Nanofluids: Physical phenomena, applications in thermal systems and the environment effects- a critical review

This paper presents a review of literature on the effects of using nanofluids (NFs) in energy systems. First, different types of NFs, including the combination of metal and non-metal particles of nanometer sizes with a base fluid, are introduced. Then, the important properties of different NFs, such as the thermal conductivity coefficient, viscosity, specific heat capacity, and electrical conductivity are discussed. Different physical phenomena associated with NF flows including wettability, aggregation, Brownian movement, migration, nanolayer at the liquid/particle interface, and diffusiophoresis are discussed. It is shown that more accurate results can be obtained by considering these phenomena in the nanofluid flow. Subsequently, hybrid NFs are introduced, and papers related to these NFs are briefly reviewed. In the main part of this review, the application of NFs in different energy systems is discussed, revealing that NFs usually feature improved heat transfer capability compared to pure fluids. Most researchers have used NFs with metal oxide nanoparticles (NPs). Moreover, the existing economic and performance analyses indicate that NFs exhibit acceptable performance and can be used as industrial fluids. Finally, the challenges involved in using NFs in different industries are identified. Most notably these include the high cost of producing NPs, the increase in the pumping power due to an increase in viscosity, the precipitation and agglomeration of NPs with time, the reduction in the thermal conductivity coefficient at high volume fractions, increased wear in these systems, and the effect of NPs on the environment. The use of NFs decreases energy consumption, emissions, waste production, raw material usage, and, hence, damage to the environment.