Enhancement Mechanism Of Superhydrophobic Nano-coatings And Their Anti-condensation And Anti-frosting Characteristics

It is one of the most common phenomena in energy conversion that gas-liquid-solid phase transitions.Dewing and frosting behaviors are the most intuitionistic performances from these phase transitions.It is very unfavorable to the heat exchanger represented by air conditioning.Even it has been became the main bottleneck to restrict its development and application.Therefore,it is of great significance to solve or relieve the dewing and frosting problems on the heat transfer surface.And it is one of the key topics to be solved in low-temperature heat transfer and refrigeration fields at present.In this paper,the technical route was proposed that structuring condensation dewdrops self-jumping effect on the low temperature heat transfer surface to effectively postpone the accumulation of condensates for realization of the enhancement of low-temperature heat transfer characteristics.The surface strengthening mechanism,self-migration properties of condensates and its influence mechanism for condensation heat transfer were focused by controllable fabrication of superhydrophobic nanostructure and development of large-scale preparation technology.It is verified for the first time that the surface effectively improves the low-temperature heat transfer of the air conditioning heat exchanger.Which has great development and application prospect due to the breakthrough of the dewing and frosting problems from the material level.The specific research contents and results are as follows.The surface wear resistance of superhydrophobic composite coatings has been obviously improved based on organic-inorganic hybrid and organic resin bonding technology.It mainly depends on the wettability between nanoparticles and resin,distribution of nanoparticles in resin reinforcer and the mechanical stability of the resin reinforcer.Thereinto,the precision regulation of resin semi-curing time is very important to enhance the mechanical stability of organic resin bonding.We prepared FEVE/EVA superhydrophobic composite coating by soft hard resin composite technology,which keep the superhydrophobicity of surfaces after enduring of 70 cycles of sandpaper abrasion,heating treatment in the range of-198℃to 150℃,freeze-thaw cycle for 20 times and immersing in various corrosive liquid for 2 weeks.A novel preparation technology was developed to fabricate flexible FEVE/EVA superhydrophobic film with comprehensive performance in translucence,tailor,stretch,high and low temperature resistance,impact resistance,wear resistance and pollution resistance.And the wettability model of the film was established under the stretch condition.It provides research foundation and technical guidance for constructing of robust superhydrophobic surface.Al₂O₃ nanoarrays,SiO₂ nanoporous and CNT/SiO₂ multi-nanostructures were fabricated to explore the self-migration characteristic of dewdrops based on the demand of condensed dewdrops self-jumping effects.And we build the jumping modes of condensed dewdrops with spontaneous and triggered mergers.Here,we compare the condensation characteristics on superhydrophobic coating with jumping and non-jumping effects after condensed of 1000 s.The results show that untreated surface has been complete covered by dewdrops and to form water film,below 10%of dewdrops with size of 10-50μm were observed on the SiO₂nanoporous coating and their on the SiO₂ nanosphere or nanoagglomerate coating is about 30%.Moreover,over 50μm of dewdrops on the SiO₂ nanosphere or nanoagglomerate coating is about20%.Combine with calculation of the thermodynamic stability of Cassie state on superwettable surface,the microdrops will be formed as Cassie state on the superwettable surfaces with more roughness under micro-nano-scale at low-temperature environment.Which provides a research foundation for the design and application of anti-dewing surface.Frosting and defrosting characteristics were systematically characterized under low temperature heat transfer based on superhydrophobic nanoarrays and nanoporous surfaces.The mechanism of anti-frosting and the mode of self-migration of frost crystal were built on superhydrophobic surfaces.The results show that the height of frost layer on the Al₂O₃nanoarrays superhydrophobic surface is about 70%of the one on the hydrophile surface,revealing that the more excellent superhydrophobicity on the surface the more difficult frosting by combining with theoretical calculation.Under initial stage of frosting,condensed dewdrops self-jumping effects not only delay frosting effectively but also enhanced phase change heat transfer of surface.Defrosting tests show that the frost crystal on the superhydrophobic surface will emerge horizontal self-migration characteristic.Beyond that,we firstly observe the vertical self-migration effects of frost crystal on the superhydrophobic surface and develop its motion model.It affords an important supplement for the rapid defrost theory of superhydrophobic surface,and provides a new thinking and research foundation for the design and application of anti-frosting on superhydrophobic surface.Base on the above studies,we develop the larger-scale preparation technology of superhydrophobic air conditioning heat exchangers and systematically evaluate their condensation heat transfer characteristics.Compare with the commercial hydrophillic Fan Coil Unit（indoor condenser）,the refrigerating capacity from superhydrophobic one increased by about 8.06%and 5.01%under rated and condensed output working conditions,respectively.By simulating outdoor conditions,the frost heavy on superhydrophobic heat exchanger after frosting 60 min is about 81.82%on hydrophillic one,and then its total heat transfer increased by about 16.54%.When the defrosting,the defrosting time from superhydrophobic heat exchanger is about 52.78%from hydrophillic one,and its defrosting heat consumption decreased by about 47.20%.FEVE/EVA enhanced superhydrophobic composite coating was fabricated on the metal panel of radiant panel heat exchanger by two-step method.The condensation temperature was effectively reduced,which could supplied more cooling capacity in the absence of condensation.It provides research foundation and technical guidance for the design and application of different types of superhydrophobic heat exchangers,and is expected to be applied in real life and production.