A Numerical Review on Stability-Oriented Hybrid Nanofluids for Heat Transfer Enhancement in Flat Plate Solar Collectors (FPSCs)

applications due to their structural simplicity and economic feasibility. However, the inherently low thermal conductivity of conventional working fluids significantly restricts their heat transfer performance. In recent years, hybrid nanofluids, formulated by dispersing two or more dissimilar nanoparticles in a base fluid, have emerged as promising alternatives owing to their synergistic thermal characteristics and improved stability compared with mono nanofluids. This study presents a comprehensive numerical review of recent computational fluid dynamics (CFD)-based investigations on FPSCs employing hybrid nanofluids, with particular emphasis on nanoparticle stability, volumetric concentration, and their coupled thermo-hydraulic effects. The governing mathematical models, thermophysical property correlations, stability considerations, turbulence treatments, and performance indicators reported in the literature are systematically analyzed and compared. Key parameters including Nusselt number, thermal efficiency, pressure drop, exergy efficiency, and thermo-hydraulic performance factor are critically evaluated. The reviewed studies consistently demonstrate that hybrid nanofluids can substantially enhance heat transfer and collector efficiency at relatively low volumetric concentrations, primarily due to increased effective thermal conductivity and improved particle dispersion. Nevertheless, excessive concentrations lead to viscosity growth, flow instability, and elevated pumping power, which offset thermal gains. Most numerical investigations converge toward an optimal concentration range of approximately ۰.۰۳-۰.۰۷%, within which maximum thermo-hydraulic benefits are achieved while maintaining acceptable stability. Furthermore, carbon-assisted and MXene-based hybrid formulations exhibit superior performance at even lower concentrations, highlighting the importance of nanoparticle selection and stability control. Finally, existing research gaps and future directions are outlined, emphasizing the need for multiphase modeling, temperature-dependent property formulations, long-term stability analysis, and techno-economic assessments to facilitate the practical deployment of hybrid nanofluids in solar thermal systems.

کلیدواژه ها:

Hybrid nanofluids ، Flat plate solar collectors ، Nanofluid stability ، Volumetric concentration ، Computational fluid dynamics (CFD) ، Heat transfer enhancement ، Thermo- hydraulic performance ، Renewable energy