Abstract
Over the last decade, numerous studies have established the superior thermophysical properties of nanofluids compared to base fluids. However, while studies have focused majorly on the nanofluids’ thermophysical properties and to a great extent on the forced convective heat transfer performance, limited experimental studies exist on the natural convective heat transfer performance of nanofluids. Also, the corrosion effects of nanofluids on materials used in thermal systems have been largely ignored when compared to heat transfer properties. It is important to note that according to the author’s knowledge, no comprehensive work has been dedicated to studying the natural convection performance and corrosion effect of graphene nanoplatelet-based nanofluid for heat transfer application. Consequently, the present project was formulated to address some of these issues.
The effects of various surfactants, including Sodium dodecylbenzene sulfonate (SDBS), Sodium dodecyl sulfate (SDS), Gum Arabic (GA) and Tween 80, on the stability and thermophysical properties of graphene nanoplatelets (GNP) nanofluids was experimentally studied. The nanomaterial and surfactant mixing ratios for stable dispersion of GNP were optimized by evaluating the nanofluid stability using zeta potential measurements and visual observation over a period. The effects of the various surfactants on the corrosion behaviour of copper heat exchanger materials were also evaluated. The corrosion properties were evaluated using electrochemical techniques, including open circuit potential and potentiodynamic polarization measurements. The scanning electron microscope, electron dispersive spectroscopy and x-ray diffraction were employed to perform post-corrosion analysis on the copper sample. Also, a detailed experimental investigation of the effects of GNP loading on the thermal conductivity and viscosity of the nanofluid was conducted at different temperatures. The nanofluids were charged and subjected to differential heating in a cavity at different temperature gradients (20, 25, 30, and 35°C) to study the natural convection heat transfer performance at different concentrations. In order to improve the thermo-convection performance of the GNP nanofluid, GNP was hybridized with alumina at different mixing ratios (25:75, 50:50, and 75:25) to formulate a stable hybrid nanofluid. The hybrid nanofluids’ thermal properties and natural convection performance were investigated compared to the single GNP nanofluid at the same concentration of 0.1 vol%.
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All the surfactants-based nanofluids with optimized GNP-surfactant ratio exhibited stability for at least four weeks. Compared to water, GNP nanofluid with Tween 80 produced the highest thermal conductivity enhancement of 8.96% at 45°C, while that of SDBS, SDS and GA exhibit an enhancement of 5.50%, 6.45% and 5.65%, respectively. Also, SDS, GA and Tween 80 inhibited corrosion of copper in GNP nanofluids by 25.93%, 42.99% and 14.35%, respectively, while SDBS greatly promoted corrosion. Further, an increase in GNP concentration and temperature caused an increase in the nanofluid’s thermal conductivity. However, the nanofluid’s viscosity decreased at an elevated temperature while it increased with an increase in concentration. Also, an increase in concentration caused the natural convective heat transfer performance to depreciate. This suggested that GNP nanofluid is not ideal for heat transfer without forced motion.
Notwithstanding, in comparison to water and mono-GNP nanofluid, the hybridization of GNP with alumina in the formulation of the hybrid nanofluid with different mixing ratios improved the natural convective heat transfer performance. A maximum Nu enhancement of 3.17%, 6.44% and 5.43% was observed for hybrid nanofluid with a mixing ratio of 25:75, 50:50, and 75:25, respectively. This enhancement was attributed to the lower viscosity increase of the hybrid nanofluids compared to the mono nanofluid. In conclusion, the lower viscosity and higher Nu enhancement of the hybrid nanofluids indicate its potential for application in thermal systems compared to mono nanofluids.