Shear-rate dependent effective thermal conductivity of H2O+SiO2 nanofluids

Abstract

Effective thermal conductivity (ETC) of water-based silicon dioxide nanofluids in shear flow fields (flow shear rate range was 0–820 1/s) was measured using a rotating Couette apparatus. The results show that the ETC of the nanofluids in shear flow fields is significantly higher than that in static states. For the flow shear rates lower than a critical value (infinite-shear rate), the ETC asymptotically increases with increasing the flow shear rate; for the flow shear rates higher than the critical value, the ETC displays a plateau value (infinite-shear thermal conductivity). The increase of the ETC with shear rate is more obvious as increase the nanoparticle diameter and the nanoparticle volume fraction. For 16 different measured nanofluids, the infinite-shear rates vary from 445.0 to 712.1 1/s, while the infinite-shear thermal conductivities increase by 9%–17% comparing with the zero-shear thermal conductivities. The conventional ETC prediction correlation proposed for the suspensions containing micro-sized particles is not suitable for the nanofluids qualitatively and quantitatively. Finally, an exponential correlation is proposed based on our measured data to predict the ETC of nanofluids considering the effects of flow shear rate, nanoparticle diameter, and nanoparticle volume fraction.