Critical review on the stability and thermal conductivity of water-based hybrid nanofluids for heat transfer applications

Thermal conductivity is undoubtedly the most significant physicochemical property for evaluating the thermal efficiency of nanofluids. In addition to thermal conductivity, stability is another crucial factor that must be assessed, as maintaining long-term stability in closed-circuit and continuous-cycle applications remains a major challenge. Notably, stability is closely linked to thermal conductivity in nanofluid applications. Recent studies on nanofluids have explored the incorporation of multiple types of nanoparticles known as hybrid nanofluids into heat transfer fluids such as water, ethylene glycol (EG), oil, and refrigerants. These hybrid nanofluids aim to enhance the thermal properties of conventional base fluids. However, nanoparticles at the nanoscale exhibit a strong tendency to aggregate owing to substantial van der Waals interactions, resulting in sedimentation and clogging, which compromise stability. This aggregation negatively impacts both stability and thermal conductivity. This review provides an in-depth analysis of the latest advancements in water-based hybrid nanofluids, incorporating various nanoparticle hybridizations, including metals, metal oxides, carbon-based materials, and plant-based nanomaterials such as nanocellulose in combination with synthetic nanoparticles, an area that remains relatively unexplored. Furthermore, this review discusses characterization techniques and strategies for improving the stability and thermal conductivity of hybrid nanofluids, including chemical modifications such as the addition of surfactants or surface functionalization as well as physical modifications such as optimizing the volume fraction of hybrid nanocomposites, selecting appropriate nanoparticle types and sizes, adjusting ultrasonication time, and modifying pH levels. This review is based on recent studies published between 2019 and 2024.