The effect of temperature and solid concentration on dynamic viscosity of MWCNT/MgO (20–80)–SAE50 hybrid nano-lubricant and proposing a new correlation: An experimental study ☆ Amin Asadi a,b, ⁎, Meisam Asadi a,b, ⁎, Mohammadhosein Rezaei c , Marzieh Siahmargoi c , Fahime Asadi c a Young researchers and Elite Club, Semnan Branch, Islamic Azad University, P. O. Box: 35196-97951, Semnan, Iran b Energy and Sustainable Development Research Center, Semnan Branch, Islamic Azad University, P. O. Box: 35196-97951, Semnan, Iran c Department of Mechanical Engineering, Semnan Branch, Islamic Azad University, P. O. Box: 35196-97951, Semnan, Iran abstract article info Available online 05 September 2016 The main objective of the present paper is to investigate the dynamic viscosity of MWCNT/MgO (20–80)–SAE50 hybrid nano-lubricant. The experiments carried out in solid concentrations ranging from 0.25% to 2% and temper- atures ranging from 25 °C to 50 °C. The results revealed that the nano-lubricant shows Newtonian behavior in all the studied temperatures and solid concentrations. Furthermore, the experimental results showed that the dy- namic viscosity decreased as the temperature increased. It is also revealed that increasing the solid concentration leads to increasing the dynamic viscosity of the nano-lubricant in all the temperatures. The maximum increase in dynamic viscosity took place at the solid concentration of 2% and temperature of 40 °C by 65% while the mini- mum increase was at the solid concentration of 0.25% and temperature of 25 °C by 14.4%. Finally, applying curve fitting method on the experimental data, a new model to predict the dynamic viscosity of the studied nano-lubricant in terms of temperature and solid concentration has been proposed. © 2016 Elsevier Ltd. All rights reserved. Keywords: Dynamic viscosity MWCNT MgO Solid concentration Temperature Nano-lubricant Correlation 1. Introduction A suspension of nano-size particles in conventional fluids, such as water, ethylene glycol, and oil, is called nanofluid. Since they possess high thermal conductivity and thermal performance in comparison with working fluids, they have grabbed the attention of many researchers in recent years [1–8]. From 1995 on which Choi [9], for the first time, introduced nanofluids, the applications of nanofluids have been widely developed in various thermal systems such as heat exchangers, heating and cooling engines, electronic devices, and so forth. Undoubtedly, viscosity and thermal conductivity of nanofluids are two crucial factors which have direct effect on heat and mass transfer. In this ground, many researchers conducted various studies on thermal conductivity of nanofluid containing different nanoparticles (metal, metal oxide, and carbon nanotubes) [10–16]. Their results indicated that the thermal conductivity of nanofluids shows considerable enhancement by adding the nanoparticles and this enhancement re- sults in improving the heat transfer rate. As for the viscosity of nanofluids, a considerable amount of literature has been published based on theoretical [17–20] and experimental studies [21–24]. Based on these studies, it can be concluded that many factors such as temper- ature, solid volume fraction, suspension techniques, particle size and shape, surfactant, type of nanoparticles, and base fluid has direct effect on the viscosity of nanofluids. On the other hand, many experimental papers have been published in recent years on a new class of nanofluids which called hybrid nanofluids [25–28]. Hybrid nanofluids are contain- ing metallic and metal-oxide nanoparticles which can be able to im- prove the thermophysical properties of working fluids. Furthermore, hybrid nanofluids containing carbon nanotubes (CNTs), which has unique thermal properties, with metal-oxide nanoparticles have been considerably studied by researchers [29–31]. Since engine oils are used in various industrial and engineering ap- plications, the crucial factor in energy saving would be improving the thermophysical properties of them. Moreover, many researchers have paid careful attention to rheological properties of them. In this regard, a new and innovative type of nanofluid called nano-lubricant (engine oil containing nanoparticles) has been introduced in recent years. Asadi and Asadi [32] have conducted an experimental study on rheolog- ical behavior of MWCNT/ZnO–engine oil nano-lubricant. Their results revealed that the dynamic viscosity of the nano-lubricant decreases as the temperature increases. They also proposed a new correlation to International Communications in Heat and Mass Transfer 78 (2016) 48–53 ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding authors at: Young Researchers and Elite Club, Semnan Branch, Islamic Azad University, P. O. Box: 35196-97951, Semnan, Iran. E-mail addresses: Aminasadi64@gmail.com (A. Asadi), Meisamasadi91@gmail.com (M. Asadi), Mohamadhossin_rezaee@yahoo.com (M. Rezaei), Marzieh.siahmargoii@hotmail.com (M. Siahmargoi), F.asadi.teacher@gmail.com (F. Asadi). http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.08.021 0735-1933/© 2016 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt