Effect Of External Magnetic Field On Heat Transfer Of Nanofluids In Microchannels

With the rapid development of microfluidic systems,the enhanced heat transfer of nanofluids in microchannels has received strong attention from researchers at home and abroad in the fields of biology,aerospace,automotive and optoelectronics.Due to the continuous miniaturization of power electronic equipment,how to effectively solve the heat dissipation per unit surface area has become an urgent problem to be solved..At the same time,existing research results show that under the action of an external magnetic field,the heat transfer enhancement effect can be further improved.However,the mechanism of heat transfer enhancement is not clear after applying the magnetic field,and the best heat transfer effect cannot be achieved by setting the arrangement and size of the magnetic field reasonably.Therefore,under the external magnetic field,the heat transfer to the nanofluid in the microchannel will become a good research topic.Firstly,the research status of nanofluids is reviewed,including stability,viscosity,thermal conductivity of nanofluid and heat transfer enhancement by nanofluid.On the basis of the available data published,it is found that the smaller the nanoparticle size,the larger the volume fraction,and the greater the mass fraction of the dispersant,the viscosity of the nanofluid will be greater.The viscosity of nanofluids has a significant effect on heat transfer.However,most of the experimental researches on heat transfer by nanofluid are carried out in a macro-scale tubes,in which turbulent flow conditions are available.Considering the small size of the heat dissipation chip in electronic devices,laminar flow should be taken into account in the researches on the heat transfer enhancement by nanofluids in a microfluidic chip for cooling.Such research results is very rare,and further discussion and analysis are needed.Secondly,a series of basic equations of fluid flow are briefly introduced,the mechanical equations and numerical simulation methods of microfluid mechanics are briefly described,and several large models of flow are also introduced.And introduce the method and boundary conditions of the model established in this paper.Finally,the force of particles in the fluid is analyzed,which lays the foundation for the following research.Then,the numerical simulation of the heat transfer of the magnetic nanofluid in the microchannel is studied.Considering that the size of the microchannel is very small,the effect of the laminar flow field on the heat transfer is mainly considered.By establishing a reasonable and effective numerical model and The numerical simulation method analyzes the influence of the presence or absence of a magnetic field and different magnet arrangements on the heat transfer of the magnetic fluid.Obtained the magnetic flux density maps,flow field and temperature field distribution characteristics under different arrangements,and also introduced the changes of the lower wall temperature and the volume force in the x and y directions under different magnetic field strengths.Then,the influence of the heat transfer of the nanofluid in the microchannel under the single-sided magnetic field arrangement,the double-sided corresponding magnetic field arrangement,and the double-sided staggered magnetic field arrangement was analyzed.The conclusion is that the double-sided staggered magnetic field has the best heat transfer effect on the nanofluid,and the heat transfer effect under the single-sided magnetic field is the least obvious.This is because under the bilateral magnetic field,the nanoparticles can move in two directions,and the disturbance generated is more turbulent.The magnetic field area under the interlaced magnetic field is enlarged,so the effect is more obvious.Finally,on the basis of Chapter 3,the influence of factors such as magnet poles,magnetic field strength,magnet spacing,and nanofluid volume fraction on the heat transfer of nanofluids is analyzed.Numerical simulations have found that when the temperature range is in the low temperature zone,the magnetic force received by the nanoparticles is several orders of magnitude larger than the Brownian motion force and the thermophoretic force,so the Brownian force and the thermophoretic force received at this time can be ignored.Only need to consider the effect of the magnetic field.When the magnetic poles are arranged as NSN poles,the magnet spacing is 0.02 mm,and the heat transfer effect of the nanofluid is better.When the Reynolds number is between 0.1 and10,the heat transfer effect of the nanofluid is enhanced with the increase of the magnetic field strength and the volume fraction of the nanofluid.But when B=0.05 T and the volume fraction of nanofluid is 3%,the heat transfer effect of nanofluid is the most obvious.