Experimental And Numerical Study Of The Heat Transfer Characteristics Of Nanofluids Under High Electric Field

The heat transfer caused by working fluid exist in various industrial process.For instance,power,metallurgy,petroleum,chemical engineering,aviation,electronics,nuclear energy,etc.,in which heat transfer is very important.Therefore,improving the heat transfer characteristics of working fluid and studying a new active heat transfer enhancing technology are two important development directions.Among the numerous heat transfer enhancement technologies,The nanofluid and EHD?electrohydrodynamics?are two of the most promising fields.In order to study the technical means to achieve this purpose,experimental and numerical simulation study has been carried on the nanofluid,EHD and the combination of the two technology.The multiphase models have been employed to simulate the forced convection and natural convection heat transfer of nanofluids.Specifically,the mixture model and Eulerian model have been applied to study the effects of nanoparticle concentration,Reynolds number and Rayleigh number on the Nusselt number and resistance coefficient,and the calculated values have been compared to the experimental results.The simulation work have indicated the good applicability of the multiphase models to heat transfer of nanofluids,and from macro perspective,the nanofluid used in the process of forced convection and natural convection heat transfer did not show a strong nano effect.Two step method has been used to prepare Al₂O₃-transformer oil nanofluid and nano suspension with different mass concentrations.The scale and morphology characteristics and physical properties have been studied.Moreover,the thermal conductivity measurement device under electric field has been built to study the thermal conductivity enhancement values of nanofluids to pure fluid with different electric field,temperature and concentration.Besides,the thermal conductivity model of nanfluid under electric field have been built.The experimental and calculated results show that the electric field enhance the equivalent thermal conductivity of nanofluid greatly,and the enhancement effect goes stronger with the increase of voltage,concentration and temperature.In addition,the calculated values of equivalent thermal conductivity by the model show a good agreement of the experimental results.Experimental device of heat transfer in the closed enclosure under electric field have been designed and built,which is used to study the effect of electric field,electrode,nanoparticle concentration and heat flux on the heat transfer characteristics of pure working fluid,nanofluid and nano suspension.The experimental results indicate that the electric filed can enhance the electro convection heat transfer of pure transformer oil,and the enhancement effect goes stronger with the increase of voltage.However,the electric field can deteriorate the natural convection heat transfer of pure transformer oil at first,and weakened with the increase of voltage,and has a weak enhancement at last.Furthermore,the electric field also has a strong enhancement effect on the heat transfer of nanofluid and nano suspension,which have a better performance with the increase of voltage,concentration and temperature.While,the nano suspension has a best concentration of nanoparticle in heat transfer process.The lattice Boltzmann method has been conducted to build the physical model coupled electric field,flow field and temperature field.In addition,the thermal conductivity of nanofluid under electric field has been employed to furthermore research applicability of the thermal conductivity model to predict the heat transfer characteristic of nanofluid in enclosure under electric field.Specifically,the effects of voltage on the electric convection and mixture convection heat transfer in enclosure have been studied,which include the fluids of pure working fluid and nanofluid.Besides,the effects of voltage,heat flux and nanoparticle mass concentration on heat transfer enhancement factor have also been discussed.Moreover,the relative influences of electrophoretic force,dielectrophoretic force and electrostrictive force on flow and heat transfer of pure working fluid have been studied.Furthermore,the electrophoretic force and dielectrophoretic force on nanoparticle in basefluid have been compared and discussed.Lastly,Form the microscopic perspective,motion of nanoparticle in basedfluid has been calculated,and discussed the effects of electric field,nanoparticle diameter,Zeta potential,temperature on the motion of nanoparticle in basedfluid.The numerical values indicate that the present electric field,flow field,temperature coupled model can simulate the heat transfer of pure transformer oil in the enclosure under electric field well.Besides,the results discussion show that the electric field break the flow looping mechanism of natural convection in the closed enclosure,which result in the deterioration effect.For the nanofluid,the electric field,flow field,temperature coupled model and the model of equivalent thermal conductivity of nanfluid under electric field can also predict the heat transfer of nanofluid in the enclosure.Moreover,the calculated results show that the electrophoretic force on nanoparticle dominate the most important effect.Besides,the motion of nanoparticle in the basefluid is enhanced with the increase of voltage,Zeta potential,and temperature,and decrease of diameter of nanoparticle.