Importance of nanofluid evaluations at identical velocity and pumping power: a specific evaluation for Ag-MgO/water hybrid nanofluid flow through a pipe under turbulent regime

In this study, heat transfer and fluid flow characteristics of Ag-MgO/water hybrid nanofluid flow through a pipe were numerically investigated under turbulent regime at identical Reynolds number, velocity and pumping power. To model the flow, the standard k turbulence model was used. In the analyses, Reynolds number was in the range from Re=10000 to Re=100000 and velocity ranged from V=0.3 m/s to V=3.0 m/s. As a result, it was found that the enhancements in convective heat transfer coefficient were obtained to be 23.72% for identical Reynolds number, 6.27% for identical velocity and 0.44% for identical pumping power. Nanofluids had higher velocities compared to their base fluid to be able to compare them at identical Reynolds number. It was found that this velocity differences can already cause a convective heat transfer enhancement of 16.29% without nanoparticle addition. Nanofluids have higher performance evaluation criteria than unity at identical Reynolds number while they have lower values than unity at identical velocity and pumping power. It can be concluded that the results obtained for identical Reynolds number are extremely optimistic and not realistic. Nanofluids should be examined at identical velocity or pumping power for a fair comparison. Keywords: Entropy generation Heat convection Heat transfer enhancement Nanofluid Turbulence