Stable and well dispersed functionalized graphene–ethylene glycol (EG) + distilled water nanofluids having graphene nano-sheets (GnS) volume concentration between 0.041 and 0.395 vol. % are prepared without any surfactant. Graphene nano-sheets are prepared from high purity graphite powder by Hummers method followed by exfoliation and reduction by hydrogen gas. Thus, obtained hydrogen exfoliated graphene (HEG) is then functionalized using acid. The graphene nano-sheets are characterized using XRD, TEM, Raman spectroscopy, and FTIR spectroscopy. Thermal conductivity and viscosity measurements are performed both as a function of graphene loading and temperature between 10 and 70 °C. Thermal conductivity enhancement of ∼15% for a loading of 0.395 vol. % f-HEG is observed at room temperature. The measured nanofluid's thermal conductivity is explained well in terms of the expression derived by Nan et al. (J. Appl. Phys. 81, 6692 (1997)), which considers matrix-additive interface contact resistance of mis-oriented ellipsoidal particles. The viscosity of the prepared f-HEG nanofluids and the base fluid (EG + distilled water) displays non-Newtonian behaviour with the appearance of shear thinning and nearly 100% enhancement compared to the base fluid (EG + DI water) with f-HEG loading of 0.395 vol. %. Known theoretical models for nanofluid's viscosity fail to explain the observed f-HEG volume concentration dependence of the nanofluid's viscosity. Temperature dependence of the studied nanofluid between 10 and 70 °C is explained well by the correlations proposed earlier for nanofluids with spherical nanoparticles. Electrical conductivity of the f-HEG nanofluids shows significant enhancement of ∼8620% for 0.395 vol. % loading of f-HEG in a base fluid of 70:30 mixture of EG and distilled water.

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Various suspensions containing Al2O3 nanoparticles (<50 nm) in a car engine coolant have been prepared using oleic acid as the surfactant and are tested to be stable for more than 80 days. Thermal conductivity and viscosity of the nanofluids have been investigated both as a function of concentration of Al2O3 nanoparticles as well as temperature between 10 and 80 °C. The prepared nanofluid, containing only 0.035 volume fraction of Al2O3 nanoparticles, displays a fairly higher thermal conductivity than the base fluid and a maximum enhancement (k nf/k bf) of ∼10.41% is observed at room temperature. The thermal conductivity enhancement of the Al2O3 nanofluid based on engine coolant is proportional to the volume fraction of Al2O3. The volume fraction and temperature dependence of the thermal conductivity of the studied nanofluids present excellent correspondence with the model proposed by Prasher et al (2005 Phys. Rev. Lett. 94 025901), which takes into account the role of translational Brownian motion, interparticle potential and convection in fluid arising from Brownian movement of nanoparticles for thermal energy transfer in nanofluids. Viscosity data demonstrate transition from Newtonian characteristics for the base fluid to non-Newtonian behaviour with increasing content of Al2O3 in the base fluid (coolant). The data also show that the viscosity increases with an increase in concentration and decreases with an increase in temperature. An empirical correlation of the type log(μnf) = A exp(−BT) explains the observed temperature dependence of the measured viscosity of Al2O3 nanofluid based on car engine coolant. We further confirm that Al2O3 nanoparticle concentration dependence of the viscosity of nanofluids is very well predicted on the basis of a recently reported theoretical model (Masoumi et al 2009 J. Phys. D: Appl. Phys. 42 055501), which considers Brownian motion of nanoparticles in the nanofluid.

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