Nanoscale Heat And Mass Transport Of Liquid-Solid Suspensions And Mixture Fluids

In nanoscales,a number of forces that can be neglected in macroscales play an important role in fluid flows,causing the fluids exhibit many special behaviors,e.g.vastly increased viscosity near walls.Accordingly,nanofluidics was emerged,which has a rapid development in the past decade.For fluids containing nanometer-sized matters or fluid confined in a nanoscale space,the role of volume forces is weakened while the role of surface forces is strengthened due to numerous new liquid-solid interfaces.Consequently,the transport characteristics of fluids have a great dependence on the interface characteristics and display many distinctive behaviors,such as anisotropy and space size dependence.This dissertation aims to investigate the heat and mass transport characteristics of liquid-solid suspensions and mixture fluids in nanoscales.The results can not only provide theoretical guidelines for the engineering application of new working fluids and the design and manufacture of nanofluidics devices,but also promote the applications of high-efficiency heat transfer technologies and nanofluidics technologies in the fields of electronics cooling,gas separation,and hydrogen storage etc.Firstly,the heat transport characteristics of nanofluids in shear flows are investigated using an established rotating Couette apparatus and a proposed molecular dynamics?MD?model.Then,the heat transport characteristics of nanofluids and binary mixture fluids confined in nanochannels are studied using MD simulation method.Finally,the mass transport characteristics of mixture gases in graphene nanopores are analyzed using MD simulation and theoretical method.The main conclusions are as follows:The enhancement mechanism of the effective thermal conductivity?ETC?of nanofluids in shear flows is found,the influence law of various factors and a ETC prediction correlation are obtained,and finally the effect of nano-sized particles on the heat transport characteristics of liquid-solid suspensions is revealed.For the flow with shear rates lower than a threshold value,the ETC increases with increasing shear rate;for the flow with shear rates higher than the threshold value,the ETC displays a plateau value.The special relationship between ETC and shear rate is related to the gradually disappearance ofnanoparticle agglomerates in shear flows.The increase of ETC with shear rate is more obvious as the nanoparticle diameter and volume fraction increase;the increase is weakened as the temperature increases and accordingly the ratio of infinite-shear thermal conductivity to zero-shear thermal conductivity decreases,because the nanoparticle agglomerates are hard to form at higher temperatures.It is found that the effect of nano-sized particles on the heat transport characteristics of liquid-solid suspensions is very obvious and the conventional ETC prediction correlation for ordinary liquid-solid suspensions containing micro-sized particles is not suitable for nanofluids either in qualitative or quantitative terms.Based on the MD simulation,a directional rotation of nanoparticle is observed,which promotes the relative motion of fluid atoms surrounding the nanoparticle and thus enhances the heat transport of nanofluids.The anisotropy and increase of heat transport in nanofluids and binary mixture fluids confined in nanochannels are found,the underlying mechanisms and influence law are also obtained,and finally the effect of nano-size on the heat transport characteristics of the two fluids in nanochannels is revealed.Comparing to pure fluids,the nanofluids exhibit totally different flow behaviors in nanochannels,e.g.fluctuations in the number density distribution both near the walls and in the center of the channel,and non-linear velocity profiles in Couette flow.The heat transport of nanofluids in nanochannels is obviously anisotropic,i.e.the thermal conductivity in the height direction of the channel is much smaller than that in the direction parallel the channel wall,which is caused by the weak motion of fluid atoms in the height direction.It is concluded that the interaction between wall atoms and fluid atoms?especially particle atoms?is the main reason for the anomalous enhancement of the thermal conductivity in the direction parallel the channel wall comparing to macroscales.The thermal conductivity increases with decreasing channel height or nanoparticle diameter?volume fraction?.Similar to nanofluids,the heat transport of Ar-Kr binary mixture fluids in nanochannel is also anisotropic,and the thermal conductivity in the direction parallel the channel wall is higher than its macroscale value.The thermal conductivity of mixture fluids is also related to the volume fraction of Kr atoms.The higher the volume fraction is,the higher the thermal conductivity is.In the Poiseuille flow driven by an external force,the thermal conductivity in the flow direction is further increased.The permeations of four different gas molecules?H_e,H₂,N₂ and CH₄?through graphene nanopores are investigated thoroughly,two mechanisms of permeation are detected and corresponding theoretical models are created,and finally the effect of nano-size on the mass transport characteristics of mixture gases in nanopores is revealed.The permeation flux increases with increasing the size of graphene nanopores,which is related to molecular size and the intensity of molecular adsorption onto graphene surface.Direct flux and surface flux are identified and their relative contributions are quantified.It is found that for gases that adsorb onto the graphene?e.g.N₂ and CH₄?,significant contribution to the flux across the graphene comes from the surface mechanism,by which molecules cross after being adsorbedonto the graphene surface.The theoretical models for direct flux and surface flux are created,respectively;the direct flux can be described accurately using kinetic theory of ideal gas;the surface flux can be predicted based on the surface diffusion equation and Fick law.Meanwhile,molecular permeation is strongly affected by pore functionalization,of which the influence mechanism is also obtained.