Molecular Dynamics Study Of Roughness And Wettability Effects On Boiling

Boiling is an effective method to remove heat from devices such as micro-and nano-electro-mechanical systems,which have huge amounts of heat being generated in limited space in a short time.The understanding of boiling heat transfer is important for many industrial processes involving bubble clustering,chip cooling,and thermal detonation.With the development of nanostructure surface technology,boiling heat transfer at the nanoscale has become one of the research hotspots.Mechanisms of nucleation boiling at the micro-and nanoscale are critical to the development of these technologies and the promotion of industrial processes.Molecular dynamics simulation(MD)method provides an effective tool to study the nucleation boiling phenomena at the nanoscale.Against this backdrop,this paper investigates the mechanisms for the nanoscale phase-change heat transfer and provides insight for promoting boiling heat transfer on a surface with temperaturedependent wettability using molecular dynamics simulation method.First,the twoand three-dimensional analysis of surface potential energy are used to address the mechanisms of surface wettability and surface roughness on nanoscale boiling characteristics of liquid molecules.Moreover,the synergistic effect of various thermodynamic parameters on boiling heat transfer is investigated by introducing the surface temperature and liquid film thickness.Based on this,a new surface with temperature-dependent wettability is proposed,to further promote the boiling heat transfer and bubble nucleation process.The main research work of this paper is as follows:First of all,a three-dimensional analysis of surface potential energy is used to address its relationship to surface wettability and nanoscale pool boiling characteristics of liquid molecules.It is found that the major reason for the enhanced heat transfer by increasing the surface wettability is essentially the improvement of surface potential energy,of which the influence is summarized using logic order.Moreover,the nanoscale boiling of water on copper surfaces with various wettability is investigated,which reveals that the increase in surface wettability can improve the critical heat flux under high superheat conditions in both nano-and macroscale systems.Besides,a nondimensional analysis is finally implemented to correlate the normalized surface potential energy(P),the Interfacial thermal conductance(N),and the normalized critical heat flux(J).Then,the effect of surface roughness on the explosive boiling of water film over the copper surfaces with nanochannels having different heights and spacing is investigated.A two-dimensional surface potential energy is used to address the relationship among the roughness ratio,interaction potential energy,and boiling characteristics of the liquid molecules on the structured surfaces.A critical channel spacing is proposed to address this effect on boiling heat transfer on surfaces with increasing roughness ratio by reducing the channel spacing.It is concluded that,when the channel spacing exceeds the critical value,the major reason for the enhanced heat transfer by increasing the roughness ratio is essentially the improvement of the surface potential energy by increasing the heat transfer area.Further investigations on nanochannels with different heights and spacing indicate that the critical channel spacing of 1.444 nm is also suitable for more nanochannel surfaces.Besides,this nanoscale boiling of water film over copper surfaces with different wettability under various surface temperature conditions is investigated.It shows that the water molecules on the hydrophilic surfaces have higher temperature gradient and initial heat flux,shorter onset time of boiling,and lower Kapitza resistance than those on the hydrophobic surface.Meanwhile,the temperature gradient,the heat flux in the water films,and the onset time of explosive boiling are promoted by elevating the surface temperature.These indicate that the hydrophilic surface under high surface temperature conditions is most favorable for enhancing the explosive boiling heat transfer.Analysis of the onset of boiling reveals that the effect of wettability on the explosive boiling is more prominent under low surface temperatures but is limited under high surface temperatures.Moreover,the effect of liquid film thickness and surface roughness ratio on the boiling of water over the copper plates in terms of hydraulic diameter defined based on the overall cross-sectional area available for the flow of water film is investigated.It is found that the reduction of the hydraulic diameter by increasing the surface roughness ratio can substantially promote the performance of rapid boiling,because of the increase in heat transfer area,together with a decrease in the Kapitza resistance.As a combined effect of the film thickness and the Wenzel roughness ratio,the hydraulic diameter can determine the onset time of boiling and the equilibrium temperature,while it can also determine the heat transfer rate and the Kapitza resistance if the film thickness or the surface roughness is fixed.Last,bubble nucleation on a surface with temperature-dependent wettability is studied.The surface has an incipient nucleation time shorter than the mixedwettability surface and the hydrophilic nanostructured surface.The interaction energy per unit area at the solid/liquid interface and inside the liquid water molecules are calculated to address the promotion of bubble nucleation over the surface with temperature-dependent wettability.The hydrophilic nanostructured surface with high interaction energy can transfer more heat to the liquid water before the bubble nucleation.When the water molecules absorb enough energy,the hydrophilic nanostructure becomes hydrophobic with low interaction energy to promote bubble nucleation.The potential energy distribution of liquid molecules is then calculated to clarify the mechanism of bubble nucleation on the surface with temperaturedependent wettability:the nanostructure causes lower absolute potential energy,and hence provides fixed nucleation sites for initial generation of bubble nucleus and shortens the incipient nucleation time.Besides,more nanostructured surfaces with temperature-dependent wettability with different wettability transition temperatures are investigated.The impacts of the wettability transition temperature on the incipient nucleation time is studied to achieve the optimal value.Finally,a nondimensional analysis is implemented to compare the heat transfer performance between the macroand nanooscale systems.This work reveals the mechanisms of the effect of the surface roughness and wettability on nanoscale boiling,and perfect the theoretical knowledge system of nanoscale boiling heat transfer.Besides,a surface with a smart wettability transition has been proposed.This study provides support and theoretical guidance for the design,manufacture and performance optimization of micro-nano systems and devices.