Thermal synergy at the nanoscale: A review on hybrid nanofluids

In recent years, hybrid nanofluids (HNFs)—suspensions of dissimilar nanoparticles in base fluids —have attracted significant research interest due to their potential to enhance thermal performance through synergistic effects between constituent nanoparticles. A comprehensive understanding of their thermophysical behavior is essential for successful integration in advanced heat transfer and energy systems. This review presents a detailed and up-to-date analysis of recent advancements in the characterization and modeling of HNFs, with a balanced emphasis on both theoretical approaches and experimental findings. The novelty of this work lies in its systematic comparison of classical models and modern formulations, as well as its synthesis of diverse experimental data into a coherent framework. Unlike previous reviews, this paper critically examines the role of particle composition, shape, concentration, and interaction mechanisms on thermal conductivity, viscosity, and stability. Key challenges such as nanoparticle agglomeration, measurement inconsistencies, and scale-dependent behavior are highlighted. In addition, limitations of existing predictive correlations are discussed. The review concludes by identifying current research gaps and outlining promising future directions, aiming to support the development of more accurate models and effective applications of HNFs in thermal management technologies.