Molecular Dynamics Study On Wettability And Mechanism Of Boiling Heat Transfer Enhancement In Nanofluids

With the gradual miniaturization and compactness of the size of micro/nano electronic devices,and the soaring heat flow density,solving the heat dissipation and cooling problem of micro/nano electromechanical systems has become the hot issue in heat transfer enhancement fields.Boiling phase transition is a highly efficient heat exchange method with high heat transfer coefficient and small heat transfer temperature difference,so it has received widespread attention.With the development of micro/nanoscale heat transfer research,nanofluids have been widely used to enhance heat transfer due to their superior thermal conductivity compared to the basic fluid,and the phase change heat of nanofluids has also been widely used in the thermal management of micro/nano electronic devices.The heat transfer process of fluids at the microscale is closely related to the wetting properties of nanofluids.There are interactions between nanoparticles and base fluid molecules,and between nanoparticles and solid walls.Nanofluids have more unique wetting properties than base fluids.Nanoparticles can change the physical properties of the nanofluid,such as viscosity,rheological properties and surface tension,etc.,so that the equilibrium contact angle of the nanofluid on the wall surface,the evaporation mode and other wetting properties are changed,thereby changing the wetting of the nanofluid.mechanism.In addition,due to the solid-liquid interface interaction force,surface roughness,and the hydrophilic and hydrophobic characteristics will inevitably affect the wetting characteristics of the droplets,which in turn affects the boiling phase transition process of nanofluids on the solid wall.Therefore,it is of great significance to study the wetting and heat transfer characteristics between nanofluids and solid walls to obtain the microscopic mechanism for further strengthening the cooling of microelectronics.The boiling phase transition of micro/nano equipment occurs on the micro/nano scale,where the randomness and influencing factors of boiling bubbles are uncertain,and the boiling heat transfer mechanism is more complicated.Therefore,it is necessary to analyze the bubble nucleation and kinetics.Traditional macroscopic experiments mostly rely on speculation and analysis of the mechanism of nanofluid influencing boiling heat transfer,which lacks theoretical basis.Molecular Dynamics simulation(MD)method accurately describes and analyzes the respective properties and structural characteristics of nanofluids and solid walls from a microscopic point of view,and studied the influencing factors one by one,effectively revealing the wetting and boiling heat transfer characteristics of nanofluids.In this paper,the MD method is used to study the spreading and micro-dynamic wetting mechanism of nanofluids on rough and smooth walls,and analyze the effects of nanoparticle and wall wettability,nanoparticle volume fraction,and wall roughness on the wetting mechanism of nanofluids.On this basis,the boiling heat transfer characteristics of the nanofluid liquid film were further analyzed,the vaporization phase transition behavior of the nanofluid liquid film on the metal copper wall and the graphene membrane is studied.The main research is as follows:(1)Considering the two wall cases(fence-shaped rough wall and smooth wall),the influence of nanoparticle surface wettability,volume fraction and wall wettability on dynamic spreading characteristics of nanofluidic droplets on solid walls were investigated.As the nanoparticle changes the viscosity and gas-liquid surface tension of the nanofluid,the equilibrium contact angle of the droplet,the evaporation mode and other wetting mechanisms are changed.On the microscopic scale,the surface tension,viscous force and separation pressure of the air-liquid interface become the dominant forces in the dynamic wetting process.Based on the scaling rate analysis of the dynamic spreading radius of the droplet with the spreading time,it is revealed that on the rough wall surface,increasing the hydrophilicity of the nanoparticle surface and the nanoparticle volume fraction will inhibit the wetting and spreading of the droplet,and the spreading mechanism will change,that is,it is dominated by surface tension and separation pressure to be dominated by viscous force.It provides a basis for actively regulating the physical and mechanical properties of nanofluid dynamic wetting.(2)Under pool boiling state,the effect of nanofluid thick liquid films on boiling heat transfer is studied.The influence of nanoparticle size,nanoparticle and wall wettability on boiling heat transfer was investigated.The results show that,compared with the basic fluid,the nanoparticle shortens the boiling start time,advances the bubble nucleation time,the nanofluid has a greater heat flux density,and the hydrophilic nanoparticle is easily adsorbed on the near wall surface.After being heated by the wall surface,the increase The temperature and heat flux density of the molecular layer near the particles enhance the boiling heat transfer.The strengthening effect increases as the wettability of the particles increases and the particle size decreases.The potential energy of the solid-liquid interaction on the hydrophobic wall is weak,and the attraction of the wall to the water molecules is small,and the water molecules near the wall are loosely distributed,which leads to an increase in the contact thermal resistance of the solid-liquid interface and reduces the heat transfer between the solid-liquid.The solid-liquid interaction potential of the hydrophilic wall is strong,and the molecules near the wall are arranged closely,which decrease the resistance of the solid-liquid interface.Therefore,the hydrophilic wall is more conducive to the energy transfer between solid and liquid,and the nanofluid absorbs more energy from the hydrophilic wall,and the boiling phenomenon occurs earlier,which is more conducive to heat transfer.(3)Under pool boiling state,the effect of wall wettability on the boiling nucleation of thin nanofluid films was investigated.On the basis of the previous two works,a hydrophilic-hydrophobic composite wall was established,and the method of enhancing the boiling heat transfer of nanofluid liquid film was studied by adjusting the ratio of the hydrophilic side and the hydrophobic side of the composite wall.The results show that the boiling heat transfer mechanism of nanofluid thick liquid film and thin liquid film is different.The accumulation of energy inside the liquid film is the main reason for the boiling phenomenon of the liquid film.The hydrophilic or hydrophobic wall surface under the same wall superheat degree has a large accumulation of energy inside the thick liquid film,a large temperature gradient along the thickness direction,and a boiling phenomenon on the solid wall surface,which coheres with the nucleation theory.This theory is not applicable to thin liquid film,because the internal energy accumulation is small,the temperature inside the liquid film is evenly distributed,and only evaporation occurs on the surface of the liquid film.However,the boiling phenomenon of the thin liquid film on the hydrophilic-hydrophobic combined wall shows that the combined wall can control the phase change characteristics of the thin liquid film and strengthen the boiling heat transfer characteristics.Due to the violent atomic disturbance at the hydrophilic-hydrophobic interface,the liquid film fractures.The temperature and cohesive energy of the liquid film at the interface are greater than the hydrophilic and hydrophobic sides,thereby enhancing heat transfer.The higher the proportion of the hydrophilic side in the composite wall,the greater the average temperature of the liquid film and the energy accumulated inside the liquid,and the more intense the boiling phenomenon.(4)Study the boiling heat transfer of the nanofluid liquid film on the single-layer graphene film.The results show that the heat transfer enhancement effect of the deposited nanoparticles mainly depends on the heat transfer from the graphene film to the particles.There is an adsorption layer of water molecules around the nanoparticle,and the heat flux and thermal conductivity of the adsorption layer are larger than other regions,which is beneficial to the heat exchange inside the nanofluid.The thermal conductivity of the particles themselves also affects the boiling heat transfer.With the larger diameter of the particle,the particle has the greater heat flux and the evaporation rate,and enhanced the heat transfer performance.The phonon frequency matching of the nanoparticles and graphene is well matched,which strengthens the heat transfer efficiency between the graphene film and the fluid.