【作者】 侯普秀；

【导师】 虞维平；

【作者基本信息】 东南大学 ， 制冷与低温工程， 2006， 博士

【摘要】 结霜现象广泛存在于制冷、低温工程、航空航天等领域,霜层的存在带来诸多不利的影响。本文从实验以及理论两个方面对霜层生长现象进行研究,明确了影响霜层生长过程的各种环境因素,提供了霜层生长的预测,为除霜以及抑制霜层的研究建立基础。首先设计了实验装置,整个实验系统由制冷装备、测温设备、图像采集系统以及环境温湿度控制系统四部分组成。通过这套系统,可以调整环境温度湿度,冷表面的温度以及表面接触角,来研究不同条件下霜层的生长状况。实验研究表明,冷表面温度主要影响霜层的高度以及冰晶体的形态;相对湿度对霜层的高度以及密度都有明显影响;在表面添加涂层后,车蜡涂层表面以及丙三醇表面上霜层出现时间较晚,霜层高度明显降低,冰晶体分布比较稀疏。对成核过程的分析表明,过饱和度以及表面接触角对于成核速率有着明显的影响。在过饱和度较低的时候,随过饱和度的增加,成核速率变化较为迅速;当过饱和度较高的时候,成核速率变化较为缓慢。吸水性涂层抑制霜层的原理在于通过改变局部的水蒸气过饱和度来影响成核速率,进而影响霜层的生长速度。憎水性涂层抑制霜层生长的原理则在于高接触角表面使得成核难度增加。采用计盒维数方法对霜层生长图像分析表明,充分生长的霜层在一定尺度范围内具有分形特征。这一特性说明霜层的结构是具有层次性的,在此存在一个尺度,当度量尺度大于此尺度时,霜层可以看作是均匀的;当小于这个尺度时,霜层满足自相似性,这一尺度内霜层的形成过程可以采用渗流结构来描述。以此为基础建立霜层结构模型,计算霜层的导热系数以及扩散系数。模拟计算的结果表明,孔隙率较高的时候,霜层的导热系数变化比较平缓;当接近渗流临界值的时候,霜层导热系数随密度的增加迅速上升。当孔隙率比较小的时候水蒸气在霜层中的扩散系数上升比较缓慢;当孔隙率较高时,孔隙率的增加有可能将多个较小的相邻孔隙合并为大孔隙,从而使得有效扩散系数迅速上升。最后在实验观察的基础上,提出了考虑附着概率的霜层生长的一维随机模型。对于前期的霜层,这个模型可以比较准确地预测霜层的生长。对于后期的霜层,这个模型对于冰晶体沉积量的预测比较准确,对霜层高度的预测则出现了较大误差。对于后期的误差,可以采用前期的实验图像来修正。修正预测的霜层高度以及晶体沉积量都比较符合实际。更多还原

【Abstract】 Frost is a familiar phenomenon in refrigeration, cryogenics and spaceflight, it brings many adverse affect. In this paper, frost growth was studied by experimental and theory method, the environmental factors influence frost growth was confirmed and the forecast of frost growth was given, all this work establish a foundation for restrain the frost growth and defrost.First, the experimental equipment was designed, all this system was consisted in four part:Refrigeration equipment, thermoscope, picture-collection system and temperature-humidity control system. By this system, environment temperature and humidity, cold surface temperature and contact angle can be adjusted for observing frost growth at different circumstance. The result indicate that cold surface temperature mainly influence frost height and shape; humidity have effect both on frost height and density; when copper surface was covered by wax and C 3 O3H8 layer, frost growth on both surface was delayed, frost height obviously lower than copper surface, ice crystal distribution was also thin.The study on nuclear process indicates that super saturation and contact angle has obviously influence on nuclear speed. Nuclear speed increase rapidly while super saturation was lower; and when super saturation was higher, it increase slower. The theory of hydrophilic surface restrain frost growth was that nuclear was delayed by changing local super saturation, as for hydrophobic surface, it was because high contact angle make nuclear hard to carry on.The Analyses on frost image by box-counting method express that full growth frost was a fractal structure ranging in some scale. This character implicate that frost structure was arranged, there was a scale, when measurement unit was lager than this scale, the frost was uniform; otherwise, frost was self similar, frost growth can be described by percolation model. Simulation result indicates that the conductivity increase slowly at higher porosity; it increase rapidly when porosity was near to critical value. Vapor diffusion coefficient increase slowly at low porosity; but it increase swiftly at higher porosity for one pore maybe unite更多还原