The Effect Of Magnetic Field On Particle Sorting And Heat Transfer In Microchannels

With the rapid development of microfluidic systems,the mass and heat transfer of nanofluids in microchannels have drawn increasing concerns in various domains including bioengineering,materials and environmental engineering.In terms of cells,viruses and drugs sieving,the sorting of fine particles in microchannels will be an extraordinary research topic.Meanwhile,it is quite promising for the application of nanofluids due to their diverse merits,examples include elevated heat exchanging effect,miniaturization and so forth.Firstly,the particle sorting in microchannels by domestic and foreign scholars is summarized in detail.The advantages and disadvantages of different types of sorters were analyzed from the aspects of active and passive sorters.It is found that the researches on the separator with similar particle size are not mature enough and needs further exploration.Besides,the studies on the separation of non-magnetic particles or active cells are not comprehensive.Some field separation procedures are complex and may possess inactivation rate.Thus,negative magnetic separation technology is required,especially the combination of negative magnetic separation technology and inertia separation technology,which could achieve better separation effect.Regarding to nano-magnetic fluid heat transfer,the influence of magnetic field distribution on fluid heat transfer is not enough,which needs to be summarized and investigated.Secondly,a series of basic equations of fluid mechanics,brief analyzes of used conditions and numerical simulation methods of microfluidic equations,methods and boundary conditions of the two-phase flow model were introduced in this thesis.Ultimately,the force of particles in the flow field was analyzed to provide theoretical basis for the later research.Then,the negative magnetic swimming particle sorter was simulated by numerical simulation software,as well as the influence of magnets number,magnetic pole selection,magnetic field strength,inlet flow rate and inlet flow rate ratio on particle sorting were analyzed.In order to further improve sorting effect of the particles,contraction expansion channels of negative magnetophoresis and inertial coupling were designed,and the effects of magnetic field strength and shrinkage expansion ratio on the sorting were analyzed.Finally,the negative magnetic swimming inertial coupling sorting chip was subjected to PIV experiment and compared with the numerical simulation results.As a result,3 magnets arrangement with NSN performed the highest efficacy;In the presence of a non-uniform magnetic field,larger strength of magnetic field gave rise to greater lateral offset distances,larger size and deflection distance of the non-magnetic particles.Considering actual situation and size of the microchannel,magnetic field with about 0.7×105 A/m is reasonable;As the inlet flow rate decreases,the separation effect of non-magnetic particles increases,but relatively small sample flow rate is adverse to productivity;As the flow ratio increases,the particle bandwidth becomes narrower and the band spacing increases.Considering the sample flow rate comprehensively to make bandwidth small and spacing between the strips large adequately,the inlet flow rate ratio was determined to be about 4;Shrinkage-expansion use in the sorting channel is beneficial to increase the lateral migration distance of the particles,the spacing of the particle bands was significantly increased with the reduction of shrinkage-expansion ratio.Therefore,0.5 of shrinkage-expansion ratio is reasonable in view of the pressure drop.Finally,the numerical simulation of magnetic field effect on heat exchanging of magnetic fluid in the microchannel,NSN and NNN arrangement,single-side arrangement and double-side arrangement of three magnets,upper and lower heteropolar and upper and lower homopolar on local heat transfer coefficient in microchannel were analyzed.In summary,the local heat exchanging effect of magnetic fluid can be improved under the action of magnetic field;The magnets arranged in two sides of NSN have better heat transfer effect on magnetic fluid,and the local heat exchanging produced by the arrangement of upper and lower poles is more stable;The staggered magnetic field arrangement expands the heat exchanging area without changing the trend of overall heat exchanging coefficient compared with the corresponding magnetic field.