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Investigation of Hydrophobic Coatings in Air Cooled Condensers for Heat Transfer Application
AuthorCheng, Kuok Kong
AdvisorKim, Kwang J
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In recent years, industries are replacing traditional fluoride based refrigerants for different alternative such as hydrocarbons and water based refrigerants in HVAC applications due to environmental concerns. Air cooled condensers are widely used in HVAC applications and there are ongoing researches to improve the thermodynamic efficiencies for these devices and environmental friendly at the same time. For internal condensations, there are several ways to enhance heat transfer performances, such as coil inserts and surface micro structures. Many of the researches focus on condenser surface modifications mainly on internal machined microstructure patterns. It was found that such techniques improve heat transfer performances up to 400 % compared with smooth surfaces due to enhance mixing and drainage. However, such techniques do not work with all geometries such as micro-channels and other complicated shapes. In the field of external condensation studies, dropwise condensations (DWC) are found to enhance heat transfer coefficients about 300 % to 400 % compared to filmwise condensation (FWC) in most researches. In order to promote DWC, hydrophobic coatings are applied to the condensing surface in most cases. Coatings techniques are solution based and could be easily applied to different geometries. Techniques for promoting DWC for external condensations can be applied to internal condensation as well. The objective of this project is to investigate heat transfer enhancements of DWC promoters has on internal condensation via model based on external DWC and FWC heat transfer models; experiments with hydrophobic coatings inside (coatings provided by NEI Corporation) as well as finned non-coated finned copper and aluminum tubes were conducted with water, n-hexane and n-heptane were carried out to investigate the effects of such promoters; experimental results were compared with establish correlations and models developed in this project. Models developed in this project suggest that DWC promoters could potentially enhance heat flux and heat transfer coefficients compared with FWC. It was found that hydrophobic coatings enhance heat flux and internal heat transfer coefficients by 20 % and 50 % for water and such coatings have negligible effects on n-hexane and n-heptane. Also, most established correlations over predict heat transfer coefficients at low mass flow in water, n-hexane and n-heptane.