The Effect Of Magnetic Field On The Convective Heat Transfer Of Magnetic Nanofluids

This paper investigated the thermophysical properties of nanofluids（Fe₃O₄-water,carbon nanotubes（CNTs）-water and Fe₃O₄-CNTs-water）prepared using various surfactants.The optimal surfactants needed to prepare stable nanofluids were explored at the same time.Moreover,Fe₃O₄nanoparticles and CNTs were mixed in the ratio 3:1 to synthesize the new Fe₃O₄-CNTs-water nanofluid.The effects of the volume fraction and temperature of the nanoparticles on the viscosity of the nanofluid were experimentally analyzed.Based on the present measurements,new empirical formulas were proposed for Fe₃O₄-water,CNTs-water and Fe₃O₄-CNTs-water nanofluids to provide accurate predictions for the nanofluid viscosity.Results indicate that better stability,smaller viscosity and higher thermal conductivity are obtained,when the surfactants tetramethylammonium hydroxide,sodium dodecyl sulfate and sodium laurylsulfonate are added into the Fe₃O₄-water,CNTs-water and the Fe₃O₄-CNTs-water nanofluid,respectively.The effect of magnetic field on the laminar convective heat transfer of magnetic nanofluids was studied by experimental method in this paper.Specifically,the effects of the Number and spacing of the ring magnets on the convective heat transfer of Fe₃O₄-water nanofluids were investigated in laminar flow regime.The experimental results were combined with the simulation results of magnetic flux density and magnetic force acting on nanoparticles to explain the potential mechanism of heat transfer enhancement observed under given conditions.Comparing the theoretical predictions with the experimental data,it is found that the two have good consistency and the error is less than 10%,which verifies the rationality of the experimental results.Studies have found that heat transfer of magnetic nanofluid can be significantly improved by increasing the Number of magnets.Under the condition of single magnetic sleeve,the magnetic nanofluid heat transfer is enhanced by26.5%and 54.5%at Re=391 and 805,respectively.The heat transfer characteristics of Fe₃O₄-CNTs-water hybrid nanofluids in heated straight tubes were exhibited experimently in this paper.The effects of Reynolds Number（476-996）,volume fraction and Number of magnets were considered in this study.The experimental results show that compared with no magnetic field,the maximum local heat transfer of 1.44%Fe₃O₄-CNTs-water nanofluid increased by 61.54%in the magnetic field and Re=996.The results also show that compared to the base fluid（deionized water）,the average heat transfer coefficient of Fe₃O₄-CNTs-water nanofluids increased by 67.9%and20.89%,respectively,in the presence of magnetic field and no magnetic field.Finally,the effect of magnetic field on the laminar convective heat transfer and pressure loss of Fe₃O₄-water nanofluid（Reynolds Number of 89-388）in the plate heat exchanger was experimentally analyzed in this study.The heat transfer capability of Si O₂-water nanofluids in heat exchanger was demonstrated.It is found that the average heat transfer of the two nanofluids is better than that of the base fluid.Compared with Si O₂nanoparticles,Fe₃O₄nanoparticles have better heat transfer enhancement effect.In the absence of magnetic fields,the heat transfer and pressure drop of 1 wt.%Si O₂-water nanofluid in the range of Reynolds Number 89-388 averagely increased by 15.79%and9.8%,respectively.The average Nusselt number and pressure difference of 1 wt.%Fe₃O₄-water nanofluids are 17.62%and 11.25%higher than those of base fluids,respectively.Under the three sets of magnets,the heat transfer of the heat exchanger is most significantly improved by 1 wt%Fe₃O₄-water nanofluid（31.63%）.The maximum pressure loss（24.73%）of the heat exchanger is also caused in this case（three sets of magnets）.