Molecular Dynamics Simulation Of Fluid Flow And Heat Transfer And Nucleate Boiling In Microscale

With the rapid development of micro-nano technology,the fluid behavior of flow and heat transfer and phase change plays an important role in the design and application of micro-nano systems and devices.It is a key problem that restricts the conversion,storage and transfer of energy in energy power systems.At present,new problems and phenomena of heat and mass transfer of fluid have not been well revealed and applied,which is mainly related to scale micronization.Thus,further research is needed.However,it is difficult to reveal complex interactions between particles at the molecular level through experimental methods or theoretical research.Thus,the molecular dynamics method has become an effective method to explore microscale phenomena and mechanisms.Therefore,in this work,molecular dynamics simulations were conducted to study the influence of heat and mass transfer of fluid in microscale.Starting from the construction of heat transfer model of single-phase fluid in microscale,physical nature of solid-liquid temperature boundary is revealed.Coupling relationship between flow and heat transfer of single-phase fluid was further studied,and new phenomenon of fluid confined in nanochannel was explored.Based on flow and heat transfer of single-phase fluid,the behaviors of phase change of fluid were studied,analyzing the influence mechanism of nucleation of liquid in nano-confined space and nucleate boiling of thin film on surface.The results provide important information for the design,manufacturing and performance optimization of micro-nano systems and devices.In order to investigate the problem of temperature boundary at solid-liquid interface at microscale,the heat transfer of liquid film confined in microchannels with various wettability was studied.The size effect on thermal resistance and temperature jump at the solid-liquid interface was examined.The dependence of thermal resistance on microchannel height exhibits two regimes:monotonically increasing dependence for the small channel and keeping constant thermal resistance for the large channel.The size thresholds of the two regimes of the thermal resistance are both sensitive to the solid-liquid interaction strength,which decrease with the increase of solid-liquid interaction strength.Furthermore,with the increase of the microchannel height,the temperature jump at the solid-liquid interface monotonically decreases and eventually approaches to the non-jump temperature boundary on macroscopic scale.The coupling mechanism of flow and heat transfer of fluid in asymmetric wetting nanochannel was investigated.Poiseuille flow was simulated using argon as the working fluid.It is found that for pure heat conduction,positive heat flow is transferred from hot wall to cold wall,which is called positive heat transfer.However,due to viscous heating,the convective energy transferred in asymmetric wetting system always behaves temperature rise and asymmetric temperature jumps and velocity slip on the two walls.The temperature distribution and heat flow of fluid can be adjusted by velocity slip and the temperature jump at solid-liquid interface.On the one hand,lowering hot wall wettabilities steepens liquid temperature gradients and strengthens positive heat flow in nanochannel.On the other hand,the internal temperature reversal phenomenon of fluid is realized and heat flow direction is switched from positive to negative,by(1)keeping superhydrophilic hot wall but lowering cold wall wettabilities,(2)keeping super-hydrophilic-hot-wall/hydro phobic-cold-wall but increasing external forces applied to liquid,and(3)keeping super-hydrophilichot-wall/hydrophobic-cold-wall but lowering hot wall temperatures.The findings provide novel working principle for nano-devices behaving temperature sensitive nature.The phase change behavior of fluid in nano-confined space was investigated.A complete bubble nucleation regime map is established,which is controlled by surface wettability and fluid state.The nucleation modes of liquid included four regimes,a non-bubble-nucleation regime for ?>0.8psat and three regimes for ?<0.8?sat,where ?sat is the saturation liquid density.With the decrease of wall wettability,homogeneous,heterogeneous and Leidenfrost regimes consecutively occur,where two critical transition values are given as ?=0.3 and 0=0.7,respectively.For homogeneous nucleation and Leidenfrost,the phase change systems behave symmetrically.For heterogeneous nucleation,the system behaves asymmetric fluid densities to cause symmetry breaking and randomly distributed nucleation site.The nucleate boiling of thin liquid film on homogeneous wetting solid wall was simulated to investigate the influence of nucleation characteristics of thin liquid film at microscale.From the point of view of nucleation kinetics,with the increase of wall wettability and liquid film thickness,a large amount of energy of fluid in the vicinity of solid wall is accumulated rapidly,promoting the rapid occurrence of nucleate boiling.The incipient nucleation waiting time is shorten and bubble growth rate is increased.From the point of view of the difficulty of nucleation boiling,the apparent superheat for initial nucleate boiling increases with the increase of wall wettability.It is obviously different from the classical nucleation theory,where bubbles generate easily on the hydrophobic wall.However,considering the interface effect in microscale,the effective superheat for initial nucleate boiling decreases with the increase of wall wettability.As the thickness of the liquid film increases,the apparent superheat for initial nucleate boiling increases and the effective superheat decreases,so the nucleate boiling of thin liquid film at microscale behaves significant size-dependence features.Based on the microscale interface effect and the linear distribution characteristics of fluids near the solid wall,a theoretical model of microscale nucleation was established.Considering the microscale interface effect,the dependence of availability nucleation criterion on wall wettability followes the classical nucleation theory.The results clarified the difference and relationship of nucleate boiling between the microscale and macroscopic scales,the two laws are essentially unified.The phase change of thin liquid film on the heterogeneous wetting wall was studied to reveal the microscopic mechanism of residual gas on the nucleate boiling and the competition mechanism between the non-uniform wall wettability and nanostructure.The residual gas in the hydrophobic cavity promotes the occurrence of nucleate boiling.The onset temperature for the solid wall with a hydrophobic cavity is 1.09 ?/kB,which is 0.04 ?/kB lower than that with a hydrophilic cavity.For the hydrophobic cavities,the residual gas causes a piston-like effect at the vapor-liquid interface.The stronger the cavity hydrophobicity,the stronger the piston-like effect,which increases the cavity pressure and shortens the incipient nucleation waiting time.However,the dependence of bubble growth rate on the cavity hydrophobicity is almost negligible.For the hydrophilic cavity,when the contact angle of the nanocavity is greater than 18°,the smooth surface wettability plays an important role.The bubble occures firstly on the smooth surface and the incipient nucleation waiting time and bubble growth rate keeps almost unchanged with variation of wall wettability.When the contact angle of the nanocavity is less than 18°,the nanostructure played an important role.The bubble generated only in the cavity,and the incipient nucleation waiting time is shortened with the increase of wall hydrophilicity.