| Summary: | Metal-oxide based nanofluids have grabbed many researchers' attention because of their better dispersion stability and enhanced thermophysical properties for potential heat transfer applications. This work presents an experimental investigation of well-dispersed ZnO nanoparticles' dispersion stability and thermophysical properties in deionized water. Aqueous ZnO nanofluids with different mass concentrations (0.012, 0.024, 0.036, and 0.048%) of nanoparticles were synthesized using a typical two-step method with and without using surfactants. Sodium hexametaphosphate and acetylacetone were used as stabilizing agents in this study. Under optimal operating conditions and surfactants' concentration, acetylacetone-based nanofluids were found stable for more than 60 days. Viscosity and thermal conductivity of nanofluids have been investigated in the 20–60 °C temperature range. Both viscosity and thermal conductivity of nanofluids increased with increasing nanoparticle loading, while with an increase in temperature, viscosity decreased, in contrast to an increase in nanofluids' thermal conductivity. At a fixed temperature and concentration of nanoparticles, maximum enhancement in viscosity and thermal conductivity was recorded as 16.75% and 23.70%, respectively, for sodium hexametaphosphate stabilized nanofluids. The existing well-known theoretical models failed to predict the nanofluids' viscosity and thermal conductivity; however, the proposed new correlations well projected the experimental findings.
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