Mechanism Study Of Liquid Film Evaporation Heat Transfer Enhancement At The Nanoscale

With the increasing demands for miniaturization and integration of the electronic devices,the characteristic scale of the electronic devices has been reduced continuously,and the heat dissipation per unit area has been increased constantly.The effective thermal management of the microelectronic devices is essential to make sure that the devices are at a stable and efficient operation.Single-phase free convection heat transfer and single-phase forced convection heat transfer cannot meet the demands of high heat flux generation at the micro/nano-scale.Phase-change heat transfer is the preferred cooling technique to overcome the micro/nano-scale high heat flux generation problems due to the latent heat and the heat dissipation capacity of 1-3 orders of magnitude exceeding the heat flux of single-phase convection heat transfer.However,due to the reduction of the characteristic scale of the surface,the curvature radius of the liquid-phase free surface is on the same order of magnitude as the characteristic scale of the surface.The liquid-phase surface will show a curved shape and the nanoscale phase-change heat transfer mainly depends on the evaporation heat transfer process in the liquid-phase meniscus region.Therefore,exploring the mechanisms of evaporation heat transfer in the meniscus region and enhancing the process of evaporation heat transfer in the meniscus region are of great significance for understanding the nanoscale phase-change heat transfer.Nanoscale liquid film evaporation heat transfer process is a non-equilibrium dynamic process affected by multiple factors.Based on the molecular dynamics simulation method,this paper investigates the evaporation heat transfer process of liquid films on the smooth and rough surfaces,studies the structure optimization method for two-dimensional rough surfaces enhancing evaporation heat transfer from the mathematical perspective and analyzes the relevant mechanism of the optimization method.Firstly,the molecular dynamics simulation of the evaporation process of liquid argon film on smooth copper surface and the corresponding influencing factors are investigated.Introducing the simulation model construction and simulation methods,and then analyzing the effects of the truncation radius and time step on the simulation process.Based on the calculation accuracy and calculation time,the truncation radius and time step are selected as 3.5 σAr-Ar and 5 fs,respectively.The effects of the film thickness,the surface temperature,the surface material,and the surface wettability on the evaporation process are studied.The simulation results of different liquid film thicknesses show that the increase in the thickness of the liquid film weakens the evaporation process of the liquid film,and as the thickness of the liquid film increases,the number of evaporated atoms and the heat absorption of the liquid film also increase when the evaporation process reaches equilibrium state.Nevertheless,the evaporation rate decreases,and the heat transfer rate increases slightly.This is because the increase in the thickness of the liquid film will bring a significant increase in the thermal conduction resistance of the liquid film and thus weaken the evaporation process.The evaporation process of liquid film is a process that absorbing energy to overcome the potential energy among liquid atoms.The increase in surface temperature will provide more energy to the liquid film to overcome the potential energy and increase the kinetic energy,which will make a larger number of liquid-phase atoms to undergo the evaporation process.As the surface temperature increases,the number of evaporated atoms and the heat absorption of the liquid film will increase when the evaporation process reaches the equilibrium state.The simulation results of different surface materials show that the surface material will influence the initial evaporation process,and the number of evaporated atoms and the heat absorption of the liquid film are different when the evaporation reaches equilibrium state,which is because of the different solid-liquid energy parameters of different material surfaces.The increase in the energy parameter will enhance the evaporation process of the liquid film on the surface.When the value of the energy parameter is large enough,the increase of the value of the energy parameter will no longer enhance the evaporation process of the liquid film.The simulation results of different surface wettability show that the increase of surface wettability will enhance the evaporation process of the liquid film.As the surface wettability increases,the heat flux and evaporation rate of the evaporation process will increase significantly.There is a liquid-phase region with a mass density close to 1.4 g/cm3 always exists ou the near surface region of hydrophilic surface,and a liquid-vapor coexistence region with a mass density of 0.7-0.8 g/cm3 always exists on the near surface region of hydrophobic surface.Different mass-density regions of different wettability surfaces cause different thermal resistances between solid and liquid,which is the reason for the different heat transfer characteristics of different wettability surfaces.Further analysis shows that the difference in solid-liquid interfacial thermal resistance between different wettability surfaces is the main reason for the different heat transfer characteristics of different wettability surfaces.Rough surface can enhance the evaporation heat transfer process of liquid film effectively,and with the decrease of the characteristic scale of surface,the influence of rough surface on evaporation heat transfer process is more significant among the range of nanoscale.Considering the diversity of rough surfaces,the new sinusoidal nanostructured surfaces are constructed to investigate the evaporation behavior and solid-liquid interfacial thermal resistance of the liquid films on different types of sinusoidal nanostructured surfaces.The simulation results show that different types of sinusoidal nanostructured surfaces have different heat transfer performances.Using the specific sine function expressions to quantify the surface shape of the sinusoidal nanostructures,and the speed of the evaporation process is related to the relevant parameter values(amplitude B and period P)describing the shape of the function.The influence of the functional parameter values of the evaporation process is that as the amplitude B increases and the period P decreases,the liquid film temperature increases faster,and as the heat transfer surface area increases,the number of non-evaporated atoms also increases.Moreover,as the increase in the amplitude B and decrease in the period P,the solid-liquid interfacial thermal resistance and the mismatches of vibrational density of states between solid-liquid will also decrease.The improvement of heat transfer enhancement at the solid-liquid interface caused by changes of phonon modes at the solid-liquid interface explains the mechanism of the enhancement in the evaporation heat transfer due to rough surfaces.At the same time,although the increase of heat transfer surface area can enhance the process of evaporation heat transfer,the increase rate of heat transfer enhancement rate decreases,which indicates that the heat transfer surface area cannot increase infinitely to enhance evaporation heat transfer.Considering that the relation between heat transfer area and heat transfer enhancement rate,based on the problem of finding the optimal surface structure when the heat transfer area is constant,studying the evaporation behaviors of the liquid films on the different types of two-dimensional rough surfaces.Rough surfaces are classified mathematically through rectangular coordinate systems,and the rough surfaces are classified into convex,concave,and concave-convex surfaces according to the maximum value and derivative of the function expressions.When the heat transfer area is constant,the evaporation processes of the liquid films with the same number of argon atoms on different types of rough surfaces show that all types of rough surfaces can enhance the processes of evaporation heat transfer,but rough surfaces with the same heat transfer area still have different heat transfer performances.All rough surfaces in the study have different defined"sectional areas of the liquid phase",and the sectional area of the liquid phase has a non-linear positive correlation with evaporation heat transfer performance.Analyses of the number percentage of high temperature argon atoms and the temperature distribution of argon atoms at specific time in the early and middle stages of evaporation show that rough surfaces with larger sectional areas of the liquid phase also have more high temperature argon atoms.In addition,the argon atoms in the sectional area of the liquid phase play a role in energy storage and heat transfer.A larger sectional area of the liquid phase means that more argon atoms participate in the process of energy storage and heat transfer,thereby enhancing evaporation in the initial heat transfer process.Finally,based on the positive correlation between the sectional area of the liquid phase and the heat transfer performance,it is possible to analyze and solve this type of heat transfer enhancement problem from the mathematical point of view directly,which is when the heat transfer area is constant,solving the maximum sectional area of the liquid phase mathematically.By converting the problem of optimal heat transfer performance into a conditional extreme value problem,the maximum sectional area of the liquid phase is solved to obtain the optimal surface structure.For concave surfaces,the optimal concave structure is an arc-shaped surface.For convex surfaces,the optimal convex structure is a surface composed of two identical circular arcs.For concave-convex surfaces,the heat transfer performance is less than those of the optimal concave surface and the optimal convex surface.According to the mathematical model of the optimization method,corresponding physical model of the optimal surface is established,and the same boundary conditions and simulation methods are used to simulate the evaporation process of the optimal surface.The accuracy of the optimization method is verified by comparing the heat absorption,the number of evaporated atoms,the heat transfer rate,and the evaporation rate.The generality of the calculation results and the optimization method is discussed in three aspects:the types of the nanostructured surfaces,the materials of the solid and liquid atoms,and the wettability of the surface.Meanwhile,a new discrete linear interface construction method is proposed to accurately capture the liquid-vapor interface at the nanoscale.Additionally,explaining the mechanisms of the optimization method.By analyzing the potential energy per liquid atom near the liquid-vapor interface,it can be found that the different nanostructured surfaces with different defined sectional area of the liquid phase do not affect the potential energy distributions of liquid atoms near the liquid-vapor interface.It indicates that the change of the sectional area of the liquid phase will not affect the heat transfer process at the liquid-vapor interface.However,the solid-liquid interfacial thermal resistance decreases as the sectional area of the liquid phase increases,and the solid-liquid interaction energy per unit area increases as the sectional area of the liquid phase increases,which both illustrate that the increase of the sectional area of the liquid phase will enhance the heat transfer at the solid-liquid interface.Moreover,the area of the liquid-vapor interface increases as the sectional area of the liquid phase increases and thus accelerates the evaporation heat transfer process.