State-of-the-art review of hybrid nanofluids for heat exchanger performance enhancement

The hybrid nanofluid (HNFs) is considered a new concept in the area of fluid technology that offers promising potential for use in advanced thermal systems because of the better thermophysical characteristics. This article reviews the use of hybrid nanofluid in different kinds of heat exchangers such as smooth tube, double pipe, shell & tube, plate, and micro channel heat exchangers. In addition, the review discusses the drawbacks of conventional heat transfer fluids, as well as the benefits of using hybrid nanoparticles. Different types of nanoparticles like metallic, metal oxides, carbonaceous, ceramics, and magnetic particles are considered along with their respective methods of preparation and stabilization such as one-step, two step preparation, ultrasonication, use of surfactants, and variation of pH level. Nanoparticles’ material, concentration, Reynolds number, and geometry of the heat exchanger are of great concern to thermal effectiveness. Moreover, the thermodynamics of the processes with respect to exergy and entropy generation is discussed to assess system efficiency and irreversibilities. It can be seen from the studies gathered that the HNFs possess great potential with regard to improving thermal conductivity and exergy efficiency when compared to traditional working fluids. Nonetheless, there are several problems yet to be addressed. Key research gaps have been highlighted along with proposed future research directions related to stability improvement, optimization of nanoparticle hybrids, AI-based modeling, and combined numerical-experimental approaches for sustainable thermal management applications.