Study On The Condensation Frosting Characteristics Of Copper Surfaces With Different Wettability

Air source heat pump is a kind of clean energy equipment,its promotion plays an important role in improving the energy utilization efficiency and realizing energy saving and emission reduction.However,the accumulation of frost on the heat transfer surface when it is used for heating may cause serious problems.Due to the advantages of energy saving,environmental protection,high efficiency,and low cost,anti-frosting technology with surface modification has a good application prospect.But in fact,the anti-frosting technology with surface modification and the defrosting technology have some shortcomings.In this paper,the condensation and freezing characteristics of droplets and the frosting and defrosting characteristics of copper surfaces with different wettability were studied.The detailed contents are shown as follows:Theoretically analyzed the nucleation and the growth of condensation droplets and freezing process at the initial of condensation frosting.The influence of surface characteristics on the droplet growth and the freezing time were analyzed,which provided reasonable explanation for the phenomenon that hydrophobic surface can delay frosting.In this paper,combined modification of metal vapor vacuum arc(MEVVA)ion implantation and laser texturing were used to fabricate the anti-frosting copper surface.With the high energy of laser,micro and nano structures were established on the surface.With the implantation of carbon ions,the chemical composition of the surfaces was changed which may enhanced the mechanical property of copper surface.This combined modification method fabricates the superhydrophobic copper surface without changing the heat transfer performance of the metal.This method has a wide field of application owing to the simple,environmentally friendly process.Based on the analysis and fabrication of the anti-frosting surface,the freezing process and the growth of the initial frost crystals have been studied by experiments.A single droplet was set as the experimental subject,the temperature variation and the morphology change of the droplet during freezing were analyzed.The frozen droplets were regarded as pure solid and the influence of the dissolved air was ignored in the relative research.Experimental investigation shows that the dissolved air may increase the opacity of the frozen droplet and have an effect on the temperature and morphology variation during the freezing process.A physical model of freezing droplet was established,considering the influence of the dissolved air,the formation of the tip singularity of frozen droplet and the shape of the freezing droplets were studied by calculation and experiments.The tip angle decreased with the content of dissolved air,and promoted the growth of the initial frost crystal.The water vapor molecules tend to deposit at the tip of frozen droplet and form the initial frost crystals.The frost crystals grow faster on the sharper frozen tip.The growth of the frost layer on vertical surfaces with different wettability and its influence factors have been studied by experiments.The effect of the ambient conditions,the surface temperature,the air speed and the surface characteristics were studied.The frost thickness increased with the relative humidity and decreased with the surface temperature.The effect of air speed was different during frosting process,before the full growth period,the frost thickness first increases and then decreases with the air speed.While,during the full growth period of the frost layer,the exists of the air inhibits the growth of frost.The super-hydrophobic surface with combined modification has advantages in anti-frosting.Compared with the untreated copper surface,when the thickness of the frost layer reaches 1.5 mm,the growth of frost can delay by 43%.In addition,the defrosting characteristics of the frost layer and the retention of defrost water on surfaces were observed and analyzed.Compared with the untreated surface,the hydrophilic surface and the sliding surface,the super-hydrophobic surface had the shortest defrosting time and the minimum retained water.By establishing the model of a single block of frost-water mixture during the deformation process,it is found that the surface contact angle and the area of the single block are positively correlated with the difficulty of the droplet shedding.Combined with the movement characteristics of the frost layer during the defrosting process,the effect of surface characteristics on the dust removal performance was studied.It is found that the super-hydrophobic surface had better dust removal performance and the dust removal rate is 2 to 4 times higher than that of the other three surfaces.The scraping force of frost layer on different surfaces have been studied by experiments.The scraping force of frost layer under different frosting time/test distances were tested by the scrape test bench.As the increase of the frost thickness,the scraping force of frost layer on the mechanical machined surface and the hydrophobic surface gradually increased,the scraping force is smaller than that of untreated surface.For untreated surface,machined surface and hydrophobic surface,when the test distance is greater than 0.2 mm,the scraping force of the frost layer tends to be stable,while,the scraping force within 0.2 mm increases exponentially.According to the structure of the initial ice layer and the structure of the porous frost layer,the calculation model of frost adhesion strength was established.The results show that,due to the porous structure of frost,the adhesion strength of frost layer on copper surface is about3.6 k Pa,which is much lower than that of ice layer(150～500 k Pa).The actual contact area between the frost layer and the surface decreases with the contact angle,which induces a smaller adhesion strength of the near surface frost layer.The hydrophobic surface is more favorable for mechanical scraping of the frost layer.