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 parameterswere selected with the aim to optimize nanofluid stability, which was analysed by Dynamic LightScattering (DLS). Thermal conductivity and dynamic viscosity were experimentally determined with aTPS 2500S hot disk device and an AR-G2 rotational rheometer, respectively. Temperature and concentra-tion 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. Mouromtseffnumbers were calculated to evaluate the effectiveness of the presented nanofluids comparing with thebase fluid. The heat transfer performances of the nanofluid at 1% mass concentration and base fluid wereexperimentally determined by using an assembly, which includes a testing section based on auniform-heat-flux boundary condition. Local and overall heat transfer coefficients were obtained at theinlet temperatures of 293 and 314 K for different power supplies and flowing speeds, corresponding tolaminar and/or laminar-to-turbulent transition Reynolds numbers. The results were expressed in adimensionless way through the Nusselt number. Finally, the goodness of different semi-empiricalequations for laminar, laminar-to-turbulent or turbulent flow conditions was tested.