Fabrication And Condensation And Frosting Properties Of The Surfaces With Nano Arrays And Heterogeneous Wettability

Condensation and frosting is common phase transition in various cryogenic and refrigeration equipment or cold surface of the system. In most cases, it not only needs to expend energy, but also increases thermal resistance and decreases the heat transfer efficiency of heating surface, so far as to cause black up in system, having a strong impact on work efficiency and safety. Take the air-condition, the most energy-intensive household appliances, as example, condensation on indoor evaporator fins when refrigerate or exsiccate in summer or frosting on outdoor condenser fins when heat in winter is primary cause of energy-extensive consumption. The effective desorption of dewdrop and frost one fin surface is always a problem demanding prompt solution in air-conditioning industry, meanwhile is a research hotspot in the field of condensation heat transfer, sea water desalination and air source heat pump. In the base of super-hydrophobic surface, combining super-hydrophilicity surface, we carried out the research of fabrication of the surface with nano arrays and heterogeneous wettability, and further, made a deep analysis of the condensation and frosting characters, aimed at realizing the desorption and controlling of the dewdrop and frost. The main research content of this paper is as follows:First, highly-oriented ZnO nanorod arrays on Cu, glass and PET were fabricated by chemical oxidation of ZnO seed layer deposited by a sol-gel method. When modified by FAS, the samples became super-hydrophobic and the CA is greater than 150° and the RA is less than 4°. We studied the influence and its mechanism of solution concentration for crystal seed layer and thereafter growth time for nanorods on the morphology of nano-arrays, and then further explained the change of wettability and transparency. By precisely controlling-the fabrication of the nano arrays, we obtained a super-hydrophobic surface high average transmittance, the concrete parameter is as follows:the solution concentration is 0.1 M and growth time is 0.5h, the average height of ZnO nano arrays is 750nm, average diameter is 60nm, average distance is 60nm, average transmittance is 95%, the CA is 153.7°,the LA is 2.7°.Then, based on the super-hydrophobic surface on Cu samples, super-hydrophilic microcells were fabricated with template photocatalysis technology and then the heterogeneous wettability surfaces were obtained. Researches show that the geometrical features of the super-hydrophilicity microcells in heterogeneous wettability surfaces has a marked impact on the condensation process. Compared with super-hydrophobic surface, the super-hydrophobic area in heterogeneous wettability surface has the characteristic of dropwise condensation, however the super-hydrophilic area has the characteristic of filmwise condensation and it significantly improves the amount of dew, which increases with the proportion of super-hydrophilic area. The amount of dew of heterogeneous wettability surfaces with round super-hydrophilic area is twice as much as that of super-hydrophobic surface. Further, heterogeneous wettability surfaces with cuneiform super-hydrophilic area has higher amount of dwe,70% more than that with round super-hydrophilic area. Thus it can be seen, there are both super-hydrophobic characters and super-hydrophilic characters on heterogeneous wettability surfaces. Soon afterwards, higher nucleation rate and high desorption rate can be acquired by majorization of the physical dimension of super-hydrophilic area, thus increasing surface heat transfer efficiency.Similar to the condensation, when frosting on heterogeneous wettability surfaces, higher proportion of the super-hydrophilic area can cause higher frost accumulation and the cuneiform geometrical features can promote frosting. The results of defrost experiment indicate that there is higher residual water accumulation on heterogeneous wettability surfaces with round super-hydrophilic area when compared to super-hydrophobic surface but lower compared to hydrophilic surface. By contrast, the residual water accumulation and the fraction of coverage is lower, showing better desorption character. Analysis suggests, the heterogeneous wettability surfaces comprehensive the advantage of super-hydrophobicity and super hydrophilicity. It has the characters of favorable desorption and efficient nucleation, making for increasing heat transfer efficiency. Meanwhile, it can further increasing desorption efficiency of dewdrop and frost by the Laplace pressure difference of wedge hydrophilic region.In this paper, controllable fabrication technology of super-hydrophobic surface and local optional modification technology were learned and heterogeneous wettability surfaces were fabricated, significantly improving the condensation and frosting rate. Also, the wedge geometric building was studied, promoting desorption of dewdrop and frost. The above researches not only lay the foundation of bionic nanotechnology used in low temperature field, but also push energy-saving and cost-reducing technology. Meanwhile, it offers a new thought to exploit heat-transfer surface, being of great significance.