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Journal of Enhanced Heat Transfer
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ISSN Druckformat: 1065-5131
ISSN Online: 1026-5511

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Journal of Enhanced Heat Transfer

DOI: 10.1615/JEnhHeatTransf.v14.i3.70
pages 257-268

Visual and Theoretical Analyses of the Early Stage of Frost Formation on Cold Surfaces

Xiaomin Wu
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, P. R. China
WanTian Dai
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, P. R. China
XiaoFeng Shan
Tenneco China Technical Center, No. 3218, North Jiasong Rd., Shanghai 201814, China; and Belcan (Shanghai) Aviation Technology Inc., Shanghai, China
Weicheng Wang
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, P. R. China
LiMing Tang
Department of Chemical Engineering, Tsinghua University, Beijing 100084, P. R. China

ABSTRAKT

Meso-scale visual observations were conducted to investigate the process of frost formation on both bare and hydrophobic coated copper surfaces, which had contact angles of 56° and 110°. The experiments were carried out for −20−0°C surface temperatures, 19−22°C ambient air temperatures, and 15−85% relative humidities. The tests showed that the frost formation on cold surfaces was not a simple process of transition from steam directly to frost, but actually followed five steps: the formation of condensate droplets, droplet growth including coalescence of the supercooled droplets, freezing of the droplets, formation of initial frost crystals on the frozen droplets, and growth of frost crystals accompanied by collapse of some of the crystals. Compared to the bare copper surface, the hydrophobic surface had a sparser distribution of condensate droplets but larger droplet sizes, delayed droplet freezing and frost formation, and a smaller frost height, all of which support the observation that the hydrophobic surface retards frost formation and growth. The frosting phenomenon was also analyzed theoretically. The initial vapor condensation before frosting was explained based on the free energies for nucleation. For condensation of steam on cold surfaces below 0°C, the Gibbs free energy barrier for water nuclei is smaller than that for ice nuclei, so condensate droplets appear before frost on cold surfaces. Further, since the hydrophobic surface has a higher Gibbs free energy barrier for nucleation than the bare surface, the droplets form more readily on the bare surface.


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