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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]

The heat transfer and flow characteristics of the air-water cross flow over cambered ducts were experimental and numerical investigated. The sequence of their Core Volume Goodness Factor (CVGF) is cosinoidal, parabolic, circular, trapezoidal and rectangular ducts successively from superior to inferior. Cambered ducts have more uniform temperature difference distribution than the equal cross section duct, and it has the minimum temperature difference in inlet and outlet of the cosinoidal duct. With the optimal overall heat transfer performance, the cosinoidal duct is superior to that of the rectangular duct by 7.3%–28.1%. In the cosinoidal duct, the smaller the amplitude is, the better the heat transfer performance is. The thickness of the thermal and velocity boundary layers adjacent to the wall surface decreases constantly with increased Reynolds number. In the near wall region, n = 5 um , the main heat transfer area is the peak and middle areas, but with weaker heat transfer performance in the trough region. Although gradually expanding cambered duct slows down the flow velocity, the structure form decreases the pressure drop loss during the flow process. While improving the convective heat exchange capability of the upstream, the heat transfer area of the downstream is also improved to boost the overall heat transfer performance. • Performance in divergent cambered ducts are better than that of equal cross section duct. • The cambered ducts have uniform temperature difference distribution. • The smaller the amplitude is, the better overall heat transfer performance is. • Temperature gradient in the peak region is bigger than that in the trough region. [ABSTRACT FROM AUTHOR]