Characterization of Frost Growth on a Membrane
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The formation of frost in heat/energy exchangers is inevitable in cold and humid environments, and can cause economic losses and technical problems in frosted exchangers. In recent decades, there has been much interest in developing anti-frosting surfaces to reduce these losses and problems. In HVAC (Heating, Ventilation and Air Conditioning) systems, air-to-air semipermeable membrane energy exchangers have shown to be less susceptible to frosting compared to impermeable plate heat exchangers due to the moisture transfer through the semipermeable membrane. However, there is no data available in the open literature on the initiation and growth of frost on semipermeable membranes. In this Ph.D. research, frost formation on a membrane is studied quantitatively and qualitatively in order to understand the frost initiation and growth on a membrane. Therefore, frost growth on a semipermeable membrane and an impermeable plate are characterized experimentally, in order to find the effect of moisture transfer through the semipermeable membrane on (1) frosting limit, (2) the process of the frost formation, and (3) the properties of the frost layer. Imaging techniques are used to measure the thickness of the frost layer and delineate the frost formation process. The results presented in this thesis show that the frost limit on a semipermeable membrane is approximately 5 to 8°C lower than the frost limit for an impermeable plate. Furthermore, it is found that the fundamentals of the process of frost formation on the two surfaces (impermeable plate and semipermeable membrane) are similar: condensation, freezing, and frosting. However, large and distinct water droplets form on the membrane that remain in the liquid phase for a longer time compared to an impermeable plate. Also, the lateral growth of ice crystals that connect frozen droplets is very clear for the semipermeable membrane. The results also show that frost formation is not eliminated on the semipermeable membrane under severe conditions. The frost growth on the semipermeable membrane is very slow for the first 30 min to 60 min of the test, compared to the frost growth on the impermeable plate. Finally, it is shown that the rate of frost growth decreases as the rate of the moisture transfer through the semipermeable membrane increases.
DegreeDoctor of Philosophy (Ph.D.)
Committeebugg, James; Noble, Scott; Zhang, Lifeng
Copyright DateAugust 2018
frost formation, semipermeable membrane, moisture transfer, delay