Study On Magnetic Nanofluids Forced Convection Heat Transfer And Particle Fouling Characteristics

As a new type of working medium,magnetic nanofluids are one of the possible ways to achieve energy saving and promote"carbon neutralization",which have aroused widespread concern in the field of heat transfer.In order to comprehensively discuss the feasibility of the application of magnetic nanofluids in heat exchange system,based on Fe₃O₄-H₂O-AG magnetic nanofluids,the thermo-hydraulic properties and particle fouling properties were explored in detail through forced convection heat transfer experiments and particle fouling deposition experiments.The main contents are as follows:first of all,for the forced convection heat transfer experiment,the effects of magnetic field form,magnetic flux density and other variables on the thermo-hydraulic properties of the working fluids were studied.Meanwhile,the thermo-hydraulic comprehensive characteristics of the system were evaluated based on the first and second laws of thermodynamics.Secondly,for the particle fouling deposition experiment,the effects of variables such as magnetic field form and magnetic flux density on the fouling performance of working fluid particles were mainly revealed,and analyzed by"qualitative"and"quantitative"methods.The anti-fouling rateηwas introduced to evaluate the scale inhibition properties.The conclusions are as follows:（1）Thermal-hydraulic properties of magnetic fluids excited by magnetic field:the vertical field is positively correlated with the enhancement of heat transfer effect,while the horizontal field is on the contrary,and the parallel or vertical field will adversely affect the flow resistance.Based on the comprehensive analysis,it was found that the fully developed turbulence,high mass fraction and high magnetic flux density vertical field correspond to excellent thermo-hydraulic characteristics.When each variable reaches the best value,the Nu is increased by 50.2%,while f is only increased by 20.5%,and the corresponding R3 index is 2.896.（2）Thermal-hydraulic properties of working fluid under the influence of twisted turbulator:with the increase of perforated ratioξ,Nu_e/Nu₀ and f_e/f₀ will increase slightly in the initial stage,and begin to decrease after reaching the peak value.Whenξ=100%,the ratio returns to 1.0.With the change of Re,theξcorresponding to（Nu_e/Nu₀）_max migrates,but theξcorresponding to（f_e/f₀）_max does not.Meanwhile,through comprehensive analysis,it was found that the perforated ratioξcorresponding to the maximum heat exchange intensity or maximum energy utilization efficiency is not the same,which should be paid more attention to when selecting twisted turbulator.（3）Particle fouling deposition properties of magnetic working fluids:high Reynolds number prolongs the induction period of fouling deposition and reduces the asymptotic value of thermal resistance in the stable period,which has an excellent effect on restraining the formation of particle fouling.The increase of mass fractionωwill lead to the augment of particle deposition rate,but its effect on the enhanced heat transfer capacity of working fluid is much higher than that on fouling characteristics.（4）The properties of particle fouling deposition under the excitation of magnetic field:under the vertical field,the induction period of fouling deposition will be shortened,the asymptotic value of thermal resistance in the stable period will increase,andηwill be lower,showing poor particle fouling properties,and with the increase of B,the fouling properties also worsen,while the parallel field shows better particle fouling characteristics（η>1）.（5）Particle fouling deposition properties under the influence of twisted turbulator:the turbulator has an obvious effect on improving the scale inhibition properties of working fluids.Under the action of the twisted turbulator,the corresponding working conditions of the magnetic field have better properties.For the vertical field,the negative effect of particle deposition can be reduced,while for the parallel field,it can strengthen the effect of restraining deposition.（6）The vertical magnetic field has better enhanced heat transfer characteristics,but its scale inhibition properties are lacking,but the addition of twisted turbulator can not only enhance heat transfer,but also strengthen its scale inhibition properties;parallel magnetic field has good scale inhibition characteristics,but the enhanced heat transfer capacity is slightly insufficient,while the addition of twisted turbulator can not only greatly improve the enhanced heat transfer characteristics,but also optimize the scale inhibition characteristics at this time.In a word,the coupling adoption of magnetic field and twisted turbulator makes it possible for Fe₃O₄-H₂O-AG magnetic nanofluids to be widely used in heat transfer system,which provides a new idea for the design of magnetic nanofluids heat transfer system.