| Frosting on the heat exchangers has always been one of the important problems faced by many fields,especially for heat pump heating system.During winter in heat pump and air conditioning system,the generation and accumulation of frost layers reduce the heat transfer efficiency of heat exchangers and increase the energy consumption of heat pump.Moreover,the severe frosting also affects the safety of the equipment.Superhydrophobic surfaces,which have been paid more attention,are considered as an effective method of frost suppression.However,the mechanism of frost suppression on superhydrophobic surface is still unclear.In addition,it’s not practical to realize the surfaces frost-free operation only via superhydrophobicity because frost will happen anyway.Therefore,this study carried out experimental and theoretical research on the mechanism of condensation frost formation on superhydrophobic surface.Also,the combination of liquid desiccant and superhydrophobic surfaces for frost suppression is performed.Firstly,we observed the condensed halo phenomenon formed by the freezing of droplets in the initial frost propagation of condensation frosting on self-prepared superhydrophobic,hydrophobic and hydrophilic surfaces by constructing a visualized experimental platform.Compared with hydrophilic and hydrophobic surfaces,the condensed halos on the superhydrophobic surface have larger diffusion radius and condensation density,and last a longer duration,which is beneficial to delay frost formation.Besides,based on the theory of phase transition kinetics and gas diffusion mass transfer,the mathematical models of droplet freeze and condensed halo formation were established.The influence of surface temperature,ambient relative humidity and surface wetting properties on the condensed halos were analyzed.The mechanism of the droplet freezing process on superhydrophobic surface is comprehensively revealed,which provides a research basis for the design and application of high-efficiency frost-resistant superhydrophobic surface.Secondly,by constructing three high-reflectivity surfaces with different wetting properties and using,a clear visual observation of the frost propagation phenomenon during condensation frosting is realized on self-prepared high-reflectivity surfaces via the micro-frost experimental test platform.It is found that the frost propagation process is divided into two different stages,namely the frost propagation in the droplet and the ice bridge propagation between the droplets.Through the theoretical model and experimental tests,the intra-droplet propagation velocity on the surface with different wetting properties was obtained.Under the experimental conditions,the frost propagation speed in the droplets on the superhydrophobic,hydrophobic and hydrophilic surfaces was ~80μm/s,~400μm/s and ~900μm/s,respectively.The mathematical model of ice bridge propagation between droplets is established,and the law of ice bridge speed between droplets is obtained by experimental observation on different surfaces.In addition,the overall frost propagation speed of different wettability surfaces was tested experimentally,indicating that the size and distribution characteristics of the condensed droplets are the main reasons for the slow frost propagation speed of the superhydrophobic surface.Thirdly,in order to study the effect of surface wetting property and fin spacing between vertical heat exchanger on the growth rate of frost,a test platform for frost growth between vertical heat exchangers was constructed.heat exchangers were prepared.The plate heat exchangers with superhydrophobic,ordinary and superhydrophilic wetting properties are used for frosting experiments between two vertical heat exchangers.In the experiment,the dynamic characteristics of the frost growth between the heat exchangers were realized by continuous shooting with a high-definition digital camera.Results show that the frost growth rate of the superhydrophobic heat exchanger is significantly lower than that of the superhydrophilic and ordinary heat exchangers.For the frost growth between heat exchangers with different fin spacing,there is a critical point of growth speed of the frost(xcutoff).Before reaching the critical point,the thickness of the frost layer is a linear rapid growth stage,and the frost growth slows down once fin spacing is less than the critical point.The critical point of the frost growth between the heat exchangers measured in this paper is xcutoff=1mm.Fourthly,a frost-resistant dry zone based on local pressure regulation of the LiCl desiccant solution was artificially constructed on the surface.The mechanism and influencing factors of the dry zone around the LiCl solution were studied by preparing the surface with different properties and using the micro-frozen test platform and the piezoelectric pulse micro-droplet distribution system.At the same time,a mathematical model for suppressing the formation of dry zone of condensation and frosting around the desiccant droplets was established,which theoretically revealed the mechanism and key influencing factors of the formation of the dry zone around the desiccant droplets.Biphilic surface for desiccant droplet arrangement was prepared,and the preliminary experiment of anti-frosting with desiccant drop and superhydrophobic surface was carried out,which realized frost-free operation of super-hydrophobic surface.This study provides a theoretical basis and guide of combination superhydrophobic surface with solution dehumidification.Finally,a frost-free air source heat pump system based on liquid desiccant dehumidification and compression-assistant regeneration is proposed.A test platform for the heat and mass transfer performance between air and desiccant solution under winter conditions was constructed.The effects of solution concentration,temperature,flow rate,ambient air temperature,humidity and flow rate on the heat and moisture transfer performance between air and solution were studied experimentally.Besides,the overall mathematical model of the frost-free heat pump system was established,and the effects of air temperature,humidity,solution regeneration temperature and air/solution flow ratio on the performance of the frost-free system were analyzed.The proposed frost-free air source heat pump system is compared with the performance of the reverse-cycle defrost heat pump system under the same operation conditions.It is found that the average COPaverage of the frost-free heat pump performance is 36.5-61.19% higher than that of the reverse-cycle defrost heat pump. |
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