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Theoretical analysis, numerical simulations, and experimental studies are used to comprehensively investigate the changes in the heat transfer characteristics of Fe3O4–water magnetic nanofluid under different types of magnetic fields and the heat transfer mechanism, providing new ideas for its practical application. By combining the fundamental conservation theorem and the Boussinesq approximation, dimensionless control equations have been established. The analysis of the variation pattern of the calculated values of the source terms under different boundary conditions, as well as the order of magnitude analysis, concludes that: the enhanced trend of Brownian motion and thermophoretic motion is the main reason for the enhanced heat transfer capability of the magnetic nanofluid; where the thermophoretic motion contributes slightly more to the heat transfer; the increase in Joule heat is the reason for the further enhancement of the heat transfer capability by the magnetic field. Volume fraction, temperature, and magnetic field strength are positively correlated with the average convective heat transfer coefficient. The reason for magnetic fields to enhance heat transfer is revealed by the deflection of the magnetic nanofluid by the magnetic body flow under local magnetic fields to form localized reflux. The alternating magnetic field further enhances the ability of magnetic fields to enhance heat transfer in magnetic nanofluids. Overall, the effect of the applied magnetic field on the motion of the magnetic nanoparticles causes the disruption of the thermal boundary layer, which is responsible for the enhanced heat transfer capacity of magnetic nanofluids. [ABSTRACT FROM AUTHOR]

Nanofluids have been widely used in various fields because of their unique thermal conductivity. This paper reviews and evaluates the thermal conductivity of nanofluids according to the recent literatures on the thermal conductivity of nanofluids. The results showed that in recent years, with the support of multiple disciplines, many new nanofluids have been prepared, such as cellulose nanofluids, graphene nanofluids, etc. These nanofluids have even better thermal conductivity. In terms of the prediction of thermal conductivity of nanofluids, neural network prediction has high accuracy and low time cost, so it is believed that it will become the main prediction method in the future. Although the experiments and studies of nanofluids have been carried out, there is no comprehensive review on the thermal conductivity of nanofluids from various aspects. In order to solve this problem, the research on nanofluids in recent years is reviewed from three aspects: influencing factors, prediction models and applications. The prediction models of thermal conductivity of nanofluids are introduced emphatically and each prediction model is evaluated. The neural network prediction models in recent years are discussed in detail. The purpose of this paper is to give a complete and detailed description of the thermal conductivity of nanofluids, as a reference for researchers to understand and study the thermal conductivity of nanofluids, and to update the latest research results in a timely manner. It is hoped to point out the direction for the research of nanofluids in the future. [ABSTRACT FROM AUTHOR]

Numerical simulations were adopted to explore the heat transfer and flow characteristics of magnetic nanofluids under different magnetic field intensities, volume fractions, and magnetic field directions. Due to these parameters have a significant effect on thermal hydraulic performance and the mechanism of this enhancement are not yet clear, a turbulent model of Re-normalization group (RNG) k-ε is used to analyze the variation of thermal boundary layer and particle motion. The obtained results found that the effect of heat transfer enhancement was small under a weak magnetic field, but it increased considerably under a strong magnetic field. Besides, the convection heat transfer coefficient initially increased and then decreased with an increase of volume fractions of magnetic nanoparticles. Moreover, the directions of the magnetic field have a significant effect on convective heat transfer coefficient, which results in 8% increase when the direction is perpendicular to direction of flow. [ABSTRACT FROM AUTHOR]