An experimental study to determine the maximum efficiency index in turbulent flow of SiO2/water nanofluids

In this work, heat transfer and pressure drop characteristics of nanofluids flowing through a horizontal circular tube have been investigated experimentally. The test tube was made of stainless steel type 304 with an inner diameter of 7.1 mm. The working fluid was SiO2/water nanofluid where the average diameter of nanoparticles was 7 nm. Nanofluids at three different volume concentrations of 0.5, 1, and 2% have been prepared and tested. The experiments have been performed for Reynolds numbers ranging from 3800 to 12000, inlet temperatures of 25, 30, and 35 °C where a constant heat flux was imposed on the tube. The effects of particle volume concentrations, inlet temperature and mass flow rate on convective heat transfer and pressure drop characteristics have been evaluated. The results revealed that with increasing Reynolds number, volume concentration, and inlet temperature the heat transfer coefficient and Nusselt number increased. Moreover, pressure drop increased with increasing volume concentration; conversely, decreased with increasing inlet temperature. The efficiency index reached its maximum quantity (i.e. 1.6) at Reynolds numbers higher than 9000, the volume concentration of 2%, and inlet temperature of 35 °C. On the other hand, the minimum values of efficiency index were obtained for Reynolds numbers less than 7000, the volume fraction of 0.5%, and inlet temperature of 25 °C. Finally, new correlations for predicting the Nusselt number and friction factor of SiO2/water turbulent flow have been proposed.