Experimental Investigation of Thermal Conductivity and Viscosity of SiO₂/Multiwalled Carbon Nanotube Hybrid Nanofluids.

In this study, the thermal conductivity and viscosity of SiO₂/multiwalled carbon nanotube (MWCNTs) hybrid nanofluids are investigated. The volume fraction of the nanofluids varied in the range of 0.5% to 2%, while the SiO₂ to MWCNTs volume proportion is either 95-5 or 90-10. The nanofluids are synthesized using a wet chemical method and a two-step technique is used to disperse nanoparticles in glycerol (base fluid). The thermal conductivities and viscosities of the nanofluids are measured using a modified transient hot-wire method and falling ball viscometer, respectively. The colloidal stability of the dispersion was investigated visually. Effective application of an ultrasonic disruptor and a suitable surfactant (gum arabic) enhance the dispersion behavior. When the effects of temperature and volume fraction on the thermal conductivity and viscosity of SiO₂/multiwalled carbon nanotube (MWCNTs) hybrid nanofluids are studied, the results showed that the thermal conductivity of nanofluids increased with an increase in the volume fraction and temperature. Further, their viscosities increased with an increase in the volume fraction but decreased when the temperature increased. The thermal conductivity and viscosity of the hybrid nanofluids increased by 16.7% and 105.4%, respectively, at a volume fraction of 2% and volume proportion of 90-10. The experimental results are compared with those predicted by classical theoretical models. Two correlations for thermal conductivity and viscosity of hybrid nanofluids are proposed on the basis of the experimental results.