Thermal conductivity and viscosity of Al 2 O 3 nanofluid based on car engine coolant

Various suspensions containing Al 2 O 3 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 Al 2 O 3 nanoparticles as well as temperature between 10 and 80 °C. The prepared nanofluid, containing only 0.035 volume fraction of Al 2 O 3 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 Al 2 O 3 nanofluid based on engine coolant is proportional to the volume fraction of Al 2 O 3 . 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 Al 2 O 3 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(μ nf ) = A exp(− BT ) explains the observed temperature dependence of the measured viscosity of Al 2 O 3 nanofluid based on car engine coolant. We further confirm that Al 2 O 3 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.