Experimental study on enhanced heat transfer and flow performance of magnetic nanofluids under alternating magnetic field

Abstract

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.