Research On Effects Of Interfacial Wettability And Nanofluid On The Convective Heat Transfer Mechanisms In Nanochannels

With the rapid development of micro/nano electronic technology,electronic devices are widely applied in many fields such as communications,national defense,mechanical engineering,aerospace,environmental engineering and biomedicine.Moore’s Law states that the integration level of electronic devices doubles every 18 months,and correspondingly the performance of electronic devices doubles.The increasing heat density,the high level of integration and the miniaturization are challenging the thermal management of these nanoelectronics,since that the thermal issues may endanger the performance and life span of the devices potentially.Owing to the high heat transfer performance,compact structure and easy integration packaging,micro/nanochannel cooling technology stands out from the various heat dissipation strategies.However,when the system reaches the micro/nanoscale level and the system spatial scale becomes comparable to the mean free path of the fluid atoms,the Navier-Stokes equation and the continuum hypothesis are no longer valid,and the size effects appear at the interface,which result in that the heat and mass transfer mechanisms at nanoscale deviate from those at macroscale.Therefore,it is necessary to further explore the flow and heat transfer mechanisms at nanoscale in order to further develop the micro/nanochannel cooling technology.First,this thesis investigates the effects of interfacial wettability on the fluid flow and heat transfer in nanochannels.Different from the flow and heat transfer in macrochannels,temperature jump and velocity slip appear at the interface of nanochannels.Due to the inadequate thermal and flow development in nanochannels,the temperature jump and velocity slip lengths in the entrance region are larger than those in the fully developed region.The local Nusselt number at the entrance is influenced by both the Kapitza resistance and boundary layer effect,and is closely related to the interfacial wettability,which differs from that at macroscale.With the surface wettability enhancing,the heat transfer capability and pressure drop will be raised.It is found out that the quasi-solid fluid atoms induced by the higher surface wettability act as the "phonon bridge" to promote the heat transmission between the fluid and the walls.Besides,Momentum exchange between the fluid and the walls also becomes more abundant as interfacial wettability increases.The Colburn factor j and friction factor f are adopted to conduct the comprehensive evaluation of the flow and heat transfer characteristics in nanochannels.The results demonstrate that χ＝1.00-1.75 is a favorable range to obtain the optimum convective heat transfer performance in nanochannels.Then,the effects of interfacial wettability on convective heat transfer between cold and hot fluids in nano heat exchangers are studied.By enhancing the interfacial wettability of nano heat exchangers,the heat transfer performance can be promoted due to the fact that the strong interfacial wettability causes more near-wall fluid atoms to contact with walls and take part in the heat transport at the interface.Moreover,the quasi-solid fluid layers emerge at the interface when the interfacial wettability enhances,which gives rise to the "phonon bridge"effect between the fluids and walls and improve the heat transfer.In addition,the Kapitza resistance and velocity slip play important roles in the convective heat transfer of cold and hot fluid sides.When the interfacial wettability of nano heat exchanger is weak,the convective heat transfer of the hot fluid side is superior to that of the cold fluid side because of the smaller Kapitza resistance.When the surface wettability of the nano heat exchanger is strong,the larger velocity slip leads to better heat transfer on the cold fluid side than that on the hot fluid side.Finally,this work researches the effects of nanoparticle suspension and deposition on the nanofluidic convective heat transfer using the molecular dynamics method.The random movement of nanoparticles in the based fluid can enhance disturbance the nanofluid atoms and improve their heat transfer,which result in the enhancement of nanofluid heat transfer ability.Moreover,the nanoparticles can collide with argon fluid atoms and have the self-rotation in nanofluid,which benifits the nanofluid convective heat transfer.Besides,by increasing the heat transfer area and disturbing the near-wall fluid flow,the deposited nanoparticles act as the fins and improve the local heat transfer.This work also finds that the deposited nanoparticles expend the low potential energy regions near the walls and more fluid atoms are attracted to participate in the heat transport at interface,which benefits the local interfacial heat transport.