Experimental and Theoretical Investigations of the Thermal Conductivity of Erbium oxide/Ethylene Glycol Nanofluids for Thermal Energy Applications The thermal conductivity of Er 2 O 3 /ethylene glycol (EG) nanofluids was investi- gated at different concentrations of Er 2 O 3 nanoparticles in the temperature range of 20–50 °C. The findings showed that the volume fraction of Er 2 O 3 nanoparticles and temperature affect the thermal conductivity. The thermal conductivity increases with increasing Er 2 O 3 concentration and temperature, and the Er 2 O 3 /EG nanofluid showed higher thermal conductivity than the base fluid. Precise correlations are proposed to forecast the thermal conductivity of the Er 2 O 3 /EG nanofluid relative to the base fluid. These results are promising for using Er 2 O 3 /EG nanofluid in solar thermal applications. Keywords: Erbium oxide, Nanofluid, Thermal analyzer, Thermal conductivity, Transient hot wire method Received: March 28, 2022; revised: August 20, 2022; accepted: September 06, 2022 DOI: 10.1002/ceat.202200159 1 Introduction Recently, the primary purpose of studies on nanofluids has been to develop their thermophysical characteristics as heat transfer fluids. Indeed, heat transfer processes play a crucial role in electronic engineering with applications in electricity production, transportation, engines, microelectronics, solar thermal collectors, heating and cooling processes, and chemical procedures [1]. Convective heat transfer is influenced by three factors: the convective heat transfer coefficient, the temperature difference [2], and the heat transfer area [3]. Since convective heat transfer takes place between a solid and a fluid surface, it follows that the convective heat transfer coefficient is depen- dent on both the thermophysical properties of the fluid as well as the velocity and physical characteristics of the solid. The heat transfer coefficient is affected by thermal conductivity. To improve the performance of common fluids, many researchers have created novel types of heat transfer fluids. Thermal con- ductivity is directly related to the heat transfer coefficient, which in turn leads to improved convective heat transfer. Meanwhile, there has been a remarkable rise in employing nanofluids based on water, ethylene glycol (EG), and engine oil in recent years because of their comprehensive utilization in the thermal energy transition [4]. The organic compound EG is a dihydric alcohol used in various engineering systems, includ- ing antifreeze, diesel electric generators [5], industrial heat exchangers [6], electronic devices [7], and other industrial products, especially in applications where the operating tem- perature is close to the freezing point of water [8]. The addition of EG to fluids can lower the freezing point and increase the boiling point. However, EG has lower thermal conductivity than water; therefore, it can affect the heat transfer performance of the fluid. By adding nanoparticles with high thermal conductivity to EG, this problem can be solved. According to Younes et al. [9], base fluids with relatively low thermal conductivities and high viscosities have generally high- er thermal conductivities and better stability. Several studies on the impact of EG-based nanofluids in improving heat transfer have been conducted [10, 11]. Incorporating nanoparticles in base fluids dramatically boosts thermal conductivity compared to typical heat transfer fluids. Chem. Eng. Technol. 2022, 45, No. 00, 1–12 ª 2022 Wiley-VCH GmbH www.cet-journal.com Monther Alsboul 1,2 Mohd Sabri Mohd Ghazali 2, * Mohamed R. Gomaa 3,4, * Aliashim Albani 5 – 1 Dr. Monther Alsboul Physics Department, College of Science, Al Hussein Bin Talal Univer- sity, Maan 71111, Jordan. 2 Dr. Monther Alsboul, Prof. Mohd Sabri Mohd Ghazali mohdsabri@umt.edu.my Faculty of Science and Marine Environment, Universiti Malaysia Terengganu, Kuala Terengganu 21030, Malaysia. 3 Prof. Mohamed R. Gomaa Behiri@ahu.edu.jo Mechanical Engineering Department, Faculty of Engineering, Al Hussein Bin Talal University, Maan 71111, Jordan. 4 Prof. Mohamed R. Gomaa Mechanical Engineering Department, Benha Faculty of Engineering, Benha University, Benha 13512, Egypt. 5 Prof. Aliashim Albani Renewable Energy & Power Research Interest Group (REPRIG), Eastern Corridor Renewable Energy (ECRE), Faculty of Ocean Engineering Technology and Informatics, Universiti Malaysia Terengganu, Kuala Terengganu 21030, Malaysia. Research Article 1 These are not the final page numbers! ((