Experimental Studies On Mechanism Of Frost Formation On Cold Surfaces And Its Control

In air crafts and most industrial applications such as heat pump, and refrigerators, frost is undesirable. Our literature survey shows that most of the previous studies dealt with a fully grew frost layer. However, few studies have been done to investigate the frosting phenomena at the early stage of frost formation. The present study seeks to clarify the fundamental nature of frost formation at initial stage, and the effect of surface wettability on frost behavior.Frost formations on a horizontal flat copper surface were experimentally investigated using the microscopic observation technology. The experiments were carried out for -20 ~ 0℃ copper surface temperatures, 22 ℃ air temperature and 15 ~ 85 % air relative humidity. It is found that the frost formation on a cold surface generally tracks a course like the following: formation and growth of water drops, freezing of the sub-cooled drops, formation and growth of initial frost crystals on the frozen drops, growth of frost layer and melting of frost crystals and layer. The super-cooled condensate drops freeze earlier for lower copper surface temperature and higher air humidity, while smaller freezing drops are seen for lower copper surface temperature and lower air humidity. The initial frost crystals are classified into four groups, including the irregular-type crystal, flake-type crystal, needle and pole-type crystal, feather-type crystal. It is pointed out that the test surface temperature and the air relative humidity are primary factors affecting the frost crystal shape. The surface temperature of the frost layer has effect on the new frost crystals forming on top of it. The early state of frost formation on both hydrophobic and bare surfaces had been investigated microscopically. The contact angle was 56° for the bare copper and 98° for the hydrophobic coating. Surface contact angle played an important role in the fundamental nature of the initiation stage of the frost formation phenomena. The characteristics of frost in terms of freezing onset time, freezing radius of super-cooled water drops, surface occupying ratio of ice particle are presented and discussed. The experimental results support that a hydrophobic surface can retard the frost formation and growth, the effect of surface wettability on frosting character becomes more obvious at higher surface temperature, this is especially true for Ts > -12℃.This information during the early frost growth period is useful in the numerical modeling of frost formation. It can be predicted that larger contact angle of cold surface can retard the frost formation and growth longer.