Effects of shear rate and temperature on viscosity of alumina polyalphaolefins nanofluids

In this paper, the shear rate and temperature dependencies of viscosity of alumina nanofluids have been investigated experimentally. The alumina nanofluids are suspensions of alumina nanospheres or nanorods in polyalphaolefins (PAO) lubricant. The base fluid PAO has a Newtonian behavior. To the first approximation, nanofluids of volume fractions ϕ=1% and 3% nanospheres as well as nanofluid of ϕ=1% nanorods can be considered as Newtonian fluids because their viscosity shows very weak shear rate dependence. However, our measurement clearly indicates that these nanofluids demonstrate certain non-Newtonian feature due to the addition of nanoparticles. Moreover, the relative viscosity (the ratio of viscosity of nanofluid to that of PAO) of these nanofluids has been measured to be independent of temperature. Nanofluid of a higher volume fraction ϕ=3% nanorods has an apparent non-Newtonian shear thinning viscosity and a strong temperature dependence of its relative viscosity. By reviewing the previous studies, the approximation that the viscosity is Newtonian and the relative viscosity is independent of temperature seem to hold for most nanofluids of low volume fraction ϕ and low aspect ratio nanoparticles.