Preparation And Heat Transfer Performance Of Superhydrophobic Surfaces With Different Structures

Condensation heat transfer is a common energy transfer mode in industrial production process,which is widely used in power generation,aerospace,thermal management,seawater desalination and environmental control.It is of great significance to improve the heat transfer efficiency of condensation heat transfer process for saving energy and protecting environment.The surface of superhydrophobic metal is an effective way to realize droplet condensation and improve the efficiency of condensation heat transfer.However,on the superhydrophobic surface with different structure morphology,the wetting state of droplets in the condensation process is different,and the heat transfer effect is also significantly different.In this paper,the nanostructures with different structure characteristics are constructed on the base of aluminum and copper.The heat transfer performance of samples under the suitable conditions is tested.The surface heat transfer coefficient and heat transfer flux of superhydrophobic surfaces with different nanostructures are calculated under different supercooling conditions.The micro mechanism analysis of the dynamic behavior of condensate droplets is carried out in combination with the structural characteristics,The results show that condensation enhanced heat transfer effect is more excellent in three nanostructures.The main contents and results are as follows:(1)Three kinds of superhydrophobic surfaces with different structural characteristics were prepared on aluminum and copper substrates by thermochemical vapor deposition,anodic oxidation and liquid phase growth,respectively: tightly grown coiled carbon nanotubes,rod cap like alumina nanostructures and tapered rod like Zn O nanostructures with high aspect ratio.The effects of different reaction conditions on the surface structure and wettability were investigated by single factor method.(2)The three kinds of nanostructures and hydrophobic properties were compared and analyzed.The results show that the carbon nanotube structure is closely arranged curly tubular structure;the rod cap like alumina nanostructures are arranged regularly and orderly with a certain structural spacing;the tapered Zn O nanorods are arranged more regularly and closely with small structural spacing and higher aspect ratio.The tapered Zn O nanorods with regular and compact structure have higher hydrophobic angle,up to 160 °.(3)The dynamic behavior of droplets on three different surfaces under wet air condensation was analyzed.Under the condition of wet air condensation,the droplets are superhydrophobic on the surface of three samples,Wenzel on the surface of carbon nanotubes,and Cassie on the surface of cap like alumina nanostructures and tapered Zn O nanorods.The coalescence between adjacent droplets can be observed during the condensation process.(4)The condensation heat transfer test-bed was used to test and analyze the heat transfer coefficients and heat flux of three surfaces under different supercooling conditions,and the optical images of droplets were taken during condensation.The results show that the surface of carbon nanotubes will be pinned with high viscosity at high supercooling degree.The size of droplets on the surface is large,and it is difficult to fall off even under the action of gravity.Compared with flat hydrophobic surface,the heat transfer coefficient and heat flux on the surface of carbon nanotubes are lower than that of flat hydrophobic surface.The droplet on the surface of rod cap alumina nanostructure will gradually transition to Wenzel state at high supercooling.When the supercooling is less than 13 K,the heat transfer coefficient of the surface is about 30% higher than that of flat hydrophobic surface.The heat transfer coefficient of superhydrophobic surface of Zn O nanorod structure can be increased by 60%.The liquid droplets can keep stable cascade condensation on the surface of the structure under the condition of large supercooling,which has excellent heat transfer enhancement effect.