Experimental Studies On Refrigerated Vehicles Plate Heat Exchanger Surface Of Frost Formation And Its Control

Frost phenomenon wingspread exists in some areas such as aerospace,transportation,and air conditioning heat pump.When cold storage refrigerated vehicles in transit,the storage plate will appear frosting.Frost thickening makes thermal conductivity continue to increase,while reducing the heat transfer efficiency,resulting in decreased cooling capacity.And making the temperature fluctuations in the compartment,which is unfavorable to food preservation.Therefore,the study of cold surface frosting process is very important.The characteristics of the cold surface will have a great impact on the frosting process.Therefore,this paper studies the hydrophobic coating and hydrophilic coating on the frosting process.For the first time,the application of hydrophobic and hydrophilic coatings to the surface of plate heat exchangers in refrigerated vehicles has been proposed to restrain the growth of frost,so as to improve the refrigerating efficiency of refrigerated vehicles.In this paper,the frosting process on the cold surface is the research object.And the goal is to achieve restraining the growth of frost layer.The microscopic observation of the frosting on the cold surface is carried out mainly by stereo microscope and CCD camera.And the frost layer thickness on the cold surface with different characteristics was measured.The main works are as follows.(1)In this paper,aluminum plate is used as a cold surface to experimentally simulate frost formation on the surface of plate heat exchanger in refrigerated vehicles.The cooling of the cold surface is realized by using semiconductor cooling system,which has the advantages of simple structure and flexible control.The cooling fan is used to cool the hot side of the semiconductor,which easy to install and have good heat dissipation.(2)It is found that the frost formation on a cold surface generally tracks a course like the following: formation and growth of water drops,freezing of the sub-cooled drops,formation and growth of initial frost crystals on the frozen drops,growth of frost layer and melting of frost crystals and layer.The surface temperature is differently,the surface frosting process is also different.Super-cooled condensate drops freeze earlier and smaller freezing drops are seen for lower surface temperature.And the frost crystals appear earlier.(3)In this paper,by contrasting the frosting process on the surface of aluminum,hydrophobic coating and hydrophilic coating,it is found that hydrophobic coating and hydrophilic coating can not only effectively delay the appearance of frost,but also have loose distribution of frost.By comparing the growth rate of the frost layer thickness on the aluminum surface,it has been found that the hydrophobic and hydrophilic coatings both effectively restrain the growth of the frost layer.(4)Based on the thermodynamic and crystal physics theory,the hydrophobic coating increases the phase transition energy barrier due to increase the surface contact angle,which can effectively restrain or slow down the frost on the cold surface.The reason why the hydrophilic coating restrains frosting is that on the one hand,the hydrophilic coating absorbs the surface water vapor,reduces the partial pressure of the surface water vapor,so that the nucleation driving force is reduced.On the other hand,since the hydrophilic coating absorbs the water molecules,the freezing point of the water dissolved therein is reduced.That it can restrain the growth of the frost layer effectively.