Introduction to hybrid nanofluids

Renewable sources provide eco-friendly sustainable alternatives to replace carbon-intensive energy sources. Nanotechnology has been developed to optimize and improve energy systems and efficiency of the energy conversion processes. Nanotechnology-based thermal fluids are introduced to harness solar and geothermal energy. Nanomaterials have gained extensive interest in recent years, encouraging to perform an innovative role in sophisticated applications. With the significant development of modern nanotechnology and nanomaterials, the discovery of nanofluids that contained nanosized particles dispersed in base fluids completely changed the picture. Nanofluids do not retain all favorable properties that are requisite for specialized applications. To trade-off between properties, hybrid nanofluids have been introduced to acquire enhanced properties, suitable for applications that involve remarkable thermal, optical, and rheological properties of the working fluid.Hybrid nanofluids are the novel high-class working fluids considered as a potential candidate for several applications. Hybrid or composite nanofluids are developed as a novel class of nanofluids synthesized by combining two or more nanoparticles containing metal or metal oxide or a combination of both particles in a base fluid. The preparation process is a significant step in the nanocomposites to further expand the thermal conductivity of heat transfer fluids. Thermophysical properties of hybrid nanofluids show promising enhancement as compared with mono nanofluids, especially thermal conductivity. Increment in nanoparticle loading results in enhanced values of thermophysical properties like thermal conductivity, viscosity, heat capacity, and density. Several investigations have been reported on the thermal conductivity and viscosity of hybrid nanofluids, but research on other properties like density, specific heat, thermal diffusivity, and magnetic properties are limited.