The Investigation Of Surface Characteristics Effecting On The Cold Surface Frosting Process

The air source heat pumps have widely been used in many industrial and resident building. However, owing to the very high humidity in the cold and moist areas, the moist air often frosts in many air source heat pumps, which significantly reduces the energy efficiency of the heat pumps. Moreover, the frost improve the energy consumption and the upkeeps of the systems. Therefore, it becomes very important to efficiently delay the frosting. The surface properties are the significant factors to impact the frost process. In this thesis, the effects of the hydrophilicity/ hydrophobicity, the surface roughness and the fractal dimension on the frost process were studied.Compared to the hydrophilic surface, the hydrophobic surface significantly delayed the frost process. The frost process on the hydrophilic copper surface was measured. As the copper surface was cooled to -5℃in 10 minutes, the moist air began to frost on the copper surface on 12 minutes. After the copper surface was treated by silicone, the moist air began to frost on 40 minutes. It could be concluded that the hydrophobic treatment significantly delayed the frost process.The surface roughness had an obvious effect on the frost process. At the beginning of the frost process, the smooth surface has a smaller condensed water drops, which delayed the frost process. As the surface roughness was 21.7 nm, the moist air began to frost on 8.5 minutes. As the surface roughness was 72.4 nm, the moist air began to frost on 5 minutes. During the latter period of the frost process, the smooth surface had a smaller thickness of the frost. As the surface roughness was 21.7 nm, the thickness of the frost was 4 mm. As the surface roughness was 72.4 nm, the thickness of the frost was 4.8 mm.The experimental results showed that the latter the moist air began to frost, the smaller fractal dimension in the range of 2 and 3 the surface had. As the fractal dimension increased from 2.3 to 2.7, the moist air delayed to frost on the cold surface. It indicated that the surface having the pre-three dimension could delay the beginning of the forest process.