SEMISOLID LUBRICANT-IMPREGNATED GRADIENT WETTABILITY SURFACE FOR ENHANCED CONDENSATION HEAT TRANSFER

Enhanced condensation heat transfer on vertical condenser tubes is critical for improving the efficiency of passive containment cooling systems (PCCS) in nuclear reactors. This study aims to improve condensation performance of vertical condenser tubes across a wide subcooling range and over extended operation periods by integrating a Teflon-based semisolid lubricant coating with gradient wettability, achieved using loosely wound vertical wires on copper tubes. Condensation experiments were conducted on vertical copper tubes attached with loosely wound vertical wires of varying wire diameters and interwire spacings, across subcooling levels from 5°C to 65°C. The combined surface showed superior performance compared to lubricant-only coated tubes, particularly mitigating effectiveness loss at low subcooling. An optimal configuration of 0.51 mm wire diameter and 7.5 mm interwire spacing demonstrated a 36% improvement in heat transfer effectiveness at 10°C subcooling compared to the lubricant-only tube and a 366% increase in heat transfer coefficient at 40°C subcooling compared to an uncoated tube. This enhancement is attributed to improved droplet mobility via Laplace pressure gradients along the vertical wires. The optimal surface maintained performance over a continuous 72-hour period. The proposed approach offers a simple, low-cost, and scalable solution for long-term, high-efficiency condensation in PCCS and similar applications.