation of heat transfer and pressure drop in a tube with coiled wire inserts placed separately from the tube wall

The paper presents the experimental investigation of heat transfer and pressure drop in a tube with coiled wire inserts placed separately from the tube wall in turbulent flow regime. The experiments were performed with a constant wire thickness of a = 6 mm, three different pitch ratios (P/D = 1, P/D = 2 and P/D = 3) and two different distances (s = 1 mm, s = 2 mm) at which the coiled wire inserts were placed separately from the tube wall. Uniform heat flux was applied to the external surface of the tube and Reynolds numbers varied from 4105 to 26 400 in the experiments. The experimental results obtained from a smooth tube were compared with those from the studies in literature for validation of experimental set up. The use of coiled wire inserts leads to a considerable increase in heat transfer and pressure drop over the smooth tube. The Nusselt number and friction factor increase with decreasing pitch ratio (P/D) and distance (s) for coiled wire inserts. The highest overall enhancement efficiency of 50% was achieved for the coiled wire with P/D = 1 and s = 1 mm at Reynolds number of 4220. As a result, the experimental results reveal that using these coiled wire inserts are thermodynamically advantageous at all Reynolds numbers.