Nanofluids: Key parameters to enhance thermal conductivity and its applications

Recent advances in the customization of nanofluids to obtain specific thermophysical properties have led to an increase in the need for a better understanding of the various factors that impact thermal conductivity. The term nanofluid refers to the combination of any nanoscale material with a base fluid. Although investigations of the development and application of nanofluids have expanded dramatically over the past decade, the number of commercialized applications of nanofluids is somewhat limited, with the majority of the research focused on investigations of the theory and fundamental science. This review discusses the various applications of nanofluids briefly. It focuses on a discussion of the various parameters that have been found to impact the thermal behavior in general significantly and the thermal conductivity of these nanofluids, including parameters such as particle size and shape, the pH of the fluids, surfactant, solvent type, hydrogen bonding, temperature, base fluids, and particular interest here, the alignment of the nanoparticles utilized (carbon nanotubes, Graphene, and metal oxides nanoparticles). These parameters have been found to affect the thermal conductivity of the nanofluids directly and can either increase or decrease the thermal conductivity. In contrast, other parameters, such as the viscosity, have an "indirect" effect on the thermal conductivity. While there is broad agreement that these parameters govern the thermal conductivity and hence, the heat transfer capability of the nanofluids, there is a lack of a clear consensus about the relative importance and impact of these thermophysical properties and, in some cases, actually conflicting data in the literature. This lack of clarity in the effect of these various parameters has resulted in the relatively slow adoption, implementation, and application of these nanofluids in commercial applications. The stability of the nanofluids and the nanoparticle suspension duration are other issues that limit commercialization efforts. The information presented here helps to clarify these issues and also explores the effects of factors such as nanoparticle size and type, which may lead to additional opportunities for commercial applications.