Research Paper Effect of suspending hybrid nano-additives on rheological behavior of engine oil and pumping power Ebrahim Dardan, Masoud Afrand ⇑ , A.H. Meghdadi Isfahani Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran highlights  Preparing SAE40 based nanofluids containing Al 2 O 3 -MWCNTs hybrid nano-additives.  Performing tests in concentration range of 0–1.0% and temperature range of 25–50 °C.  All hybrid nanofluid samples were Newtonian fluid at all temperatures considered.  Performing sensitivity analysis for viscosity using experimental findings.  Proposing a new correlation to predict the viscosity of the hybrid nanofluid. article info Article history: Received 7 April 2016 Revised 23 July 2016 Accepted 18 August 2016 Available online 20 August 2016 Keywords: Rheological behavior Hybrid nano-additives Al 2 O 3 nanoparticles MWCNTs SAE40 Pumping power abstract In this paper, the rheological behavior of engine oil containing various quantities of hybrid nano- additives has been examined. The experiments were performed in the solid volume fraction range of 0–1.0% and temperatures ranging from 25 °C to 50 °C. Viscosity measurements, at the shear rate range of 1333–13,333 s 1 , showed that Al 2 O 3 -MWCNTs/SAE40 hybrid nanofluid had a Newtonian behavior at all solid volume fractions and temperatures considered. Experimental results also indicated that the vis- cosity of the hybrid nanofluid increased with increasing nano-additives concentration and decreasing temperature. Results of relative viscosity of the hybrid nanofluid showed that the maximum augmenta- tion of the viscosity was about 46%. Results from sensitivity analysis of viscosity revealed that the viscos- ity sensitivity to temperature variation is minor, while it is more sensitive to the variations of solid volume fraction. Furthermore, an accurate correlation was proposed to predict the viscosity of the hybrid nanofluids for application in thermal engineering. Finally, the effects of nano-additives on the pumping power for the oil flow have been reported. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Engine oil is a type of lubricant employed in many engineering applications such as engine generators, power cars, bearings and machines including moving parts. In mechanical systems, the fric- tion between the moving parts diminishes the efficiency by alter- ing the kinetic energy to heat. The chief duty of oils is to reduce friction between parts which move contrary to each other. In addi- tion, the engine oil can cool the parts that are heated due to friction. The thermal conductivity and viscosity of engine oil are two important properties in cooling and lubricating of mechanical sys- tems, respectively. The viscosity also affects the pumping power and oil flow. It is clear that engine oils with enhanced thermal con- ductivity can improve the heat transfer rate. One of the methods to improve the thermal conductivity is dispersing nano-additives in liquids, called nanofluids [1]. Many researchers reported that sus- pending the nano-additives to base fluid significantly enhances the thermal conductivity [2–12]. They revealed that the amount of the enhancement depends on various parameters including tempera- ture and concentration. However, when the nano-additives are suspended in a base fluid to improve its thermal conductivity, the viscosity is also affected. Many analytical and experimental studies on rheological behavior of fluids containing nano-additives have been performed. For example, Batchelor [13], Drew and Passman [14] and Wang et al. [15] suggested analytical models for estimating the viscosity of nanofluids. Moreover, a summary of experimental studies on the viscosity of nanofluids is presented in Table 1. These works have http://dx.doi.org/10.1016/j.applthermaleng.2016.08.103 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: masoud.afrand@pmc.iaun.ac.ir, masoud_afrand@yahoo.com (M. Afrand). Applied Thermal Engineering 109 (2016) 524–534 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng