Vapor Condensation Nucleation Mechanism And Heat Transfer Enhancement Methods On Nanostructured Surfaces

The vapor condensation phenomenon is an important link in many applications.The research on the mechanisms related to the condensation process and heat transfer enhancement methods are important for the development of industrial applications and high-tech technologies such as seawater desalination,atmospheric water collection,and equipment thermal management.Vapor dropwise condensation involves four basic processes:nucleation,growth,coalescing,and departure,spanning multiple scales.Although a great deal of research has been conducted on the heat transfer characteristics and influencing factors of vapor condensation,various methods have also been proposed to maintain vapor droplet condensation.However,the critical liquid nucleus size is on the nanoscale,while the critical size of the condensed droplet removed from the surface in experiments is on the order of micrometers to millimeters.Due to the multi-scale characteristics of the surface morphology,there is also a complex cross-scale evolution relationship between the droplet size and the surface structure size during the condensed droplet life cycle,resulting in many new problems and phenomena during vapor condensation.For example,under a high sub-cooling degree,due to the critical liquid nucleus size being much smaller than the surface structure size,vapor molecules rapidly nucleate within the microstructure,resulting in droplets coalescing and growing,ultimately leading to water flooding,invalidating the superhydrophobic effect on the microstructure surface,and worsening heat transfer.Therefore,understanding the vapor condensation nucleation mechanism and heat transfer enhancement methods on nano structured surfaces can help to achieve accurate control of vapor nucleation sites,and to promote rapid departure of condensed droplets,thus achieving enhancing heat and mass transfer.This article focuses on the vapor condensation nucleation mechanism and heat transfer enhancement methods on nano structured surfaces,focusing on solving the key scientific issues of nucleation regulation mechanism and heat and mass transfer enhancement mechanism.The vapor nucleation mechanism and droplet growth dynamics on nano structured surfaces are studied using the molecular dynamics simulation method.The surface energy analysis method is used to determine liquid nuclei generation location.The point where the absolute value of the interaction potential between water molecules and the dimpled surface being largest is located at the dimpled bottom rather than the flat surface,indicating that the dimpled bottom is a natural nucleation site.In addition,the potential energy at the dimpled bottom decreases with the increase in surface hydrophobicity or curvature radius,making vapor nucleation more difficult.Unlike the nucleation mechanism of gas gathering inside the macro dimple,the stronger solid-liquid interaction inside the micro-nano scale dimple is the microscopic mechanism of vapor nucleating here.For convex surfaces,the point where the absolute value of the interaction potential being highest is always located on both sides of the structure.Therefore,the vapor is more likely to nucleate on both sides of the convex structure than on the top,which is different from the classical nucleation theory assumption.Although the surface topological structure’s existence is conducive to nucleate,once liquid nuclei are formed inside the dimpled or on both sides of the convex structure,as the condensation continues,if the Wenzel condensed droplets present on the surface cannot be removed promptly,water flooding will occur,worsening vapor condensation heat transfer efficiency.To address the above issues,active and passive methods are used to regulate the vapor condensation process.First,consider using a passive method to regulate vapor nucleation sites,construct a randomly mixed wettability structured surface,and observe the effects of different doping atomic properties and hydrophilic atomic doping ratios on condensation nucleation characteristics.It is found that when the surface average potential energy is the same,the difference in the properties of doped atoms would affect the surface potential energy distribution.As the difference in the properties of doped atoms increases,the local high-energy region generated by hydrophilic atoms induces rapid nucleation for vapor molecules,and condensation nucleation characteristics improve.In addition,when fixed doped atoms’ properties and increased the hydrophilic atoms doping ratio,the increment of hydrophilic atoms’ aggregation degree would increase the surface average potential energy,enhancing the solid-liquid interaction,and making it easier for vapor molecules to aggregate on the surface to form initial liquid nuclei,effectively improving the defect of relatively poor vapor nucleation characteristics when doped atoms properties are similar.Based on this,a correlation diagram between the difference in doped atoms’ properties and the vapor nucleation sites is further constructed to achieve the regulation of vapor nucleation sites.Secondly,consider using an electric field to promote condensed droplets escaping from the interior of the structure.The influence of the electric field on droplet growth kinetics is analyzed,and it is found that the deflection effect of dipoles is the microscopic mechanism that affects the droplet growth mode.When a vertical electric field is applied,as the electric field strength increases,water molecules would more easily deflect towards the electric field direction,greatly weakening the interaction force between the surface and water molecules,and limiting the condensed droplets’growth in other directions except for the vertical direction.The maximum condensed water molecules number Nmax is reduced by 26%compared to that without the electric field applied.Applying a horizontal electric field facilitates the contact of water molecules with the surface,increases the heat exchange area,and ultimately exhibits better condensation performance.In addition,with the increase of the electric field strength or the decrease of the electric field frequency,the condensed droplets’ wetting state would be changed from the Wenzel state to the Cassie state.Therefore,a new method using periodic horizontal electric fields to promote condensed droplets escaping from the interior of the structure is proposed.Finally,consider using the surface structure design to promote the rapid departure for condensed droplets,the effect of surface structure distribution on droplet coalescing dynamics is explored.It is found that with the increase of surface wettability,three typical situations would occur after droplets coalesce:the coalesced droplet bouncing off the surface;the droplet does not bounce off the surface after coalescing,but the droplet’s wetting state changing from the Wenzel state to the Cassie state;and the coalesced droplet remained the Wenzel state.When the distance between adjacent topological structures on the surface is 65.28 A,the ordered flow inside the droplets leads to less viscous dissipation during the coalescence process,and the energy conversion efficiency can reach 16.7%.In addition,as the droplet’s radius ratio decreases,when the liquid bridge strikes the solid surface,the small droplets would first begin to contract under the surface tension’s action,escaping from the interior of the structure in advance,and the centroid of the coalesced droplets would also be more likely to deviate towards the larger droplet.When the droplet radius ratio r is within the range of 0.45-1.00,the self-jumping phenomenon of coalescence droplets can be observed.