Nanofluids for heat transfer applications: a review

Since diluted suspensions of nanoparticles were first called nanofluids and presented as viable solutions for heat transfer applications, this subject has received much attention and related investigations have expanded to many paths. In order to comprehend how nanoscale-related effects could influence the macroscopic transport behavior of nanofluids under single or phase-change conditions, researchers have studied, for example, the stability of these solutions, variation of thermal and rheological properties, and the convective heat transfer behavior of a great variety of nanofillers in common fluids, mainly water. The deposition of nanofillers over heated surfaces has also been investigated due to the role of surface nanostructuring in modifying wettability, thermal resistance, and delaying the occurrence of critical heat flux. Despite the considerable number of publications regarding nanofluids, scattered results for transport properties or convective behavior of nanofluids under similar experimental conditions are often found, which hinders their applications due to a lack of comprehension on the mechanisms related to the behavior of these fluids and, consequently, to the difficulty in predicting it. In this context, this work concerns a review about the heat transfer behavior of nanofluids under single-phase flow, pool boiling, and flow boiling conditions. In general, there is a consensus that the heat transfer coefficient of single-phase flow is enhanced by the addition of nanoparticles to base fluids, although overall benefits of their application cannot be assured due to increases in viscosity. In contrast, either increase or decrease in heat transfer coefficient could be observed for pool and flow boiling conditions. Such behavior can be attributed to surface modifications due to interactions between the bare surface texture and the deposited nanoparticles; however, information on the surface texture is commonly missing in most works. Finally, the main mechanisms reported in the literature pointed out as responsible for the heat transfer coefficient behaviors are summarized, where it can be seen that modifications of transport properties and particles movements impact single-phase flow, while phase-change heat transfer is also influenced by variations of surface characteristics.