Magnetic Field And Hydrophobic Surface Improve The Convective Heat Transfer And Flow Resistance Of Nanofluids

With the developing of energy conversion equipment towards high power and integration,the demand for thermal management with high standard is more urgent than ever before.In the cooling technology of thermal equipment,convective heat transfer has the widest application.Choosing nanofluid with better heat transfer to replace traditional cooling medium can increase convective heat transfer coefficient only from the fluid side without changing the original upstream equipment design,which has been widely confirmed.It has been proved that the convective heat transfer can be improved by using nanofluid with a relatively high nanoparticle concentration.However,it is not advisable to blindly increase the particle concentration,since an extremely high particle concentration leads to sharp increase in fluid viscosity and flow resistance,aggravation of particles agglomeration,and even excessive deposition on heat transfer surface.So far,there is no relatively effective scientific practice and theory on how to further increase the convective heat transfer of nanofluid within a reasonable particle concentration and ensure that the flow resistance is controllable.For the above two core problems that prevent the wide application of nanofluid,this paper provides two feasible solutions.One idea is to match an external magnetic field for the magnetic nanofluid to further increase the convective heat transfer,the other is to combinate the advantages of nanofluid and hydrophobic surface to reduce flow resistance on the basis of increasing heat transfer.Guided by those two ideas,this paper provides specific scientific practices in theoretical analysis,experiment and numerical simulation,and enriches the nanofluid science theory.The main research contents and conclusions are summarized as follows:（1）Fe₃O₄/DI-water and SiO₂/DI-water nanofluids are prepared by a two-step dispersion method and its transport parameters are tested.The Zeta potential for the former is+39 Mv.The micro morphology test results show a good dispersion.The specific heat capacity,thermal conductivity and dynamic viscosity involving convective heat transfer are measured by experimental method,which provides necessary thermophysical parameters for the continuous phase setting in numerical simulation.A hysteresis loop of Fe₃O₄/DI water magnetic nanofluid with 2.00%concentration is measured,which prove that the magnetic nanofluid with particle size of 20 nm shows superparamagnetism.Combined with the actual size of magnetic domain,it is analyzed that the nano scale is the cause of superparamagnetism.It is predicted that the small temperature change at room temperature has no effect on magnetic nanofluid suspension stability,saturation susceptibility and magnetic moment.The effects of gravity,Brownian force and Kelvin force on particles suspension stability are evaluated,the results show that the Kelvin force has no effect on nanofluids suspension stability.（2）The forced convection and flow resistance of magnetic nanofluid under magnetic field control is studied experimentally.It is found that compared with the two-sided opposite and one-sided single row layouts,the static magnetic field with staggered on both sides layout can maximize the convective heat transfer and ensure a relative low flow resistance.For example,under this layout with Re=2400 and B=4320 Gs,the average convective heat transfer coefficient of magnetic nanofluid（?=2.00%）are increased by 6.8%and 13.4%compared with no magnetic condition and deionized water,respectively.Due to the induction of magnetic field,the nanoparticle micro motion will migrate from the low temperature gradient central region to high temperature gradient near wall region.Moreover,If the magnetic field keeps changing,the heat transfer can be further strengthened.In addition,a uniform speed approaching dynamic magnetic field is matched to the system,the dynamic increase and finally stable variation of local convective heat transfer coefficient are observed when the magnetic field is from weak to strong.When the magnetic field intensity reaches to and keep at peak,the local convective heat transfer coefficient also reaches to peak and finally tends to be stable.It is confirmed that the forced convective heat transfer of magnetic nanofluid can be indirectly controlled by altering magnetic field intensity and spatial distribution.（3）Forced convection and flow resistance of magnetic nanofluid under magnetic field concrol are numerical simulated.Based on the Euler-Lagrange multiphase flow model and motion characteristics of discrete and continuous phase,the mechanism of magnetic nanofield controlling convective heat transfer is discussed.The steady-state results show that radial velocity fluctuation,interphase velocity slip and particle aggregation near the wall are the main reasons for convective heat transfer enhancement in low Reynolds number flow.The radial pulsation of nanoparticles,velocity slip between two phases and vorticity variation near the wall are the mains reasons for convective heat transfer enhancement in high Reynolds number flow.In addition,finely divided vortex near the wall is a relatively ideal state for convection,which can be used as an evaluation criterion to judge the matching between fluid velocity and magnetic field strength.The instantaneous results show that external magnetic field can broaden fluid fluctuating velocity range,improve fluctuating velocity intensity,enhance fluid turbulent diffusion and improve vorticity characteristics in the main flow region.（4）The experimental and numerical simulation results show that nanofluid cooperating with hydrophobic surface can increase heat transfer and reduce flow resistance.A micro/nano scale plate-like silver crystal structure is constructed on the copper tube inner surface by controlling the silver nitrate solution concentration and reaction time,and then coated with0.015 mol/L benzoic acid to prepare a contact angle 139°hydrophobic copper tube.The results of Si O₂/DI-water nanofluid cooperating with hydrophobic surface show that heat transfer increase is not obvious in laminar flow but is prominent in turbulent flow.Moreover,the heat transfer and flow resistance can be optimized at the same time when the nanofluid concentration is greater than 1%.For example,heat transfer coefficient increases by 18.1%,the flow resistance decreases by 4.9%and Webb coefficient i _E is 1.20 when the nanofluid with concentration of 2.0%.Then,based on the flow characteristics,the heat transfer enhancement mechanism is discussed by numerical simulation as follows:1)The viscous effect of hydrophobic surface is weak,so that nanofluid can flow through the near wall region at a faster speed.2)Hydrophobic surface overcomes the lift effect and prevents the migration of nanoparticles from the high shear near wall region to low shear main flow region.The mechanism for flow resistance reduction are summarized as follows:1)The air captured by the hydrophobic surface micro/nano structure forms a lubricating layer between the surface and nanofluid,and making the nanofluid flow the top of the air layer in an approximate"sliding"state.2)The surface energy of crystalline benzoic acid is low and not enough to destroy contacting nanofluid surface tension.