| Frost formation is a complex heat and mass transfer process,which widely exists in natural and industrial fields.In heat exchange equipment,the frost layer increases the resistances of flow and heat transfer,reduces heat transfer efficiency.In the aerospace field,frost formation affects the aerodynamic feature of aircraft,which may lead to serious hidden dangers threatening flight safety.Therefore,it has been an important task in scientific research and engineering application to investigate the mechanism of frost formation as well as to find effective frost restrain or eliminate methods.Compared with traditional defrosting methods,the defrosting technologies with the application of magnetic field and spraying condensable additives are significant effective ways with less energy consumption,and they have a broad application prospect.However,current investigations are mainly focused on the qualitative analysis under a particular condition,and the research on the mechanism of magnetic field and condensable additive with multi-factor affecting and accurate prediction method should be further carried on.In this thesis,the freezing process was studied where wet air flows across the cold surface.By combining theoretical analysis with experimental research,the mechanism of the defrosting method with imposing magnetic field and spraying condensable additive was investigated based on phase transformation dynamics,electromagnetism.The model of system energy change in the frosting process under magnetic field was established,and the quantitative transformation relationship between magnetic energy and super-cooling degree was put forward.The variation of critical nucleation radius and critical free energy with magnetic field intensity was analyzed quantitatively under the conditions of different super-cooling degrees,ambient temperatures,and humidities.Super-cooling degree,cold surface temperature,and magnetic field intensity were considered as the main factors influencing the frosting process under the magnetic field.A quantitative correlation between the ratio of critical nucleation radius without a magnetic field to that with a magnetic field,super-cooling degree,cold surface temperature,and magnetic field intensity was established.A visualization experimental platform of frosting characteristics under a magnetic field was established,on which the processes of condensation of water vapor,formation of frost particles,frost crystal growth,and frost layer deposition on the cold surface under the magnetic field can be studied using microscope observation and analysis.The influence of magnetic field on the frosting process was obtained by combining the variation characteristics of frost crystal freezing time,frost thickness,and frost layer quality under the conditions of different super-cooling degrees,cold surface temperatures,magnetic field intensities,and relative humidities.The experimental results showed that when the magnetic field intensity was 38mT,the frost coverage on the cold surface was decreased from 85%to 42%.Super-cooling degree and ambient relative humidity were driving forces of phase transformation.When the super-cooling degree was larger,the liquid droplets were frozen and they didn’t grow up sufficiently.The freezing occurred at the earlier stage of phase transformation and the average radius of the frosting nucleus was smaller.The average radius of the frosting nucleus increases with relative humidity,and the effect of the magnetic field was more effective under higher humidity of 30%to 90%.Based on the quantitative transformation relationship between magnetic energy and super-cooling degree,the correlation formula of critical nucleation radius with the supercooling degree and magnetic field intensity was obtained.The deviation between theoretical calculation and experimental results was within 20%.The mechanism of the combined defrosting method with magnetic field and surface wettability was studied.The quantitative variation of critical nucleation radius and critical free energy with the super-cooling degree,relative humidity,contact angle and magnetic field intensity was obtained.A surface tension model of the solid-liquid interface under magnetic field was established,and the variation of water wettability with magnetic field on the cold surface was studied.The mechanism of the combined influence of the magnetic field and surface wettability on the frosting processes was obtained.At the beginning of frosting,the contact angle decreased under the magnetic field,the form of droplets on the cold surface was small molecular cluster,and the critical nucleation radius decreases.At the middle and late stages of frosting,the surface was covered by the frost layer.Although the influence of surface wettability on the frosting process was weakened,the magnetic field changed the structure of water molecules,resulting in the slow growth of the frost layer,and the magnetic field has an inhibitory effect on the frosting process.The variations of critical nucleation radius and critical free energy with magnetic field intensity,contact angle,and cold surface temperature were analyzed theoretically.The mechanism of the combined influence of magnetic field and surface wettability on the frost formation process was experimentally studied.The variations of the average layer thickness,the average radius of frost particles,and the quality of the frost layer with the magnetic field intensity and contact angle were obtained,respectively.Considering the combined effect of magnetic field and the surface wettability,the average radius of frost particles on the hydrophobic surface was the smallest,which was 50%and 30%of those on the bare surface and the hydrophilic surface under the same conditions,respectively.With the increase of contact angle,the freezing time of the frost layer became long,and the quality and density of the frost layer were decreased.When the magnetic field intensity was 38mT,the masses of frost on the bare and hydrophobic surfaces were 84.4%and 70%of that on a hydrophilic surface,and the frost densities were 93.4%and 87.8%respectively.The variation of the frosting process under the combined action of magnetic field and surface wettability was obtained.At the initial stage of frosting,due to the different spreading states of droplets on different surfaces,the state on the surface of hydrophilic materials existed in the form of liquid film,with a large contact area and sufficient heat transfer.Therefore,the condensation speed was fast.While the droplets were spherical on the hydrophobic surface,the contact area between the droplets and the cold surface was reduced,and the frost crystal growth was relatively slow.At the middle and late stages of frosting,the influence of surface wettability on frost formation was gradually decreased,and the defrosting effect of the magnetic field was enhanced.The characteristics of heat transfer and flow during frosting on ultra-low temperature surfaces(temperature was-196℃)were studied.The effects of ethanol injection ratio on the heat transfer coefficient and pressure loss in the air side were obtained under two surface temperatures.The correlations of total heat transfer coefficient with time under different cold surface temperatures and ethanol injection ratios were proposed.On the one hand,spraying condensable additive could reduce the freezing temperature of the liquid on the cold surface to restrain the frosting process.On the other hand,the condensable additive could fill the void in the original water vapor frost layer.The porosity was reduced and the frost layer was compacted,which resulted in the reduction of the thermal resistance.Ambient relative humidity and undercooling were considered to the main factors affecting the frosting state,flow and heat transfer performance of the heat exchanger.When the ambient humidity was high,the frosting mass on the cold surfaces was large.The heat transfer coefficient under the relative humidity of 71%was 2.5 times of that under the relative humidity of 27.5%,and the corresponding pressure loss increases.Sub-cooling also affected the heat transfer performance.Under the same conditions,the frost on the cold surface with lower temperature was formed faster,and the pressure loss outside the tube of the heat exchanger increased significantly.When the cold surface temperature was-33℃ and the relative humidity was 27.5%,with the injection ratios of 1:0.1 and 1:0.2,the heat transfer coefficients were increased by 30%and 60%respectively compared with no injection.When the cold surface temperature was-196℃,with the injection ratios of 1:0.4 and 1:0.6,the heat transfer coefficients were only increased by 10%and 20%compared with no injection.Only when the ethanol injection quantity was 2 to 3 times of that on-33℃ cold surface temperature,an obvious frost suppression effect could be obtained. |
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