Experimental Study On Magnetic Nanofluid Enhanced Convection Heat Transfer Under Magnetic Field

Magnetic nanofluid is a new kind of functional material formed by the stable dispersion of magnetic nanoparticles in non-magnetic substrate.Since magnetic nanofluids have both superparamagnetism and flow characteristics,its have great potential in the field of efficient and controllable energy transfer.The application of magnetic nanofluids to different types of heat transfer devices by magnetic field control is a hot topic in current research.However,there is no unified conclusion on the mechanism of enhanced heat transfer under fluid-magnetic coupling of magnetic nanofluids,and the experimental results of convective heat transfer of magnetic nanofluids under constant magnetic field are controversial.In order to make magnetic nanofluid better applied to different types of heat exchange equipment,this paper uses Fe3O4/water magnetic nanofluid to carry out experimental research on the flow and heat transfer characteristics under constant magnetic field,and further analyze the potential driving mechanism of heat transfer enhancement under the action of magnetic field.In this paper,the Fe3O4/water magnetic nanofluid was prepared by the "two-step method",and the stability experiment was conducted by combining the visual method and the transmittance method.The effects of ultrasound duration and surfactant on the stability of magnetic nanofluids are analyzed.The results show that the best ultrasonic time is 1h during preparation and the CTAB had the best adaptability with Fe3O4 magnetic particles,and the best mass ratio is 1:1.The thermal conductivity of Fe3O4/water magnetic nanofluids was measured by transient hot-wire method.The experimental results show that the thermal conductivity of magnetic nanofluids increases with temperature and particle concentration.At higher volume fractions,the thermal conductivity of magnetic nanofluids exhibits a non-linear change with concentration,which is more likely to cause aggregation and sedimentation as the particle concentration increases.The heat transfer and flow characteristics of Fe3O4/water magnetic nanofluids were experimentally studied under the effect of a magnetic field.The experimental results were combined with a magnetic flux density distribution.The effects of magnetic flux density,magnetic field gradient,and different magnetic field orientations on the local Nusselt number and flow pressure drop were analyzed.The results show that,increasing the magnetic flux density and magnetic field gradient can achieve a significant improvement in the convective heat transfer.At the Re of 1080 and the magnetic field of 415 gauss,the local Nusselt number for 0.5 vol%Fe3O4/water magnetic nanofluids increased by 4.36%,whereas the corresponding value for the magnetic field of 700 gauss increased by 7.19%.Additionally,the local Nusselt number increased by 32.0%under a magnetic field gradient of 28.6 gauss/mm.The occurrence of chain-like structure is the main reason for the increased convective heat transfer.In addition,the flow disturbance caused by the magnetic field increases the pressure drop.However,the effect is weaker than the enhancement of heat transfer.