Comparative experimental investigation on viscosity and stability of W/EG based non-Newtonian hybrid nanofluids for the heat transfer applications

This research explores the stability and rheological characteristics of hybrid nanofluids made from water-ethylene glycol (W/EG) and incorporating nanoparticles such as SiC, Al₂O₃, and multi-walled carbon nanotubes (MWCNT). The preparation involved a two-step method, and the nanoparticles were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Stability assessment showed that Al₂O₃-MWCNT hybrid nanofluids are optically more stable than SiC-MWCNT as W/EG-based Al₂O₃-MWCNT hybrid nanofluids took longer to sediment. Al₂O₃-MWCNT hybrid nanofluids exhibited superior stability in visual tests over a period of 19-21 days while SiC-MWCNT nanofluid took 12-14 days to sediment. The rheological analysis revealed that higher particle concentrations resulted in increased viscosity, with SiC-MWCNT and Al₂O₃-MWCNT hybrid nanofluids showing viscosity increases of 3.56 and 3.98 times, respectively, in comparison to the base fluid. Conversely, raising the temperature from 25°C to 55°C led to a decrease in shear stress, with reductions of 72.8% and 64.8% observed for SiC-MWCNT and Al₂O₃-MWCNT hybrid nanofluids, respectively. Furthermore, the viscosity versus shear rate trends indicated a pseudoplastic or shear-thinning nature for both hybrid nanofluids with particle volume fraction above or equal to 0.1%.