This work presents the thermophysical properties of carbon nanotubes (CNT) and CNT/Al 2 O 3 hybrid nanofluids for heat transfer applications. An equal proportion of nanoparticles CNT (50%) and Al 2 O 3 (50%) were added to the base fluid for two different concentration of 0.05 and 0.1%. Results show that addition of Al 2 O 3 nanoparticles with CNT nanofluid improves the thermophysical properties. The thermal conductivity of hybrid nanofluids improved by 20% with a maximum concentration of 0.1%, while the thermal conductivity of CNT alone improved by only 8% with the base fluid. Similarly, density and viscosity of hybrid nanofluid increased up to 7 and 10%, respectively, while comparing it with the base fluid. The result of specific heat energy capacity of hybrid nanofluids increases to about 138% than CNT nanofluid with a maximum concentration of 0.2%.

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In the present work, magnesia nanoparticles (17 nm) were synthesised by solution combustion and characterised using X-ray diffraction and scanning electron microscopy. Nanofluid prepared by magnesia dispersion in ethylene glycol and deionised water (50:50) is characterised using Fourier transform infrared, dynamic light scattering (DLS) and Zeta potential measurement. The-41 mV zeta potential confirms nanofluid stability. The average particle size in the nanofluid obtained by the DLS method is 33 nm, whereas the crystallite size is 17 nm as per the Scherrer relation. The nanofluid properties were determined under varying volume concentration (0.025, 0.05, 0.1, 0.2%) and temperature (25-60°C). The viscosity measured by using a rotational viscometer decreases with increase in the temperature, whereas the thermal conductivity and specific heat, measured by using a KD2 Pro analyser and a differential scanning calorimeter, respectively, increases. With the increase in volume concentration, thermal conductivity, and viscosity increases, whereas specific heat decreases. The maximum thermal conductivity enhancement and specific heat diminution was 15.6 and 20.5%, respectively, for 0.2% concentration and the minimum viscosity was 2.2% for 0.025% concentration at 60°C. Based on the measured data, new correlations have been proposed.