Rheology And Thermal Conductivity Anisotropy Of Magnetic Nanofluids

As a new type of heat exchange working medium,nanofluid has strong heat exchange ability.It has broad application prospects in heat storage and heat exchangers.In addition to the heat transfer characteristics of traditional nanofluids,magnetic nanofluids also have characteristics such as paramagnetism.Therefore,magnetic nanofluids can be used to control the heat transfer of magnetic nanofluids by changing the magnetic field.Due to the field of heat exchange and collection,the stability of nanofluids is one of the main reasons that limit its application.Therefore,this paper takes water-based Fe₃O₄ magnetic nanofluids as the research object,analyzes its stability qualitatively and quantitatively,and explains its stability mechanism from a microscopic point of view.On the basis of stability,its rheological properties and thermal conductivity anisotropy are analyzed and studied through experiments and numerical simulations.The subject of this research not only provides theoretical data for the application of surfactants in the stability of magnetic nanofluids,but also further reveals its potential in rheological properties and thermal conductivity,and provides feasibility for its use in solar heat storage and heat exchange.The Fe₃O₄ magnetic nanofluid was prepared by a two-step method.The Fe₃O₄nanoparticles were characterized by TEM,XRD and EDS.The results showed that the Fe₃O₄ nanoparticles were ellipsoidal,with an average particle size of about 20nm and a purity of 99.9%.In terms of stability,four methods were used to analyze the effects of surfactant types and ratios on the stability of Fe₃O₄ nanofluids.The results show that the effects of different types of surface activity are:TMAH>CTAB>CA>PVP>SDS.The effect of TMAH surface activity addition ratio from high to low is:1:4>1:2>1:1>2:1>4:1.The best ratio of stability is when the mass ratio is 1:4.The rheological properties of Fe₃O₄ nanofluids were studied by rotational viscometer,and the effects of surfactant types,mass ratio,and shear rate on their rheological properties were analyzed.The study found that among different types of surfactants,PVP has the greatest impact on viscosity,which is 54%greater than that of TMAH.As the mass ratio and shear rate increase,its viscosity will gradually increase.It was also found that the Fe₃O₄ nanofluids exhibited non-Newtonian fluid behavior in the shear rate range of 25-250s^-1,and belonged to the expansion fluid in the non-Newtonian fluid.The thermal conductivity and anisotropy of Fe₃O₄ nanofluids were explored by thermophysical analyzer and numerical simulation.The thermal conductivity change law of Fe₃O₄ nanofluids was analyzed from the angles of surfactant type,mass ratio and concentration.Studies have found that the thermal conductivity of SDS is significantly higher than other surfactants.The increase in mass ratio also has a significant effect on heat conduction.Through numerical simulation,it is found that as time goes on,the temperature distribution of Fe₃O₄ nanofluids appears irregular,and the temperature distribution changes from a circle to an ellipse.As the volume concentration of the magnetic fluid increases,the temperature in the direction perpendicular to the magnetic field gradually decreases,and the ellipse gradually becomes flat.