Study On Heat Transfer Enhancement Mechanisms Of Fe₃O₄-Water Nanofluids In Tube Under Magnetic Field

With the development of industry,the combination of traditional working fluids?such as distilled water,alcohol,oil.et al?and conventional tubes?such as circular tube?can no longer meet the increasing requirement of heat exchange.Therefore,this paper considers following aspects.Firstly,the Fe₃O₄-H₂O nanofluids are used as working fluid instead of distilled water due to their excellent thermal conductivity.Besides,the circular tube is replaced by corrugated tube because of its limited heat transfer performance.Moreover,the effects of magnetic field are also taken into consideration to improve the heat transfer performance of system.In this paper,an experimental system of water based Fe₃O₄ nanofluids forced convection is set up to investigate the influence of magnetic field on the thermo-hydraulic performances of magnetic nanofluids.The main research work and conclusions of this paper are as follows:?1?Fe₃O₄-H₂O nanofluids are prepared by two-step method and their stabilities are investigated by static observation.The physical properties of Fe₃O₄ nanofluids with different mass fractions are measured.The results show that the thermal conductivity of nanofluids can be increased by 1.5%at most,and the viscosity can increase by 5.8%at most.?2?The forced convection experimental system of water-based Fe₃O₄ nanofluids under the influence of paralleled magnetic field is established.The effects of mass fraction and paralleled magnetic induction intensities on the Nusselt number and flow resistance coefficient of nanofluids are studied.It is found that the paralleled magnetic field can weaken the heat transfer of nanofluids.Finally,a comprehensive evaluation index is applied to estimate the experiment system and the highest point of heat transfer performance is obtained,where the comprehensive evaluation index can reach1.18.?3?The forced convection experimental system of water-based Fe₃O₄ nanofluids under the influence of vertical magnetic field environment is set.The results turn out that the vertical magnetic field is beneficial for heat transfer enhancement,which is different from paralleled magnetic field.The arrangement of the magnetic field also affects the heat transfer to a certain extent.The experimental results show that the two-side staggered magnetic field has the best heat transfer performance.Considering the influence of magnetic field on heat transfer and flow resistance,the comprehensive evaluation index can reach 1.47 at best.?4?The effects of tube type on the flow and heat transfer characteristics of are discussed.By comparing the Nusselt numbers and flow resistance coefficients of the Fe₃O₄-H₂O nanofluids in the corrugated tube and the circular tube,the results show that the use of corrugated tube can enhance heat transfer performance significantly,and the maximum value of relative heat transfer enhancement ratio is 54.9%.Moreover,its comprehensive evaluation index is also much higher than that of the circular tube.The comprehensive evaluation index of the corrugated tube can reach1.47 at best,while it is only 1.15 for circular tube.