Study On Mechanism And Characteristics Of Frosting And Defrosting On Superhydrophobic Fin Surface

Air source heat pump(ASHP)is widely used as cooling and heating sources for heating,ventilation and air conditioning systems,due to its advantages of ability of both cooling and heating,energy saving,environmental protection and flexibility.Moreover,ASHP is actively promoted in "Clean Energy Instead of Coal" process in the north of China.However,frost formation is easy to occur on the surface of its fin-tube evaporator,when it is used for heating in winter.The adverse phenomenon seriously influences the heating performance and operation security of ASHP.Accordingly,exploring effective anti-frosting technology and defrosting method to maintain efficient and stable operation of ASHP,has become an important topic to promote the development of ASHP.In this paper,anti-frosting and defrosting methods for ASHP were put forward based on superhydrophobic modification of fin surface.The mechanism and characteristics of frosting and defrosting on superhydrophobic fin surface were revealed,and a novel defrosting method which combined air flow with superhydrophobic fin was developed to remove condensate droplets in the early frosting stage and cut off the frost layer growth.The detailed results are shown as follows:The microscopic behavior of frosting and frost layer property on superhydrophobic fin were firstly studied,and the theoretical system of the effects of the fin surface characteristics on frosting was established.The fins with different surface characteristics were prepared,and the frosting processes on fins were studied by visualization experiments.The behavior characteristics,including formation,growth,merging,jumping and freezing of condensate droplets,and subsequent frost crystals formation and frost layer growth,were all acquired.Experimental results show that fin surface characteristics have significant effects on condensation and freezing in the early frosting stage.Compared with the hydrophilic,bare and hydrophobic fins,the condensate droplets on the superhydrophobic fin are sparsest with lowest surface coverage rate,which indicates that the superhydrophobic fin has ability to inhibit water vapor condensation and condensate droplets freezing.The superhydrophobic fin shows the best anti-frosting performance,on which the frost thickness is 45.0%lower than that on the bare fin when the frosting process lasts 60min.Meanwhile,based on the phase transition kinetics and surface physicochemical theory,the phase driving force and nucleation energy barrier on fin were elucidated,and the relationship between the surface characteristic parameters and the nucleation density was established.The effects of fin surface characteristics on the growth and freezing of condensate droplets,frost crystals formation and frost layer growth were analyzed,and the theoretical system of the effects of the fin surface characteristics on frosting was formed.The theoretical study shows that the driving force of water vapor condensation is independent of the surface characteristics,but the nucleation energy barrier to be overcome increases with the increase of surface contact angle.And the nucleation density and the growth rate of condensate droplet all decrease with the increase of surface contact angle.At the same time,the defrosting characteristics and mechanism on superhydrophobic fin were studied.The defrosting process was microscopically visualized,and the effects of fin surface characteristics and different defrosting conditions on defrosting process and melting water retention were analyzed.The experimental results show that the defrosting processes of the hydrophilic,bare and hydrophobic fins are similar,in which the frost layer is divided into several blocks accompanying with melting,and the frost-water mixed blocks finally shrink into water droplets because of the surface tension.However,frost layer rolls up from the edge of the superhydrophobic fin at the beginning of melting and directly sheds from it.Further analysis shows that only the superhydrophobic fin has large surface contact angle and small contact angle hysteresis,can the frost layer directly shed from the surface.The fin surface characteristics have effects on the shape and mass of melting water retention.The retained water mass on the superhydrophobic fin is 90.8%and 87.3%less than that on the hydrophilic and bare fins.With respect to theoretical research,the energy conservation of melting-water shrinkage was established to reveal the variation law of surface free energy during defrosting process.Based on the "saturation theory" of melting water retention,a model was developed to predict the critical radius of retained water droplets and retained water mass.The calculation results show that the critical radius and retained water mass all decrease with the increase of surface contact angle and the decrease of the contact angle hysteresis.The study scale was then raised to the heat exchanger based on the mechanism of frosting and defrosting,and the fin-tube heat exchanger with superhydrophobic surface was prepared.The superhydrophobic heat exchanger was prepared by follow three-step process:firstly,etching in the solution to roughen the surface;then,boiling in water to create nano-arrays;finally,fluorinating to form superhydrophobicity.A frosting/defrosting experimental system was constructed to study the frosting/defrosting performances on heat exchangers with different surface characteristics.The frost mass on the superhydrophobic unit is reduced by 18.0%and 38.6%compared with the hydrophilic and bare units.The superhydrophobic unit shows the best heat transfer rate during the frosting process.In the aspect of defrosting,the defrosting time of the superhydrophobic unit is shortened by 41.7%and 43.2%,and the energy consumption for defrosting is reduced by 47.2%and 61.9%.The superhydrophobic heat exchanger not only shows excellent anti-frosting performance,but also reduces the defrosting energy consumption and improves the defrosting efficiency.The defrosting time and defrosting energy consumption on the bare unit all decrease with the increase of defrosting temperature,while it almost has no influence on the superhydrophobic unit.Based on the above study,a generalized defrosting method was put forward,by using air flow intermittently action on superhydrophobic fin to remove condensate droplets in the early frosting stage.When the air flow was applied to the superhydrophobic fin under frosting condition,most of the condensate droplets were instantaneously blown away due to the weak surface adhesion,and a few small retained droplets were evaporated quickly in seconds.The blowing model and evaporation model of condensate droplets were developed,and it was found that increasing the fin surface contact angle and reducing the contact angle hysteresis could further improve the removal efficiency of condensate droplets.An ASHP system was designed to achieve the air flow action on the superhydrophobic heat exchanger.When the air flow was applied,there was no frost on the heat exchanger after the ASHP running for 120 min under frosting condition of Ta = 0.0℃ and RH = 90%.Compared with the traditional reverse defrosting,the novel defrosting method increased the heating efficiency of the ASHP by 13.2%.The novel defrosting method can avoid the disadvantages of existing defrosting methods and is expected to realize the efficient operation of ASHP in winter.