The economic efficiency of energy conversion systems can be increased without affecting the energy conversion efficiency by increasing the heat transfer rate using active or passive approaches. The passive approach is more reliable and cost-effective than the active method since it has no moving parts. The use of nanofluids (mono- and hybrid) and turbulators are examples of passive approaches. Turbulators can be inserts, such as twisted tapes, vortex generators, baffles, or porous materials, or changes to the tube surface, such as longitudinal fins, helical fins, or fin pin arrays. The purpose is to increase performance by boosting effective thermal conductivity, efficiently mixing flow, and enhancing heat transfer rates. As a result, energy usage, costs, and CO2 emissions can be reduced.

Hybrid nanofluids have demonstrated superior thermal characteristics and stability when compared to simple nanofluids, making them suitable candidates for thermal applications, including solar thermal systems, automotive cooling systems, heat sinks, or thermal energy storage. However, the use of hybrid nanofluids and turbulators may result in an increased drop in pressure, therefore, the overall heat transfer coefficient must be evaluated to assess whether the technology is favorable. Several studies have been published by researchers that demonstrate the application of machine learning (ML) approaches to forecast the performance of complex energy systems. Nonetheless, there is an urgent need for research into the application of mono- and hybrid nanofluids in various energy systems, as well as the creation of mathematical/ML models to predict thermophysical characteristics.

This Special Issue is focused on evaluating the idiosyncratic behavior of mono- and hybrid nanofluids along with their applications in various thermal systems. Moreover, this Special Issue covers experimental and numerical studies focusing on the applications of mono- and hybrid nanofluids, which can be combined with other turbulators. The use of hybrid nanofluids is one of the most promising thermal enhancement techniques, and its combination with turbulators can lead to global optimum heat transfer conditions. We welcome both original research and review articles.

Potential topics include but are not limited to the following:

• Synthesis, preparation, and thermophysical properties evaluation of mono- or hybrid nanofluids
• Particle shape, thermophoresis, and Brownian effects of mono- or hybrid nanofluids
• Numerical and experimental mechanisms behind mono- or hybrid nanofluid enhancement
• Multi-objective optimization and hybrid machine learning approaches in nanofluid-based energy systems
• Application of mono- or hybrid nanoparticles and turbulators in heat exchangers, solar energy, HVAC, and electronic cooling or thermal management
• Coating heat transfer surfaces using mono- or hybrid nanoparticles
• Adding nanoparticles to improve combustion processes
• The use of mono- or hybrid nanoparticles for CO2 capture