Abstract: SiO2 nanoparticles with different sizes (50 nm and 500 nm) were added into the deionized water by a “two-step” method to prepare water-based nanofluid, and the dispersion stability of SiO2-H2O nanofluids was evaluated using a photometric dispersion analyser. The results showed that the instability index values of SiO2-H2O nanofluids were lower than 0.37, suggesting that SiO2 nanoparticles were stably dispersed in the deionized water. The thermal conductivities of SiO2-H2O nanofluids at 25 ℃ and -20 ℃ were measured by a hot-disk method, and the effect of nanoparticle concentration and size was investigated detailedly. The results showed that as the concentration of SiO2 nanoparticles increased or the size of SiO2 nanoparticle decreased, the thermal conductivity of SiO2-H2O nanofluids at 25 ℃ increased, whereas that of SiO2-H2O nano-fluids at -20 ℃ decreased due to the lower thermal conductivity of SiO2 nanoparticles compared with the ice. The Maxwell, Bruggeman, Yu and Choi, and Xie models were adopted to calculate the thermal conductivities of SiO2-H2O nanofluids at 25 ℃ and -20 ℃. The calculated and mea-sured values were compared to further investigate the mechanism of thermal conductivity of SiO2-H2O nanofluids. The results showed that these models could predict the thermal conductivities of SiO2-H2O nanofluids at -20 ℃, whereas the deviations between theoretical and experimental values of thermal conductivity at 25 ℃ were distinct. It suggested that the thermal conductivity of SiO2-H2O nanofluids was affected by the intrinsic thermal conductivity and Brownian motion of nanoparticles, and the Brownian motion of nanoparticles played a dominant role in particular.
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2021, Vol. 35 
