| Frosting on the cold surface will increase the thermal resistance and energy consumption of heat transfer and cause harm.For example,frost on the fan in the cold storage system can reduce the heat transfer efficiency,increase the resistance of the air flow and reduce the COP of the cold storage system;the frost on the spacecraft makes the communication satellite unable to enter the expected orbit.It can be seen that frosting has a very serious impact on refrigeration,aerospace and other fields.Many scholars at home and abroad are committed to the mechanism and experimental research of frost formation,and an effective way to suppress frost is needed.However,most scholars focused on the growth stage of frost layer,ignoring the initial stage of frost crystal formation.Freezing of droplets is the initial condition for numerical simulation of frost crystal growth and frost formation.It is very important for the freezing of droplets.1.In this paper,the freezing of droplets on the high precision temperature controlled cold and hot table was observed by microscope and compared with the freezing process of the droplet under the action of the additional electromagnetic field.The study found that:(1)The droplet will form bright stripes on the surface after freezing and the top will form convex crystals,and the frost crystals grow on the convex crystals.(2)The supercooling degrees of droplets at the electromagnetic field strengths of 0Gs,20 Gs,25Gs,30 Gs,35Gs,40 Gs,45Gs,50 Gs,55Gs,60 Gs are: 5.83°C,6.45°C,6.57°C,8.23°C,7.95 °C,7.83 °C,7.75 °C,7.68 °C,7.49 °C,8.01 °C,respectively;When the direction of the electromagnetic field is changed,the supercooling degrees of droplets are 5.83 °C,6.34 °C,6.56 °C,7.56 °C,8.34 °C,7.67 °C,7.78 °C,7.13 °C,7.42 °C,7.91 °C,respectively.Taking into account thermodynamics,crystal mechanics,crystallography and electromagnetic theory,the theoretical calculation of the increase of droplet supercooling degrees under electromagnetic field is carried out.In addition,it is observed through the microscope that the droplet freezes to form smaller crystals under the action of the electromagnetic field.2.We observed the initial frosting process on a cold platform without magnetic field and magnetic field by means of a microscope.(1)No matter whether there is a magnetic field,the initial stage of frosting is divided into: nucleation-phagocytosis and growth of droplet-contraction and height of the droplets-droplet freezing-frosting.Under the action of magnetic field,the droplets formed on the cold platform are smaller and more evenly distributed.(2)Under the action of electromagnetic field,the condensation temperature and freezing temperature of droplets decrease on the cold surface.When the intensity of electromagnetic field is 0Gs,10 Gs,20Gs,30 Gs,40Gs and 50 Gs,the condensing temperature of the droplets are 4.71?,4.62?,4.4?2,4.31?,4.21?,4.01?respectively,and the crystallization temperature of the droplets are-12.11,-13.11,-13.22?,-13.65?,-14.42? and-15.42?.When the direction of the electromagnetic field is changed,the temperature of the droplets are 4.71?,4.52?,4.32?,4.21?,4.15?,3.96?,respectively,and the freezing temperature of the droplets is-12.11?,-12.91?,-13.12?,-13.55?,-14.22?,-15.62? respectively.(3)The quality of frost on the cold surface is reduced under the action of electromagnetic fields. |
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