Investigation of the Effect of Superhydrophobic Coatings on the Performance of Heat Exchangers

Abstract

The main aim of this project was to investigate the effect of superhydrophobic coatings on the performance of aluminium louvered fin heat exchangers. A method was developed to prepare superhydrophobic coatings on the exchangers by applying layer of metallic zinc and then reacting the zinc-coated exchanger with silver to electrolessly deposit a microscopically rough silver layer. This was then treated with a polyfluorothiol compound to create a self-assembled monolayer with low surface energy, giving a superhydrophobic coating, or an alcohol terminated thiol to give a superhydrophillic coating. Initial tests of louvered-fin heat exchangers with both these coatings showed they retained significant amounts of condensed water when they were used to cool air to below the dew point. This condensed water affected both the stability of the coatings and the resistance to air flow through the heat exchangers. An alternative approach to preparing superhydrophobic coatings was then also adopted in which superhydrophobic copper powder was fixed onto the heat exchanger using a thin adhesive layer. Dynamic dip tests were conducted with the coated and uncoated heat exchangers suspended vertically and horizontally. With the normal vertical mounting the superhydrophobic zinc/silver coated heat exchanger surprisingly retained more water than the untreated heat exchanger and much more water than the hydrophilic heat exchanger. This was due to the coating interfering with the drainage of water through the narrow channels which are designed to allow water to escape in uncoated heat exchangers. Conversely, with the heat exchangers suspended horizontally, the advantage of the superhydrophobic coating was apparent since the system retained significantly less water than the uncoated control. However, under normal operating conditions the superhydrophobic coating did not give the same improvement due to flooding of the textured surface under high water condensation rates. The thermal performance of the coated and uncoated heat exchangers was measured with a simple test apparatus which was built for this purpose and allowed control of the input air temperature, humidity and velocity. This system also allowed measurement of the inlet and outlet air temperature and relative humidity as well as inlet and outlet water temperature, thus enabling overall heat transfer coefficient to be calculated. It was found that the overall heat transfer coefficient generally increased with inlet air velocity for all heat exchangers. However, comparison of the coated and uncoated heat exchangers at similar input velocities showed that the calculated the heat transfer coefficients of all the exchangers were similar. There may have been small improvements due to dropwise condensation on the surfaces but this was more than counterbalanced by the increased pressure drop observed for the coated heat exchangers due to retained water. This means that the overall performance of even the horizontally mounted superhydrophobic coated systems, which showed very favourable water shedding properties under bulk water testing, was not improved in water condensation experiments. This main source of this effect is the flooding of the superhydrophobic surfaces at high condensation rates, an effect which will need to be addressed if the potential advantages of superhydrophobic surfaces are to be realised in real heat exchange systems.

Date of Award	2017
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Steven Bell (Supervisor) & Chirangano Mangwandi (Supervisor)