On the effect of static and dynamic contact angles on humid air condensation heat transfer

Abstract

Surface modification is a widely utilized technique for enhancing condensation heat transfer. Two main properties of surfaces in manipulation of condensation heat transfer are the contact angle and contact angle hysteresis. This study focuses on the influence of contact angle (CA) and contact angle hysteresis (CAH) on humid air condensation. For this, hydrophilic and hydrophobic surfaces with varying CA and CAH values were fabricated and tested in a humidity-controlled climate chamber at different relative humidity levels. Three hydrophilic surfaces samples with a contact angle of approximately 70° and CAH values of 10°, 20°, and 42° were tested. Two hydrophobic surfaces with a contact angle of approximately 110° and CAH values of 21° and 39°were also prepared as well as a hydrophobic surface with a contact angle of 96° and a CAH of 43°. The role of CA and CAH in different stages of condensation cycle was investigated. Our findings show that while CA plays the main role in droplet nucleation, CAH has a significant impact on droplet coalescence and departure. Increasing CAH while keeping CA constant has a negative effect on condensation heat transfer in all wettability levels. However, the relationship between changes in CA while keeping CAH constant does not have the same trend in the condensation heat transfer performance for every case. Changing CAH for lower CAH values led to a greater impact on enhancing condensation heat transfer than higher values. Moreover, increasing CAH on hydrophobic surfaces had a more significant effect than on hydrophilic surfaces. Additionally, decreasing CAH had a more pronounced effect on improving condensation heat transfer than increasing CA. The findings emphasize on the importance of considering both the contact angle (CA) and contact angle hysteresis (CAH) in the surface design to have the optimum condensation heat transfer performance.