Enhanced heat transfer and frictional losses in heat exchanger tube with modified helical coiled inserts

Abstract

The application of compact heat exchangers in any thermal system improves overall performance with a considerable reduction in size and weight. Inserts of different geometrical features have been used as turbulence promoting devices to increase the heat transfer rates. The present study deals with the experimental investigation of heat transfer and fluid flow characteristics of a tubular heat exchanger fitted with modified helical coiled inserts. Experiments have been carried out for a smooth tube without insert, tube fitted with helical coiled inserts, and modified helical coiled inserts. The helical coiled inserts are tested by varying the pitch ratio and wire diameter ratio from 0.5–1.5, and 0.063–0.125, respectively for the Reynolds number range of 1400 to 11,000. Experimental data have also been collected for the modified helical coiled inserts with gradually increasing pitch (GIP) and gradually decreasing pitch (GDP) configurations. The Nusselt number and friction factor values for helical coiled inserts are enhanced in the range of 1.42–2.62, 3.4–27.4, relative to smooth tube, respectively. The modified helical coiled insert showed enhancements in Nusselt number and friction factor values in the range of 1.49–3.14, 11.2–19.9, relative to smooth tube, respectively. The helical coiled and modified helical coiled inserts have thermo-hydraulic performance factor in the range of 0.59–1.29, 0.6–1.39, respectively. The empirical correlations of Nusselt number and friction factor for helical coiled inserts are proposed.