Investigation of the effects surface alteration via coatings on the condensation heat transfer and wettability

Background: Heat exchangers are a critical component in organic Rankine cyclic (ORC) geothermal power plants as they represent more than 50% of the capital. In this study, the thermal performance of coatings devised for condensation heat exchangers have been studied. Methods For this purpose, a mock-up heat exchanger rig was designed and fabricated. The coatings used for the trials are of hydrophobic nature, in order to promote a change in the heat transfer mode from film-wise condensation to drop-wise condensation. The coatings for this study were developed by chemical vapour deposition and magnetron sputtering on aluminium substrates. The wettability and adhesion of the coatings was characterized before and after the tests. Statistical analysis was used to analyse the heat transfer related data. Results The results indicate that the silica based coating produced an improvement in the heat transfer performance indicators and the working fluid caused a deterioration of the wetting properties. However, even at the end of the tests, the coated samples performed better than the substrate material in terms of heat transfer performance. Conclusions The results of this research prove that shifting from film-wise condensation to drop-wise condensation is viable method to improve heat transfer efficacy. However, a one to one correlation could not be established with water contact angles measured in water or diiodomethane suggesting that wettability properties in ORC fluid could differ to established in these conventional fluids for contact angle measurements.