| Frost deposition is a common phenomenon in refrigeration and cryogenics,wind power generation, aerospace and other fields. The existence of the frost layer seriously influences the operational efficiency of refrigeration and air conditioning systems and the safety of aerospace aircraft flight. The residual water droplets will exist after traditional heat defrosting,these droplets will have a huge impact on the next frosting cycle. Melt water droplets will still remain on the surface and refreeze in the next frosting cycle, forming thick substrate on the cold surface. Frozen water droplets will increase the cohesive force between frost layer and cold surface, brought in negative influence on defrosting. Hence, searching for a effective method of removing the water droplets and building aneroid environment before frost to inhibit the growth of frost layer have important scientific significance and engineering application value.At present, the traditional mothed to remove the water droplets after defrosting is heatingevaporation. Although it can successfully remove the water droplets,it also brought some corrupt practice such as long time, high energy consumption and huge temperature fluctuations.In recent years, ultrasonic has been wild used in heating ventilation air conditioning and refrigeration domains such as atomization dehumidification, industrial dust removal,descaling heat exchanger,due to its good characteristics of high frequency, short wave length and energy concentration. Therefore, this article introduces ultrasonic in frost resisting and defrosting field. Using acoustics, thermodynamics, kinetics of phase transformation, interface theory and so on, this article studied the growth process of frost layer on cold surface deeply.Meanwhile, the growth and distribution regularities of melt water droplets are analyzed in virtue of the melt and frost cycles. The negative impact of water droplets on next frosting process was put forward. The possibility of ultrasonic vibration on the release of the melt water droplets from cold surface was investigated. In summary, the main research contents and conclusions in this paper are as follows:(1) Microscopic visualization experimental platform of frost layer growth on horizontal cold surface was designed and build. The whole process of the frost layer growth on cold surface was observed and described carefully. The frosting process was divided into four stages according to the characteristics during frost layer growth process and the evolution rules of frost crystals. These four stages are condensate droplets formation stage, condensate droplets freeze and formation of initial frost crystals stage, frost crystal growth stage, frost layer sufficient growth stage.(2) The theory of water vapor on the surface condensation into cold nucleation in the early period of frosting was analyzed. It was put forward that the driving force of water vapor condenses into nuclear is phase transformation driving force. Transformation driving force formula and the critical nucleation radius formula were deduced according to the theory of phase transformation kinetics and interface theory. Analysis shows that the cold surface temperature lower, the air moisture content greater, the phase transformation driving force will be greater. The water vapor will be easier to condensate on the cold surface.(3) Taking advantage of microscopic visualization experimental platform of frost layer growth on horizontal cold surface, melt-frost-melt cycle process was carried out. Then the water droplets residual phenomenon after heat defrosting was put forward. Regularities of growth and distribution of the melt water droplets was analyzed. The size distribution of melt water droplets can be regard as chi-square distribution of degrees of freedom as six. The coverage rate of melt water droplets changing with times of frosting under different cold surface temperature and different frost time was explored. Here comes to a conclusion that:when the average particle size of the droplets are under 0.1 mm, the dominant factor affecting droplets coverage rate is frost deposit amount. When the average particle size of the droplets is between 0.1mm to 1.0mm, the dominant factor affecting droplets coverage rate is droplet polymerization. When the average particle size of the droplets is over 1.0mm, the dominant factor affecting droplets coverage rate is frost deposit amount.The experiment also proves that: the exist of melt water droplets have brought serious adverse effects on the process of frosting again. First of all, defrosting droplet after frozen prompted the frost crystal growth started ahead of time. Meanwhile, The frozen water droplets became into frost layer basal which increased the cohesive force between frost layer and cold surface. It certainly fight against the removal of the frost layer. What's more, the frost layer broke the stable growth height duo to melt water droplets. The maximum height of frost layer can exceed stable growth height for more than 2 times in this experiment.(4) In order to searching for quick and efficient method of removal water droplets, The contrast experiment of removing water droplets by ultrasonic and heating evaporation was conducted. Experiments verify the feasibility of removing the water droplets by ultrasonic.Ultrasonic show many excellent qualities during the process of removing the water droplets,such as short removal time, low energy consumption and good thermal stability. The water droplets removal time by ultrasonic is no more than 1/60 of removal time by heating method.Its' energy consumption is only about 1/100 of heating method. During the droplets removal process by ultrasonic,the maximum temperature rise of the aluminium plate is just 5.7 ?.In addition, the temperature rise can recover within 3 minutes. However, the maximum temperature rise of the aluminium plate is 177.6 ? under heating. The numerical value is 31.2 times of ultrasonic vibration. What's worse, the aluminum plate temperature fluctuations didn't die down even after 25 minutes later.Experiment also showed that: the mechanism of removing water droplets by ultrasonic is not heating effect. It can be cavitation effect and mechanical effect. From the experiment of removing different size water droplets by different power ultrasonic, we can conclude that:when the ultrasonic power under 54W, the droplets removal probability decreases obviously.When the ultrasonic power upon 54W, the droplets removal probability reach close to 100%.Improving the ultrasonic power, the removal time cann't be cut down obviously. The energy consumption is also essentially unchanged. The removal manifestation doesn't increase distinctly as well. |
PDF Full Text Request