Experimental investigation of rheological properties of low concentrated TiO2/water and MWCNT-TiO2/water hybrid nanofluids

To reveal the mechanism of nanofluids in heat transfer and mass transfer, experimental investigations on the rheological properties of TiO2/water nanofluids with four solid volume fractions (0.012%, 0.238%, 1.227%, and 2.556%) and 0.152%, 0.182%, 0.213% and 0.246% MWCNT-TiO2/water hybrid nanofluids are carried out. The measurements covered in this paper are hysteresis loop; 3ITT test; temperature sweep and steady shear. It is found that TiO2/water nanofluids are featured with Newtonian behavior without thixotropy at different temperatures (278 K - 318 K). The viscosity-temperature behavior for TiO2/water could be well described by the Arrhenius equation. The relative viscosity increases by an amount proportional to the volume fraction, which is unaffected by the temperature. The maximum increment of the viscosity is 17% for 2.556% TiO2/water nanofluid. Furthermore, MWCNT-TiO2/water hybrid nanofluids with solid particles ranged from 0.182 vol% to 0.246 vol% exhibit a non-Newtonian shear thinning behavior without thixotropy, fitted to the Herschel-Bulkley model appropriately. The relative viscosity of MWCNT-TiO2/water hybrid nanofluids increases with temperature and concentration whereas it decreases with shear rate. The lowest and highest average viscosity enhancements observed are 13% and 39%, which occur at the volume fraction of 0.152% and 0.246% hybrid nanofluids respectively. Finally, based on the experimental data, a new correlation is proposed to predict the relative viscosity of MWCNT-TiO2/water hybrid nanofluids which could contribute to future works associated with the application of nanofluids.