Carbon nanomaterials such as graphene and carbon nanotubes possess great thermophysical properties which make them very good candidates for heat transfer application. However, the major challenge of these nanomaterials is their tendency to agglomerate and bundle together when dispersed in base fluids. This study reviews the homogeneous dispersion of these nanomaterials in aqueous solution with the aid of surfactants. The different surfactants and their characterization methods for stable dispersion of carbon nanomaterials have been examined. The influence of surfactants on the thermophysical and rheological properties of carbon-based nanofluids was also highlighted. The usefulness of noncovalent functionalization using surfactants is due to its ability to efficiently unbundle carbon nanomaterials and sustain homogeneity of the nanofluids without compromising the integrity of their structure. Sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfate (SDBS), Gum Arabic (GA), Triton X-100, and cetyltrimethylammonium bromide (CTAB) are the commonly used surfactants. Unlike SDS, SDBS, and CTAB, GA does not foam when agitated. Various authors have investigated the stability of carbon-based nanofluids. Both physical and chemical techniques have been used to stabilize nanofluids. Mixed surfactants were found to stably disperse nanomaterials at lower concentrations compared to individual surfactants. However, limited studies exist for long term stability of carbon-based nanofluids.
This paper investigates the thermophysical properties and heat transfer performance of graphene nanoplatelet (GNP) and alumina hybrid nanofluids at different mixing ratios. The electrical conductivity and viscosity of the nanofluids were obtained at temperatures between 15–55°C. The thermal conductivity was measured at temperatures between 20–40°C. The natural convection properties, including Nusselt number, Rayleigh number, and heat transfer coefficient, were experimentally obtained at different temperature gradients (20, 25, 30, and 35°C) in a rectangular cavity. The Mouromtseff number was used to theoretically estimate all the nanofluids’ forced convective performance at temperatures between 20–40°C. The results indicated that the thermal conductivity and viscosity of water are increased with the hybrid nanomaterial. On the other hand, the viscosity and thermal conductivity of the hybrid nanofluids are lesser than that of mono-GNP nanofluids. Notwithstanding, of all the hybrid nanofluids, GNP-alumina hybrid nanofluid with a mixing ratio of 50:50 and 75:25 were found to have the highest thermal conductivity and viscosity, enhancing thermal conductivity by 4.23% and increasing viscosity by 15.79%, compared to water. Further, the addition of the hybrid nanomaterials improved the natural convective performance of water while it deteriorates with mono-GNP. The maximum augmentation of 6.44 and 10.48% were obtained for Nuaverage and haverage of GNP-Alumina (50:50) hybrid nanofluid compared to water, respectively. This study shows that hybrid nanofluids are more effective for heat transfer than water and mono-GNP nanofluid.
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