Heat Transfer Performance and Visualization of a Thermosyphon according to Hydrophobic Coating Characteristics on the Condenser Section

This study investigates hydrophobic surface modifications on condensation heat transfer in a visualization-type thermosyphon. Three surfaces—PDMS-based SiO₂ nanoparticle coating (nPDMS), wet-etched, and amorphous fluoropolymer (AF)—were applied to the condenser section and evaluated for thermal resistance, heat transfer coefficients, and condensation behavior. While performance was similar at low heat flux, significant differences appeared at higher flux: at 4 W/cm², the AF coating reduced thermal resistance by∼20% and doubled condensation heat transfer coefficients compared to the bare surface by promoting rapid droplet departure (∼2 mm), whereas the nPDMS surface showed poor performance due to delamination and large hysteresis, and the etched surface shifted from dropwise to filmwise condensation. Overall, AF coatings demonstrated superior durability and thermal performance, highlighting their potential for practical thermosyphon applications.