Search

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]

For the serrated heat transfer surface, the heat transfer and flow properties are investigated with external vibration excitation using numerical simulation method. By using the dynamic grid to realize the vibration of the fin wall, the impacts of frequency, amplitude, wind velocity and ambient temperature on vibration-strengthened heat transfer are investigated. The results show that vibration impacts the temperature distribution around the fin, interferes with the flow of air, causes the air to form a certain angle with the fin wall, thus preventing the development of the boundary layer and achieving strengthened heat transfer. The impact of amplitude on vibration-strengthened heat transfer is smaller than that of frequency, and low wind velocity is more favorable to the role of vibration-strengthened heat transfer, and the change of ambient temperature has less impact. Compared with the stationary condition, wall vibration can improve the average heat transfer coefficient by up to 66.6%. [ABSTRACT FROM AUTHOR]

Nanofluids are important heat transfer mediums with great potential in industry applications because of its high thermal conductivity when adding metal or metalloid nanoparticles, which has better heat transfer characteristics and controllability than the base liquid. However, there are great uncertainties, deviations and anomalies in the computational model, experimental results and theoretical analysis when conducting research on the thermal physical properties, convective heat transfer performance and heat transfer mechanism of nanofluids. This poses a great challenge to the precise and active controllability of nanofluids when considering their own parameters and external excitation. Also, these inevitable problems may cause certain confusions on the follow-up study of nanofluids. Based on these phenomena, this paper systematically reviews the recent researches on heat transfer performance of nanofluids. The thermophysical characteristics, convective heat transfer coefficient, stability and heat transfer mechanism of suspension mixed nanofluids were summarized and compared. It is hoped that it can provide effective references and guidance for researchers to study the heat transfer performance of nanofluids, and also provide new ideas for future researches on the heat transfer performance of nanofluids. [ABSTRACT FROM AUTHOR]

The heat transfer and pressure drop performance of magnetic nanofluids (MNFs) under different alternating magnetic fields were investigated experimentally. The results showed that the effect of alternating magnetic field on local heat transfer coefficient is better than that of unidirectional and non-magnetic fields along flow direction. The local heat transfer coefficient in the rear of the flow channel has a greater promoting capacity than that in the front. The heat transfer performance is higher at a lower Reynolds number (Re), but worse at a higher Re. Heat transfer coefficient always increases with the increase of volume fraction with or without alternating magnetic fields. When the volume fraction is among 1 vol%-3 vol%, the greater the alternating frequency is, the better the heat transfer performance will be. When the volume fraction is greater than 3 vol%, increasing the frequency of alternating magnetic field has little effect on heat transfer performance. For magnetic nanofluids with different volume fractions and different magnetic fields, they also show quadratic variation rules. The larger the volume fraction and magnetic field frequency are, the greater the pressure drop will be. The improvement of the overall heat transfer performance increases first and then decreases when the alternating frequencies and Reynolds numbers increase. The reasons for the improvement of the heat transfer are the movement and accumulation of magnetic nanoparticles at the thermal boundary layer, and the eddy action of MNFs. • Effect of alternating magnetic field on heat transfer coefficient is better than that of unidirectional fields. • Heat transfer coefficient increases with the increase of alternating frequency and volume fraction. • Factors that enhance heat transfer are particle motion, accumulation, vortex flow, and thermal boundary layer. • The larger the volume fraction and magnetic field frequency are, the greater the pressure drop will be. • The overall heat transfer performance increases first and then decreases. [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]