Heat transfer efficiency of Al 2 O 3 -MWCNT/thermal oil hybrid nanofluid as a cooling fluid in thermal and energy management applications: An experimental and theoretical investigation Amin Asadi a,⇑ , Meisam Asadi b , Alireza Rezaniakolaei a,⇑ , Lasse Aistrup Rosendahl a , Masoud Afrand c , Somchai Wongwises d a Department of Energy Technology, Aalborg University, Pontoppidanstraede 111, DK-9220 Aalborg, Denmark b Young Researchers and Elite Club, Semnan Branch, Islamic Azad University, Semnan, Iran c Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran d Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab. (FUTURE), Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Bangmod, Bangkok, Thailand article info Article history: Received 15 August 2017 Received in revised form 3 October 2017 Accepted 9 October 2017 Keywords: Experimental study Heat transfer efficiency Thermophysical properties Convection heat transfer coefficient MWCNT and Al 2 O 3 nanoparticles abstract The main objective of the present study is to assess the heat transfer efficiency of Al 2 O 3 -MWCNT/thermal oil hybrid nanofluid over different temperatures (25–50 °C) and solid concentrations (0.125%–1.5%). To this end, first of all, the stability of the nano-oil has been studied through the Zeta potential analysis. Then, the dynamic viscosity and thermal conductivity of the nanofluid have been experimentally inves- tigated. It was found that the nanofluid showed Newtonian behavior over the studied range of temper- atures and solid concentrations. The dynamic viscosity showed increasing trend as the solid concentration increased. It is found that the minimum increase in dynamic viscosity is at the temperature of 50 °C in all the studied solid concentrations except 0.5% and 1%. As for the thermal conductivity, it showed increasing trend as the temperature and solid concentration increased. The maximum enhance- ment was at the temperature of 50 °C and solid concentration 1.5% by approximately 45%. Based on the experimental data, two new highly precise correlations to predict the dynamic viscosity and thermal con- ductivity of the studied nanofluid have been proposed. Moreover, the heat transfer efficiency of the nano- fluid has been evaluated based on different figures of merit. It is revealed that using this nanofluid instead of the base fluid can be beneficial in all the studied solid concentrations and temperatures for both the internal laminar and turbulent flow regimes except the solid concentrations of 1 and 1.5% in internal tur- bulent flow regimes. The effect of adding nanoparticles on pumping power and convective heat transfer coefficient has also been theoretically investigated. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Thermal oils have been widely used as a coolant fluid for heat transfer and energy management applications in various industries such as automotive and electrical industry. Furthermore, thermal oils are the dominant fluids in the applications of lubricating and cooling the internal combustion engines. The most important func- tions of engine oils can be summarized as: lubricating the engine parts in both the low and high temperatures, reducing the wear on different moving parts, cooling down the engines by carrying heat away from moving segments, protecting the emission sys- tems, and so forth. To this end, viscosity and thermal conductivity of oils play an important role. In other words, lower viscosity results in lower pumping power and pressure drop and on the other hand, higher thermal conductivity means higher heat trans- fer performance. Nanofluids are a suspension of nanoparticles in conventional working fluids such as water, ethylene glycol, and motor oils, which are introduced for the first time by Choi [1]. Adding the par- ticles which have higher thermal conductivity compared to that of the base fluids results in improving the thermophysical properties of the working fluids. This feature grabs the attention of many researchers in recent decade to utilize the invaluable characteris- tics of this new class of working fluids in different applications [2–14]. https://doi.org/10.1016/j.ijheatmasstransfer.2017.10.036 0017-9310/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. E-mail addresses: Ams@et.aau.dk, Aminasadi64@gmail.com (A. Asadi), Alr@et. aau.dk (A. Rezaniakolaei). International Journal of Heat and Mass Transfer 117 (2018) 474–486 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt