Study On Thermal-flow-magnetic Coupling Mechanism Of Magnetic Nanofluids

With the explosive development of big data,cloud computing,artificial intelligence and so on,the number of integrated logic gates per unit area of chips is increasing,At the same the structure of chips is getting smaller and smaller,which brings about a sharp increase in heat flux per unit area of electronic devices and equipment.In view of the limited volume and size under the condition of high heat flux density,the traditional single-phase liquid cooling technology cannot meet the needs of device cooling and temperature uniformity among multiple devices.The magnetic nanofluid with both fluidity and magnetism provides a new research idea for realizing liquid cooling and controllable heat dissipation of electronic devices through efficient and controllable energy transfer.Fe3O4 nanoparticles with a particle size of about 20nm were prepared by chemical coprecipitation method.By adding 16mg/mL sodium oleate,adjusting pH=9.5 and dispersing by 30min ultrasonic,the wrapping environment of surfactant was improved,and the nanoparticles were stably suspended in deionized water to prepare Fe3O4-H2O magnetic nanofluids with different volume fractions.The effects of volume concentration and temperature on thermal conductivity and viscosity were studied experimentally,and the correlations between thermal conductivity and viscosity with temperature were fitted.The experimental study on the macroscopic convective heat transfer characteristics of Fe3O4-H2O magnetic nanofluids was carried out under the influence of magnetic field.The influence of heat-flow-magnetic multi-field coupling on the convective heat transfer of magnetic nanofluids was analyzed.The results reveal that the end of a single rectangular magnet has a greater heat transfer effect on magnetic nanofluids.Compared with the absence of magnetic field,the local convective heat transfer coefficient(h)increases by 17.03%.The reason is that under the synergy of heat-flow-magnetic field,a clockwise vortex is formed at the end of the magnet,which accelerates the disturbance of the thermal boundary layer near the wall,and the position on the wall is the same as the flow direction,and enhances the mixing and heat transfer between cold and hot fluids.When the distance of the magnet is 136mm away from the inlet,the maximum increase of h reaches 17.39%,indicating that the micro-movement and migration of nanoparticles at the inlet leads to significant changes in macro flow and heat transfer characteristics.Adding a magnetic field to the inlet section can further enhance the heat transfer characteristics of magnetic nanofluids,and the maximum increase of h amounts to 21.41%.For the inlet section,the smaller the length of the rectangular magnet is,the more obvious the strengthening effect is.When the length of the magnet in the inlet section is 46mm,maximum h for 0.5vol%magnetic nanofluid increase up to 6.45%compared with 96mm.The coupling effect of heat flow magnetism makes the vortices in different directions formed at the front end and tail end of the magnet.For magnets with different lengths,the disturbance intensity and region are different,which makes the enhanced heat transfer effect of shorter magnets more excellent.