Abstract
Optimizing thermal management in heat transfer systems has sparked increased interest in hybrid nanofluids, particularly due to their tunable properties from nanoparticle blending. This study experimentally investigates the thermal behavior, efficiency, and entropy generation of Fe3O4/ZnO hybrid nanofluids in circular pipes at various hybridization ratios (80:20, 60:40, 50:50, 40:60, and 20:80) with a constant volume concentration of 0.0125 %. The 80:20 blend exhibited the greatest heat transfer enhancement, demonstrating a 36 % improvement in the transition regime and 9 % in turbulent flow. In contrast, the 20:80 ratio achieved a 37 % enhancement in the transition regime but only a 3 % improvement in turbulence, indicating lower thermal effectiveness at higher Reynolds numbers. The Total Efficiency Index (TEI) peaked at 1.53 for the 80:20 mixture, followed by 1.47 for the 60:40 blend. A higher ZnO fraction delayed the onset of flow transition, thus enhancing thermal regulation. Regarding pressure drop, the 20:80 blend consistently showed the highest resistance, while the 60:40 ratio demonstrated the lowest, indicating superior hydraulic performance. However, this ratio did not yield the best heat transfer results, suggesting a tradeoff between thermal and flow efficiency. The 50:50 ratio provided balanced performance in both heat transfer and pressure loss, making it a promising choice for practical applications. These findings highlight the influence of Fe3O4's magnetic properties in enhancing heat transport and the critical role of hybridization ratio in optimizing thermofluid performance. Future research should investigate the effects of surfactants, alternative base fluids, and external magnetic fields on long-term nanofluid stability and performance.