Thanks to continuous advancements in science and engineering, internal combustion engines in modern automobiles have achieved high thermal efficiencies, driven by technologies such as turbocharging, direct fuel injection, and lean-burn combustion under optimized operating conditions. However, these engines are constantly subjected to high thermal loads; therefore, to maintain engine performance and longevity, effective thermal management becomes a key factor. To achieve this, the cooling system must dissipate excess heat, maintain the engine at its optimum operating temperature, and enable rapid thermal stabilization during start-up [1].
At the heart of the cooling system lies the radiator, which serves as the primary component responsible for rejecting heat generated during engine operation. A typical radiator comprises three main sections: (1) the upper tank, which receives hot coolant from the engine; (2) the lower tank, which supplies cooled fluid back to the engine; and (3) flat tubes with attached fins that facilitate heat exchange with ambient air. As coolant flows through the tubes, it is cooled by airflow passing over the fins. However, due to the flat geometry of the tubes and fins, the heat transfer area is limited, resulting in suboptimal thermal performance. Conventional coolants, typically a water and ethylene glycol (EG) mixture, offer benefits such as anti-freezing and elevated boiling points [2], but suffer from low thermal conductivity and poor performance under extreme or rapidly varying heat loads [3].
To solve this problem, several studies have focused on enhancing fin geometry (e.g., corrugated, perforated, or multi-layer fins) to increase the surface area for heat exchange [4-6]. While effective, these approaches are approaching their practical design limits [7, 8]. In light of rising fuel costs and the demand for increased energy efficiency, downsizing the cooling system, including the radiator, is a promising direction. However, reducing the radiator's size while relying on conventional coolant leads to a further drop in thermal performance due to inherent limitations in thermal conductivity.
Nanofluids, base fluids engineered with suspended nanoparticles, represent a promising solution for thermal management challenges. Their high surface-area-to-volume ratio and superior thermal properties enable significantly enhanced conductivity compared to conventional coolants [9-11]. In the automotive sector, nanofluids have shown potential for enhancing heat dissipation while facilitating system miniaturization. For instance, Leong et al. [12] reported that using a copper/EG nanofluid in automotive radiators increased the heat transfer rate and overall heat transfer coefficient by up to 3.8%. Peyghambarzadeh et al. [13] observed a 45% improvement in cooling efficiency using water/Al₂O₃ nanofluids. Other studies involving CuO, Fe₂O₃ [14-16] or carbon nanotubes (CNTs) [17], have shown significant enhancements in the heat transfer coefficient (8-55%) and Nusselt number (up to 90.76%). Ferrão Teixeira Alves et al. [18] further demonstrated that Al₂O₃-CuO hybrid nanofluids improved the Nusselt number by 16.64%. More complex hybrid nanofluids, such as TiO₂-Cu- Ag [19], ZnO-TiO₂ [20], Al2O3/CuO/water-EG [21], CuO-MgO-TiO₂ [22], have also shown promising results, with thermal performance improvements of up to 46%.
Building on these advancements, this study investigates the convective heat transfer performance of nanofluids containing Al₂O₃, CuO, and their 50:50 hybrid mixture, dispersed in two base fluids: pure water and a 60:40 water/ethylene glycol mixture. Nanoparticle volume concentrations of 1%, 3%, 5%, and 7% are examined. A representative tube model of a car radiator is simulated using ANSYS Fluent, employing CFD techniques to analyze heat transfer performance via the Nusselt number and convective heat transfer coefficient.
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