Investigation Of Fluid Flow Heat Transfer And Phase Change Mechanisms In Nanoconfined Spaces

With the rapid development of micro/nano-electro-mechanical systems(MEMS/NEMS),the flow heat transfer and phase change at micro/nanoscale play a significant role in energy transportation,transition and storage in micro/nano systems and devices,which have received much attention from researchers in the world.However,the size effect becomes prominent when the system size reduces to the micro/nanoscale.Micro/nano flow heat transfer phenomena cannot be interpreted by the traditional flow heat transfer theories.Hence,it is urgent to investigate the flow heat transfer characteristics and phase change mechanisms at micro/nanoscale.The exploration of the flow heat transfer and phase change mechanisms at micro/nanoscale not only contributes to the innovation and application of MEMS/NEMS,but also enriches the micro/nanoscale heat transfer theory and technology.In this dissertation,the theory analysis and molecular dynamics simulations are applied to investigate the flow heat transfer and phase change mechanisms in nanoconfined spaces.The convective heat transfer characteristics at nanoscale are studied.The influence rules and intrinsic mechanisms of heat transfer enhancement and phase change in nanoconfined space are revealed.The contents and results of this dissertation can be summarized as follows:(1)Considering axial conduction and viscous dissipation,the two-dimensional energy equation is solved by a mathematical method.The energy equation and temperature jump boundary conditions are combined to calculate the dimensionless temperature and Nusselt number(Nu).Effects of axial conduction(represented by Pe),viscous dissipation(represented by Br),and rarefied effect(represented by Kn)on the convective heat transfer of the fluid in micro/nanochannels are investigated.Results show that Pe determines the impact degree of axial conduction on the convective heat transfer of the fluid in micro/nanochannels.It has a greater effect on the heat transfer when the absolute value of Br is larger in micro/nanochannels.When Kn≤0.1,the fluid flow is discontinuous and the temperature jump appears at the solidliquid interface of the channel.(2)The fractal Cantor structure,as a typical fractal geometry,is used to construct the rough surface of the nanochannel.The convective heat transfer performance in the nanochannel with the fractal Cantor structure is investigated by the molecular dynamics method.It is found that the temperature jump and velocity slip occur at the solid-liquid interface of the nanochannel.The heat transfer performance in the nanochannel with the fractal Cantor structure increases with the increase of surface wettability(χ)or fractal number(n).For larger χ,the quasi-solid layer appears near the wall,which contributes to the increase of phonon transport channels and thus promotes the convective heat transfer in nanochannels.The comprehensive convective heat transfer performance is better for the nanochannel with the fractal Cantor surface in comparison with that with the normal rough surface.When n=3 and χ=1.00,the nanochannel with the fractal Cantor surface has the best comprehensive convective heat transfer performance,which is attributed to the fact that the narrow wall gap results in the local hydrophobic effect.With the increase of roughness,the heat transfer in nanochannels increases while the flow resistance no longer increases.(3)The convective heat transfer between cold and hot fluids in nanochannels with a smooth rough surface,rectangular rough surface and triangular rough surface are investigated.respectively.Impact mechanisms of rough surfaces on the convective heat transfer between cold and hot fluids in nanochannels are studied.Results demonstrate that the temperature jump and velocity slip are observed at the wall-fluid interface for both cold and hot sides at nanoscale.For the three surfaces,with the increase of period length(P*),the convective heat transfer performance initially increases and then declines slowly.The nanochannel with the rectangular rough surface at P*=4 has the best convective heat transfer performance.The introduction of rough surfaces increases the solid-liquid heat transfer areas,which leads to the fact that more near-wall fluid atoms act as "phonon bridge" to enhance heat transfer performance in nanochannels with rough surfaces.Meanwhile,the expansion of heat transfer areas caused by rough surfaces facilitates more near-wall fluid atoms to gather at low potential regions to participate in the interfacial heat transfer in nanochannels.(4)The flow condensation heat transfer characteristics of nanochannels with different nanopillar cross-sectional areas and heights are studied.Results manifest that the condensation performance in nanochannels can be promoted by adding nanopillars.With the increase in nanopillar cross-sectional areas or heights,the condensation performance enhances.In fact,these nanopillars act as fins.which not only increase heat transfer areas,but also interrupt and redevelop thermal boundary layers.However,the time that the fluid spends to reach stability will be put off when nanopillars are added in the nanochannel.In comparison with heat transfer rates in the smooth nanochannel,the heat transfer rates increase 11.6%～35.8%at a higher nanopillar height,which improves more significantly than other cases with nanopillars(1.7%～4.1%).The preeminent condensation heat transfer performance is ascribed to the fact that nanopillars with a higher height disturb the vapor region and thus allow vapor atoms with strong Brownian motion to collide with nanopillar atoms directly,which strongly boosts the condensation heat transfer in the nanochannel.(5)The evaporation and boiling characteristics of the liquid film in nanoconfined space are explored by vibration-induced vaporization of nano liquid film.According to the vaporization characteristics during the vibration,the acoustothermal evaporization mode can be divided into evaporation,nuclear boiling and film boiling.Under the same amplitude and vibration frequency,the smooth surface and rough surface have a similar vaporization mode,while both the normalized atomization(N*)and film temperature(Tf)of rough surfaces are smaller than those of the smooth surface.With the decrease of rough structure size,both N*and Tf reduce,which indicates that the adding of a rough surface suppresses the vibration-induced liquid film evaporation and boiling performance.It is further discovered that the rough surface reduces the surface wettability and thus weakens the effect of vibration on the solid-liquid heat transfer.(6)Combined investigations of convective heat transfer in nanochannels and thermal deicing,dynamic processes of ice melting at nanoscale are studied.During the thermal de-icing on a nanochannel,the ice with a hexagonal network structure is gradually melted from the ice mantle to the core.The ice temperature and the ice energy increase while the average number of hydrogen bonds decreases with the change of heating time.The ice melting time can be calculated by the ice temperature and the average number of hydrogen bonds.In the nanochannel,a large number of fluid atoms are trapped in low potential energy regions to participate in the interfacial heat transfer,which is conducive to the rapid heat transfer between the ice and hot fluids of the nanochannel.The increase of hot thermostat temperature contributes to the rapid ice melting,but the hot thermostat temperature has little effect on the melting time once it reaches a high value.In addition,the stronger surface wettability between the ice and nanochannel reduces the ice melting time,which is also beneficial to the thermal de-icing on a nanochannel.In summary,focusing on the flow heat transfer and phase change in nanoconfined spaces,this dissertation reveals convective heat transfer mechanisms in nanochannels and elucidates effects of rough surfaces on flow and heat transfer at nanoscale.In addition,a new method of vibration-induced vaporization of nano liquid film is proposed and the effect of convective heat transfer mechanism in the nanochannel on the dynamic process of the ice melting is analyzed.This dissertation will provide significant theoretical supports for research fields of micro/nano electronic devices,MEMS/NEMS technology,thermal de-icing,etc.