| As an energy-saving and environment-friendly heat supply method,air-source heat pump(ASHP)has higher utilization efficiency than coal heating,electric heating and other traditional heat supply methods.Moreover,ASHP has other advantages of the ability of both cooling and heating,low initial cost and convenience of utilization.However,actual efficiency of ASHP heating mode in winter is lower than expectation because of frosting problem in outdoor heat exchanger.Researches about how to restrain frosting in ASHP outdoor heat exchanger and how to defrost efficiently has become an important issue for promoting ASHP development and application.In this paper,an anti-frosting method based on superhydrophobic surfaces is proposed.Visualization experiments and theoretical analyses have been conducted for revealing microscopic characteristics in early stage of frost formation process on superhydrophobic surface,which could lay the foundation for revealing frosting characteristics and anti-frosting mechanism of superhydrophobic surface.Meanwhile,effects of micro surface structure on frosting/defrosting characteristics have been preliminarily researched.The research results can provide theoretical basis for design and application of superhydrophobic surfaces.The detailed results are shown as follows:A condensation nucleation barrier model was constructed for the condensation nucleation stage of the frosting process on the superhydrophobic fin surface.Difficulty of condensation nucleation is reflected by condensation nucleation barrier which has to be overcome during nucleation,and effects of surface energy and surface structure on condensation nucleation process have been revealed.The preferential nucleation position on the superhydrophobic surface.The research provide reference for preparation of anti-frosting superhydrophobic surface.Based on single superhydrophobic surface,visualization experiment has been conducted for condensation characteristic of superhydrophobic surfaces.The morphology,growth and distribution of the condensation droplets on the surface of the superhydrophobic fins were obtained,and the surface of the fins was compared with other fin surfaces.By analyzing the coverage of the surface of the fins and the average radius of the condensation droplets,the anti-condensation effect of the surface of the superhydrophobic fins was evaluated,which provided experimental data support for the study of the subsequent stage of the superhydrophobic fin surface frosting process.For exploring freezing stage of frost formation processon superhydrophobic surface,visualization experiments have been conducted for freezing characteristic of superhydrophobic surfaces.Microscopic characteristics of single droplet freezing process on superhydrophobic surface and effects of surface characteristic and surface temperature on duration of droplet freezing process have been acquired.Moreover,heat and mass transfer model during droplet freezing period has been established,and the model has been utilized to predict droplet freezing time.Theoretical analysis results consistent with experimental results: duration of droplet freezing increases with the increase of surface contact angle;duration of droplet freezing increases with the increase of surface temperature.Inspired by the phenomenon of “ice bridge” which result in frost spreading during droplet freezing initial stage of frost formation process on superhydrophobic surface,a model about mass transfer between droplet and freezing droplet has been established.Disciplines of frost spreading on superhydrophobic surface have been revealed,and the condition of frost spreading has been researched.Effects of surface wettability and surface temperature on formation of “ice bridge” have been researched.Main effect items of frost spreading By exploring the main factors affecting the frost spreading behavior,it is clarified that the small droplet radius and the sparse distribution on the superhydrophobic fin surface are the key to delay the freezing behavior and delay the growth of frost.Based on the study of the condensation and freezing characteristics in the initial stage of frosting on the superhydrophobic fins,the mechanism of the superhydrophobic fin surface structure on all stages of the frost formation process was studied by microscopic view.The superhydrophobic surface with grooved microstructure was fabricated.Through the visualizational experiment study,the microscopic features of the frosting process on the micro-groove structured superhydrophobic surface were obtained,and the surface microstructure affected the frosting process on the superhydrophobic fin surface.The effects of surface structure on condensation stage,freezing stage and frost crystal growth stage have been revealed,and the effects of frosting time and surface temperature on the characteristics of frost layer on micro-groove super-hydrophobic surface and ordinary superhydrophobic surface were compared and analyzed.The mechanism of the superhydrophobic fin surface structure on the frosting process is revealed,which provides a research basis for the optimization of the superhydrophobic surface structure. |
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