Droplet Behaviors And Heat Transfer Of Pure Steam Condensation At Atmospheric Pressure On Superhydrophobic Nanowire-bunch Arrays

Condensation heat transfer enhancement is of great importance to realize the high efficiency and integration of energy systems.Dropwise condensation has attracted the attention of several researchers because it has an order of magnitude higher heat transfer performance than filmwise condensation.The micro/nanostructures can regulate the droplet wetting state,promote droplet jumping,reduce the droplet size distribution,and improve the heat transfer performance.However,compared with condensation of steam at low vapor pressure or containing non-condensable gas,during condensation in the pure steam at atmospheric pressure,the nucleation rate is higher,the initial nucleation radius is smaller,and nucleation tends to occur inside the nanostructures,resulting in the failure of surface regulation function and flooding condensation.In this thesis,based on the regulation of sub-processes of condensation,hierarchical nanowire-bunch arrays are designed.Then we demonstrated the mechanism of preferential nucleation,the fast growth of droplets in the microgroove,and stable jumping condensation during condensation of pure steam at atmospheric pressure.Remarkable heat transfer enhancement is achieved.AAO template-assisted electrochemical deposition method is used to prepare hierarchical nanowire-bunch arrays.From the bottom to the top of the sidewall of a nanowire bunch,the nanowire interspacing becomes smaller and smaller;The nanowires around the top of the nanowire bunch are densely arranged.There are height differences between neighboring nanowires,so numerous nanosteps are formed.V-shaped microgrooves form between nanowire bunches.In the condensation experiments of this work,the average height of the nanowire bunch is～22-25μm,and the overall solid fraction is equal to the porosity of the AAO template.The smaller the solid fraction,the more obvious the agglomeration of nanowires,and the larger the microgroove width and microgroove angle.The microgroove angles of different surfaces are 6°,27°,and 53°,and the corresponding surfaces are termed as S6,S27,and S53 surfaces.As a comparison,the nanowired surface without microgroove is also prepared.Branched-like structures are formed between neighboring nanowires and the surface is termed as B-3D surface.Environmental scanning electron microscopy(ESEM)was used to observe the microscale condensation process at different visual angles.It was found that preferential nucleation occurs around the top of nanowire bunches,which is beneficial to the formation of Cassie-state droplets.The classical nucleation theory shows that the nano-step structures and densely packed nanowires can increase the solid-liquid interface area and decrease the vapor-liquid interface area,so the nucleation energy barrier at this region is 12%-23%lower than those in other regions.The molecular dynamic simulation was used to investigate the nucleation process on the top and sidewall of the nanowire bunches.Different hybrid nanowire arrays are constructed,including hybrid nanowire arrays comprising densely packed and sparsely packed nanowires and hybrid nanowire arrays comprising densely packed nanowires and nano-steps.The simulation results show that the nuclei form at the region with densely packed nanowires and the nucleation ability of the nanowire arrays decreases with the increase of nanowire interspace.The visual observation shows that the microgroove can promote the fast growth of droplets and the regulation mechanism of droplet wetting mode on hierarchical nanowire-bunch arrays is proposed.The droplet growth rates at different regions of nanowire bunches are measured and the results show that the growth rate of the droplets on the top of the microgroove is 56%higher than that of the droplets outside the microgroove.This is because the droplets on the microgrooves contact the two sidewalls of neighboring nanowire bunches,which have a large solid-liquid interface area.Besides,the agglomeration of nanowires leads that the solid fraction of the region on the top of the microgroove sidewall is higher than that of the region on the top surface of the nanowire bunch,which is beneficial to heat transfer.The droplet wetting state is determined by microgroove angle and nucleation density.With the increase of the microgroove angle,the nano-step area for preferential nucleation and the nucleation density decreases,which is beneficial to reduce the adhesion between the droplet and the surface.For the large droplets,the height of the lower meniscus of the droplet decreases and gradually immerses in the microgroove,resulting in the increase of the adhesion between the droplet and the surface.The model analysis shows that when the microgroove angle is larger than 123°,the height of the lower meniscus is 0 and the droplet is completely immersed in the groove.Therefore,the surface with a moderate microgroove angle can maintain stable Cassie-state droplets.Through changing the surface subcooling,the reversible wetting transition phenomenon from the Wenzel state to Cassie state is found,which broadens the application range of the surface.The fast departure and surface renewal of droplets are realized through droplet jumping and the heat transfer performance is significantly improved.The S27 surface with a moderate microgroove angle can effectively regulate the nucleation density and the height of the lower meniscus of the droplets and can maintain stable jumping-droplet condensation under high heat flux.The jumping-droplet condensation mode can significantly reduce the droplet size distribution,accelerate the surface renewal frequency and reduce the thermal resistance of condensate.Compared with S6,S53,and B-3D surfaces that can’t maintain stable jumping-droplet condensation,the average droplet departure radius on the S27 surface(64.1μm)is reduced by 50%-96%;The surface renewal frequency is increased by 297%-3800%;The condensation heat transfer coefficient is increased by 21%-221%.The maximum heat transfer flux of jumping-droplet condensation is 1.06 MW/m~2,which is the highest value in the literature.Based on the morphologies of surfaces and microscale droplet growth characteristics,combined with the droplet departure characteristics and droplet size distribution model during vapor condensation in the pure steam,the heat transfer model suitable for hierarchical nanowire-bunch arrays is established.The effect of condensation sub-process enhancement on the overall heat transfer performance of the surface is analyzed.Due to the agglomeration of nanowires,the local solid fraction on the top of nanowire bunches increases.During the heat transfer process of small droplets,the thermal resistance of the nanowire layer on the hierarchical surface is 13%-84%of that of a single-tier nanowire surface,and the single-droplet heat transfer rate is 1.7-27 times of that on the single-tier nanowired surface.Then the effect of the enhancement of single droplet heat transfer on the overall heat transfer performance is analyzed.When the nucleation site density and droplet departure size are the same,the heat transfer performance of the nanowire-bunch surface is 300%higher than that of the single-tier nanostructured surface.The theoretical value of single droplet growth rate and condensation heat transfer performance of different surfaces are in good agreement with the experimental values.