An Experimental Study On Effect Of Surface Characteristics On Frost Deposition On Low-temperature Surfaces

Frost deposition is an extremely complicated heat and mass transfer phenomenon,which is widely observed in industrial operation and production and daily life.The presence of frost layer often hinders the normal operation of systems,such as increasing heat transfer and flow resistance,severely reducing heat transfer efficiency,and causing energy loss.Therefore,during last decades,exploring effective ways to suppress frost had attracted much attention of both various industries and scientists.However,most of frost suppression had been on ordinary-low temperature conditions（>-30℃）,frost suppression on low-temperature（-30^-153℃）and cryogenic-temperature（<-153℃）surfaces has been seldomly touched.Surface modification has been proven to be an effective frost-reducing method in ordinary-low temperature frosting.Based on this understanding,in this paper,the surface modification of the plain copper surface was carried out,aiming at exploring the effect of surface characteristics on frosting process in low temperature field.The main work conducted are summarized as follows.（1）Influences of super-hydrophobicity on frost deposition on low-temperature surfaces.A series of systematic frosting experiments on superhydrophobic and plain copper surfaces at low temperatures were carried out.The effects of hydrophobicity on frost coverage,frost thickness and frost crystals morphology were given.Superhydrophobic surfaces（CA=155.6°）were prepared on plain copper plate by chemical etching.And we carried out frosting experiments at different cold surface temperature and air humidity.It was found that the frost crystals on the superhydrophobic surface was much denser compared with plain copper surface when the initial frost crystals formed by desublimation.This was completely different from the condensation-freezing frost deposition mode in which the frost formation on the superhydrophobic surface was suppressed significantly during the initial period of frost deposition.This phenomenon is the first discovery in the field of the effect of hydrophobicity on frost deposition.Furthermore,it was found that the effect of hydrophobicity on frost deposition weakened with the cold surface temperature dropped or the air relative humidity increased.Hydrophobicity had a significant effect on frost crystals morphology under condensation-freezing frost deposition mode,but hydrophobicity had no effect on frost morphology under desublimation frost deposition mode.Finally,the phenomenon observed were also analyzed according to the theory of cloud physics and phase transformation principle.（2）Effects of contact angles on frost formation on low-temperature cold surfaces.A series of different contact angle hydrophobic surfaces（97.6°¹56.7°）on copper plate were prepared by chemical etching and carried out systematic frosting experiments on these surfaces at low temperatures.The effects of contact angles on frost coverage,frost thickness and frost morphology were given.And it is founded that the frost coverage of the surface is not proportional to the contact angle in the case of air temperature 25℃,air humidity 50%,and cold surface temperature-30℃.The frost crystal coverage on the surfaces with different contact angles was 140.1°>155.6°>129.6°>97.6°in turn.This was completely different from the effect of contact angle on frosting under the condensation-freezing frosting mode.Frost suppression effect enhanced with contact angle increased.Finally,the phenomena were explained from the cloud physics and phase transition principle according to the microscopic structure of each surface.（3）Development of superhydrophilic paints for restraining frosting on low-temperature surfaces.Frosting experiments with different formulations hydrophilic paint painted surfaces were carried out.The effect of component used in hydrophilic coating on the anti-frosting effect was given.The frost suppression effect on the surface of different salt components was CaCl₂>MgCl₂>NaCl>Ca（CH₃COO）₂ in the case of the cold surface temperature-20℃,the ambient air temperature 25℃ and the relative humidity 50%.And frost suppression effect enhanced with the decrease of the freezing point of salt water solution.（4）Influences of Calcium salt based superhydrophilic paint on restraining frost deposition on low-temperature cold surfaces.Finally,frosting experiments with hydrophilic coating surface added calcium salt（No.2）were conducted for different cold surface temperatures and relative humidity.The frost coverage and frost thickness of hydrophilic coating surface and plain copper surface were measured and compared.The lowest cold surface temperature with frost suppression and frosting process on hydrophilic coating surface were given.The results showed that,the hydrophilic coating surface had no significant frost suppression effect at-50℃ when the air temperature was controlled at 25℃ and the relative humidity was controlled at30⁵0%.The hydrophilic coating surface had no significant frost suppression effect at-45℃ when the air temperature was controlled at 25℃ and the relative humidity was controlled at 30⁵0%.The anti-frost effect of hydrophilic coating weakened with the decrease of cold surface temperature and the increase of air humidity.There were obvious dendritic frost crystals on the surface of hydrophilic coatings during the initial period of frost deposition in the case of the cold surface temperature-40℃.But these frost crystals were extremely unstable,which would melt into liquid state on the surface,the liquid water frozen again,and then stable frost crystals grew on the ice-water mixture.This phenomenon was the first time to be observed.