Molecular Dynamics Study Of Vapor-liquid Phase Change Heat Transfer On Microstructured Surface

In recent years,aerospace thermal control,new energy utilization,phase change energy storage and heat transfer enhancement have put forward new requirements and challenges for the heat transfer characteristics of condensation and boiling.With the development of new materials and micro/nano technology,the research on the heat transfer characteristics of fluids at the micro-scale has attracted more and more attention.However,the mechanism that affects fluid movement and heat transfer has changed significantly with the scale effect.Many new phenomena,laws and mechanisms have been produced,which need to be further studied.Since molecular dynamics method can study the complex interactions between particles at the molecular level,it has gradually become the main research method for fluid heat transfer and phase transitions at the microscale.Therefore,in this work,molecular dynamics simulation method is used to construct a nanostructured surface.The explosive boiling phenomenon on the surface is studied in-depth,aiming to reveal the influence of wettability and liquid film thickness on the onset temperature(Ts)of explosive boiling at the microscale.Furthermore,a hydrophilic nanostructured surface with hydrophobic coating at microscale is built.The influences of coating and nano-pillar parameters on explosive boiling phenomenon are explored.In addition,on the basis of nanostructured surface preparation,the influence of different structural parameters on condensation heat transfer characteristics is studied.According to a reasonable surface which can enhance the efficiency of condensation heat transfer,the phenomenon of coalescence induced droplets jumping on a mixed-wetting superhydrophobic surface is studied.In order to search methods to improve the jumping velocity,and provide theoretical support for strengthening the condensation heat transfer,the influences of stripe width,contact angle and the relative position of the two droplet centers on the jumping velocity are analyzed deeply.The explosive boiling on both flat and nanostructured surfaces has been investigated.Distinctive values of TS are summarized with different liquid film thicknesses and two kinds of surface wettability.It is found that TS decreases with the increase of film thickness on both wettability flat surfaces.However,the decreasing rates of Ts on two wettability surfaces have the significant distinction,where the difference between two surfaces of Ts with the identical film thickness(?Ts)is decreasing.All the values of Ts on nanostructured surfaces are lower than those on flat surfaces with the same film thickness,which is attributed to the enhancement of the heat transfer between the liquid film and the surface with the existence of nanostructure.In addition,as the increasing of the film thickness,Ts presents a decreasing trend on both hydrophilic and hydrophobic nanostructured surfaces.When the liquid film thickness exceeds 6 nm,the values of Ts on nanostructured hydrophilic surfaces are higher than those on hydrophobic surfaces,which is completely reverse to the variation law of the flat surface.This is because the rise rates of Tf on two wettability surfaces exhibit an opposite trend comparing to the flat surfaces.A hydrophilic nanostructured surface with hydrophobic coating is built.Explosive boiling has been studied with three coating thicknesses,two liquid film thicknesses,and two kinds of pillar width.It is verified that the coating thickness,pillar width,and liquid film thickness are all affect the onset temperature of explosive boiling.The hybrid nanostructure can decrease the onset temperature compared to the pure hydrophilic surface.With the increasing of the hydrophobic coating thickness,the onset temperature of explosive boiling decreases for both pillar widths and liquid film thicknesses.When the liquid film thickness and coating thickness ratio are the same,the onset temperature of explosive boiling on the hybrid wetting surface with larger pillar width is smaller than that on the surface with smaller pillar width.When the hydrophobic coating thickness and pillar width are the same,a larger temperature gradient is discovered with thicker liquid film,which benefits to trigger the bubble nucleation.The hydrophilic and hydrophobic nanostructured surfaces are constructed,and the condensation phenomenon on these two surfaces is investigated.The simulation results on hydrophilic surfaces indicate that the larger groove width and height increased the condensation area,inducing more liquid atoms and a quicker temperature response.With the increase of the groove height and width,the potential energy reaches stability more slowly.For the condensation heat transfer,when the groove width is small,the change of groove height has little effect,while still causing a significant variation in the heat flux with a large groove width.Furthermore,if the cold wall becomes hydrophobic,the final numbers of liquid atoms for hydrophobic surfaces are all smaller than those for hydrophilic surfaces.The difference between the small groove height and larger groove height becomes more obvious with a larger groove width.The potential energies decreases with the increase of the groove height,which shows completely opposing trends compared with those on hydrophilic surfaces.One of the strongest distinctions is that the groove height becomes a significant impact factor,which causes no condensation with a larger height.Coalescence-induced droplet jumping on mixed-wettability superhydrophobic surfaces is studied numerically by using the molecular dynamics simulation method.The effects of the strip widths,contact angles and the relative positions of the center of two droplets on the jumping velocity are deeply investigated.It is found that the jumping velocity on mixed-wettability superhydrophobic surfaces can exceed the one on a perfect surface with the contact angle of 180°.The larger the strip width is,the higher the jumping velocity is,and the less the wettability differences between two strips is,the lower the jumping velocity is,and when the width of more hydrophilic strip is fixed,the jumping velocity will increase with the width of the other strip increasing,which is contrary to the trend of fixing the width of the more hydrophilic strip and altering the other strip width.The larger the stripe width is,the higher the bounce speed is;The smaller the difference between the two wettability,the lower the bounce speed;When the width of the hydrophilic stripe is fixed,the bounce speed will increase with the increase of the width of the other stripe,which is contrary to the speed trend of changing the width of the other stripe when the width of the hydrophilic stripe is fixed.