【作者】 张新华；

【导师】 刘中良；

【作者基本信息】 北京工业大学 ， 热能工程， 2006， 硕士

【摘要】 由于霜层会增加传热热阻、降低能量的传递速率,因此结霜现象对于制冷、低温和航天技术的影响十分严重。本文主要研究亲水表面结霜和外加电场对霜的生长的影响,力求寻找一种既经济又节约能源的抑霜方法。首先对水分子和冰晶的物理特性进行了简要介绍,讨论了在电场下与冰晶生长相关的一些基本特性。为了加深对晶体生长阶段的认识,本文对某一具有确定温度的冷壁面突然与一定环境条件下的湿空气接触时冷壁面上的成核过程进行了研究。结果表明,冷壁面温度越低,水珠冻结时间越短,且冻结的冰珠越小;当冷壁面温度高于某一临界值,譬如,在我们给定的实验条件下,-12°C时,空气相对湿度越大,冷壁面上凝结的水珠越大,水珠冻结越晚,但当冷壁面温度较低时,几乎观察不到空气相对湿度对水珠冻结时间的影响;冷壁面温度高于某一临界值,譬如,在我们给定的实验条件下,-20°C时,发生凝结成霜,冷面温度低于这一临界值时,发生凝华成霜;随着实验工况的不同,观察到的霜晶呈现八种不同的形态。冷壁面上着霜在很大程度上是一种界面现象,因此,冷壁面的性状必然会对着霜过程产生重要影响。本文利用纳米SiO2/ TiO2的超亲水性制作了一种涂层。研究发现,这种涂层能够在一定程度上抑制霜层的生长,并且这种涂层牢固性好,重复使用性高。另外,这种涂层的传热热阻非常小,与具有较大热阻的霜层相比可以忽略。本文对高压静电场作用下紫铜板表面霜晶的生长进行了观察,研究发现,电场能够改变霜晶形态,控制霜层的厚度,减少结霜量;壁温越低,霜层越厚,但壁温对着霜质量的影响很小;环境温度对霜层的厚度影响不大,但对着霜质量的影响很大。通过研究电场作用下亲水表面上霜层的生长,发现亲水表面比紫铜表面能够更好地抑制霜层厚度,但使结霜质量增加。本文还从改变电极形状和电场强度出发,分别研究绝缘线电极、铜板电极以及网状电极形成的电场下的霜层生长情况,结果表明,将绝缘电极线直接固定在冷板上的电极布置方式不能控制霜层的生长,板状和网状电极电场下霜晶的生长速度明显高于无电场和线电极电场下霜晶的生长速度,能够清楚地看到细长的针状霜晶。更多还原

【Abstract】 Frost accumulation has a severe negative effect on refrigeration, cryogenic and many other engineering systems due to the increased flow and heat transfer resistance and the reduced energy transfer rate. In order to seek the effective defrosting and frost-controlling methods, frost growth on hydrophilic surface under natural conditions with and without the existence of external electric field were investigated in this thesis.The physical characteristics of water molecule and ice crystal were described, and properties relating with the ice crystal growth under external electric field were discussed.In order to understand the frost crystal growth, the nucleation process on the cold plate with constant temperature was investigated. The experimental results revealed that the lower the cold plate temperature is, the shorter the frozen time of water droplets, and the smaller the frozen water droplets. If the cold plate temperature is higher than a certain critical value, for instance, -12°C under our observed experimental conditions, the higher the air relative humidity is, the bigger the water droplets concreting on the cold plate, and the later the water droplets froze. However, if the cold plate temperature is lower than a certain critical value, the influence of air relative humidity on the frozen time of water droplets was not observed. If the cold plate temperature is higher than a certain critical value, for example, -20°C under our observed experimental conditions, frost deposition is through condensation, or condensation frosting. However, if the cold plate temperature is lower than a certain critical value, frost deposition is by ablimation, or ablimation frosting. We identified eight different frost crystal patterns under various experiment conditions.Frost formation on cold surface is a typical interface phenomenon, so the cold surface characteristics should have an important influence on the process of frost formation. Using of the strong hydrophilicity property of nano-SiO2/ nano-TiO2, a hydrophilic coating was made. We found that the coating could restrain the frost layer growth to certain extent. The coating shows a good fastness and repetitive-use performance. In addition, the coating has a very small heat transfer resistance, and the heat transfer resistance may be neglected compared with that of the frost layer. The frost growth on copper plate surface under electric field was observed in this thesis. We found that the applied electric field may change the shape of frost crystals, control the frost layer更多还原