Heat transfer, friction factor, and effectiveness-number of transfer units were estimated experimentally for different volume concentrations of Fe3O4 nanofluid flow in an inner tube of a double pipe U-bend heat exchanger with different pitch ratios (p/d) of wire coil with core-rod (WCCR) inserts. The experiments were conducted at different particle volume concentrations (0.005%, 0.01%, 0.03% and 0.06%), different Reynolds numbers (16000-29000) and different pitch ratios of WCCR inserts (p/d = 1,1.34 and 1.79). The Nusselt number, for a 0.06% volume concentration of nanofluid, is enhanced up to 9.76% and 14.76% at Reynolds number of 16545 and 28954, respectively when compared to water data with a pumping penalty of 10%. Similarly, the Nusselt number is further enhanced to 25.39% and 37.90% for 0.06% nanofluid with wire coil with core-rod insert of p/d = 1 at Reynolds number of 16545 and 28954, respectively compared to water. Nusselt number and friction factor correlations were proposed based on the experimental data. The effectiveness-number of transfer units (NTU) were calculated for water/nanofluids flow in a tube with WCCR inserts. (C) 2019 Elsevier Ltd. All rights reserved.