ZnO nanofluids have been prepared by dispersing ZnO nanoparticles in the ethylene glycol (EG) and glycerol (G) as the base fluids. Ammonium citrate, as a dispersant, has been used to improve the dispersion of nanoparticles and suppressing formation of particle clusters to obtain stable suspensions. The thermal conductivity of ZnO nanofluids has been measured as a function of the volume fraction and temperature. The thermal conductivity of ZnO/EG and ZnO/G nanofluids increases nonlinearly up to 10.5% and 7.2%, respectively, as the volume fraction of nanoparticles increases up to 3 vol.%. The thermal conductivity of a ZnO nanofluid increases nonlinearly with increasing the temperature at a constant volume fraction of nanoparticles. For the first time, we have measured the viscosity and surface tension of ZnO nanofluids. The viscosity ratio of nanofluids increases with increasing concentration and decreasing the temperature. The surface tension ratio of suspensions containing solid particles increases with increasing the volume fraction of the solid nanoparticles. The experimental data for thermal conductivity and viscosity have been compared with some existing theoretical models.

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