Effect Of Surface Wettability On Droplets Freezing In The Initial Stage Of Frosting And Defrosting Process

Frosting is a widespread phenomenon in nature and industrial field.It has bad impacts on aircraft,power grids,wind turbines and outdoor heat exchangers.So it is necessary to suppress frosting as much as possible and to defrost regularly in the actual production and life.By changing the surface wettability to suppress frosting and promote defrosting,do not need additional equipment and energy consumption,is an ideal means to reduce the risk of frost.This paper has studied the frosting,defrosting and drainage processes of the surface with different wettability.The main works and results are as follows:(1)Experimental study on droplet freezing characteristics of surfaces with different wettability during initial frosting: Comparing bare aluminum with hydrophobic and superhydrophobic surface,the time that droplets begin to freeze is the fastest in bare aluminum,followed by hydrophobic,slowest in superhydrophobic.The droplets are observed to freeze from the surface edges and defects or small dust deposits,then spread in the form of frozen waves.The higher the cold surface temperature is,the smaller the relative humidity is,the slower the droplets freeze in superhydrophobic surface.(2)Experimental study on defrosting processes of horizontal surfaces with different wettability: The defrosting processes of bare aluminum and hydrophobic surface can be divided into three stages: frost melts and cracks,ice-water mixture ruptures and merges,melting water film shrinks.But in the defrost process of the superhydrophobic surface the meltwater bounces,curls,and become a series of droplets,then droplets join into large droplets.The longer the frosting time is,the more difficultly the meltwater breaks,the larger droplets it becomes.After the defrosting,the meltwater on the bare aluminum and hydrophobic distributes over the surface evenly,but the meltwater on the superhydrophobic surface distributes dispersedly,with most of the surface area exposed to air,and its surface coverage is very small.(3)Experimental study on the characteristics of defrosting drainage of surfaces with different wettability: During the defrosting drainage processes of vertical bare aluminum and hydrophobic surfaces,the frost layer begins to melt and split into multiple liquid films,and then the liquid films shrink to irregular or ellipsoidal droplets,remaining on the surface.During the defrosting drainage processes of vertical superhydrophobic surface,the frost layer is directly peeled off before melting,defrosting process is extremely fast,and there are almost no droplets on the surface.Further study finds that in the process of superhydrophobic defrosting,the longer the frosting time is,the more difficultly the meltwater break,the frost layer peeled off more completely,and the surface after the drainge is cleaner.And the greater the surface dip,the faster the meltwater will be removed.Under the experimental conditions of this paper,the coverage of defrosting water after drainage draining of the aluminum surface is the highest,the hydrophobic surface is the second,and the superhydrophobic surface is the lowest.