Abstract
The paper reports on the convective heat transfer, friction factor, effectiveness and number of transfer units (NTU) of Fe3O4/water nanofluids flow in a double pipe U-bend heat exchanger with and without wire coil inserts under turbulent flow conditions. The heat transfer and friction factor experiments were conducted at different Reynolds numbers (16,000-30,000), different particle concentrations (0.005- 0.06%) and different wire coil inserts of coil pitch to tube inside diameter (p/d) ratios (1, 1.34 and 1.79). The experimental results indicate that, Nusselt number increases with increasing Reynolds number and particle concentrations, and decreasing p/d ratio of wire coil inserts. The Nusselt number enhancement for 0.06% volume concentration of nanofluid is 14.7% and it further increases to 32.03% for the same 0.06% nanofluid with wire coil inserts of p/d = 1 with a friction penalty of 1.162-times at the Reynolds number of 28,954 compared to water data. The experimental data was fitted using regression analysis to estimate the Nusselt number and friction factor of nanofluid as a function of the Reynolds number, Prandtl number, volume concentration, and wire coil helix ratio. The effectiveness of heat exchanger is formulated in terms of number of transfer units (NTU) for the base fluid and nanofluids. (C) 2017 Elsevier Ltd. All rights reserved.