Investigation On The Heat Transfer Of Nanofluids And The Sorting Of Fine Particle In Microchannels

Nanofluids and the separation of fine particles in suspension belong to the category of particle flow.The problem of particle mass transfer and heat transfer in microchannels exists widely in pharmaceutical.biological,food,energy,chemical and other fields,and has become a research hotspot in recent years.In terms of energy,microchannel heat sink with traditional cooling medium can no longer meet the requirement of high intensity heat dissipation,so it is necessary to develop microchannel heat sink with nanofluid cooling.In drug screening,the separation of fine particles in microchannels has a good application prospect.Firstly,the thermal conductivity mechanism of nanofluids,the application of nanofluids in the heat sink of micro-channels and the research methods are summarized.It is found that there are not enough mechanisms and influencing factors related to the enhancement of heat transfer by nanofluids,and the studies on the flow and heat transfer laws in microchannels are not in-depth enough,which still need to be further explored and studied.In addition,the microchannel particle separation technology is summarized.It is found that the single separation technology can not meet the requirements for the fine particles with the particle size of 0.5-5?m.coupling thermophoretic separation and inertial separation are needed.Secondly,basic equations,boundary conditions,flow states and influencing factors of micro-scale flow field are introduced to understand theoretical knowledge of micro-scale flow field.The two-phase flow model for the separation of nanofluids and particles is introduced in detail,and the method and process of the whole numerical simulation process are described.Then,after considering the interactions between temperature,viscosity,concentration.Brownian and thermophoretic motion,an inhomogeneous two-phase model was established to study the mass and heat transfer of Al2O3-water nanofluids as coolants in the heat sink of micro-channels.Through numerical simulation,the mechanism and rules of flow heat transfer of nanofluids in micro-channel heat sink are obtained.The results showed that:in the heterogeneous two-phase model,the heat transfer performance of nanofluids in the micro-channel heat sink was better than that of the single-phase model,and when the particle size of nanofluids was less than 20nm.the main strengthening heat transfer mechanism was Brownian motion,while when the particle size was larger than 20nm.the main heat transfer mechanism was thermophoresis;The mechanism of heat transfer enhancement caused by Reynolds number,mass fraction of nanofluid and particle size can be explained by NBT.On this basis,the optimal volume fraction and particle size can be selected.In addition,trapezoidal groove structure can destroy the boundary layer,block the full development of heat and enhance heat transfer.In terms of micro-channel heat sink structure,the optimal nanofluid was selected to optimize the position of micro-channel heat sink inlet and outlet as well as the length,width and shape of trapezoidal groove,It was found that when L1/L2=0.75,the heat transfer effect of trapezoidal groove heat sink was the worst,which should be avoided in the design.The optimal trapezoidal groove width ratio is W1/W2=1.4.Finally,The force state of particles in suspension was analyzed,and the force which had great influence on particle separation was obtained.In order to separate the fine particles of different sizes,two separators,thermophoretic separators and thermophoretic coupling separators,were designed.Numerical simulation of particle separation efficiency,movement trajectory and lateral displacement of thermophoretic separators under different wall temperature difference,inlet velocity ratio and inlet velocity ratio show that small inlet velocity ratio.large inlet velocity ratio and large temperature difference are conducive to particle separation.In terms of thermophoretic coupling separation law.although there are shrinkage and expansion regions,particle capture is not realized and the effect of inertial force is minimal.However,the temperature gradient can be changed in the shrink-expansion region,and with the decrease of the shrink-expansion ratio Ha/Hb,the temperature gradient gradually increases,thus improving the sorting effect.In addition,a MicroPIV system experimental platform was built to conduct experimental research on the thermophoretic coupling separator,so as to verify the correctness of the numerical simulation rules of the thermophoretic coupling separator.