Transport properties and heat transfer coefficients of ZnO/(ethylene glycol + water) nanofluids

New nanofluids as dispersions of dry ZnO nanopowder in ethylene glycol + water (50/50% in volume) were investigated at three mass nanoparticle concentrations up to 5%. Several sonication parameters were selected with the aim to optimize nanofluid stability, which was analysed by Dynamic Light Scattering (DLS). Thermal conductivity and dynamic viscosity were experimentally determined with a TPS 2500S hot disk device and an AR-G2 rotational rheometer, respectively. Temperature and concentration dependences on these two properties were studied by analysing the base fluid and nanofluids at 1%, 2.5% and 5% mass concentrations within the temperature range from 283.15 to 343.15 K. Mouromtseff numbers were calculated to evaluate the effectiveness of the presented nanofluids comparing with the base fluid. The heat transfer performances of the nanofluid at 1% mass concentration and base fluid were experimentally determined by using an assembly, which includes a testing section based on a uniform-heat-flux boundary condition. Local and overall heat transfer coefficients were obtained at the inlet temperatures of 293 and 314 K for different power supplies and flowing speeds, corresponding to laminar and/or laminar-to-turbulent transition Reynolds numbers. The results were expressed in a dimensionless way through the Nusselt number. Finally, the goodness of different semi-empirical equations for laminar, laminar-to-turbulent or turbulent flow conditions was tested.