Predicting Condensation and Fogging Risks in Humid Ventilated Tunnels Through a Coupled Thermo–Hygro–Fluid Model

Tunnel ventilation is accompanied by heat and mass transfer, while moisture transported by incoming airflow and seepage from surrounding rock walls leads to a highly humid environment within the tunnel, thereby hindering normal operation. This study presents a fully coupled thermo–moisture–flow model capable of simulating the spatiotemporal distribution of temperature and humidity fields in humid tunnels. The model integrates airflow transport, heat transfer, wall surface evaporation, moisture migration, and latent heat effects within a unified framework. Analysis of the key factors influencing the tunnel humidity field reveals that low surrounding rock temperatures and high wall moisture availability are the dominant factors promoting humidity accumulation, whereas inlet airflow conditions significantly influence the development of the humidity field. Based on the spatial evolution of relative humidity, condensation distance and fogging distance are proposed as characteristic indicators for quantifying high-humidity risk. Parametric analyses reveal that increasing ventilation velocity effectively suppresses humidity growth when the inlet air is relatively dry, whereas dehumidification should be prioritized when the inlet relative humidity exceeds 0.7. Furthermore, predictive equations for condensation and fogging distances were established through multivariable regression analysis, providing a rapid method for assessing humidity-related hazards under different ventilation conditions. The findings improve understanding of coupled heat and moisture transfer between humid tunnel walls and ventilating airflow and provide practical guidance for humidity control in underground ventilation systems.