Abstract:
Condensation of water on metallic surfaces is critical for multiple energy
conversion processes. Enhancement in condensation heat transfer efficiency
often requires surface texturing and hydrophobicity, usually achieved through
coatings, to maintain dropwise condensation. However, such surface treatments face conflicting challenges of minimal coating thermal resistance,
enhanced coating durability, and scalable fabrication. This study presents
a thin (≈2 µm) polytetrafluoroethylene–carbon nanofiber nanocomposite
coating that meets these challenges and sustains coalescence-induced
jumping droplet condensation for extended periods under highly demanding
condensation conditions. Coating durability is achieved through improved
substrate adhesion by depositing a submicron thick aluminum primer layer.
Carbon nanofibers in a polytetrafluoroethylene matrix increase coating
thermal conductivity and promote spontaneous surface nanotexturing to
achieve superhydrophobicity for condensate microdroplets. The coating
material can be deposited through direct spraying, ensuring economical
scalability and versatility for a wide range of substrates. No other coating is
known for metallic surfaces that is able to sustain jumping dropwise condensation under shear of steam at 111 °C flowing at ≈3 m s−1
over the surface
for 10 h and dropwise condensation for an additional 50 h. Up to ≈900%
improvement in condensation heat transfer coefficient is achieved compared
to conventional filmwise condensation.
