Under-Ceiling Temperature Distribution in a Small-Radius UTLT: Effect of Transverse Fire Location

High-temperature smoke remains a primary threat in tunnel fire safety. The curved walls of small-radius Urban Traffic Link Tunnels (UTLTs) significantly alter smoke flow patterns and temperature distribution. Furthermore, no quantitative model exists to assess the impact of transverse fire location variation on temperature distribution in small-radius UTLTs. To address this gap, this study integrates experimental and numerical methods to specifically investigate the influence of curvature radius and transverse fire location on ceiling temperature distribution. Key findings demonstrate: (1) Wall-adjacent fires exhibit substantially higher temperatures than non-adjacent scenarios, resulting from restricted air entrainment (increasing flame height) combined with wall thermal constraint effects. (2) Competition between centrifugal and inertial forces consistently produces a higher maximum temperature rise beneath the convex ceiling versus the concave side in curved sections. (3) A novel dimensionless parameter Rcs is derived from smoke control volume force analysis. This parameter quantifies the coupled effect of curvature radius and ventilation velocity on convex-concave ceiling temperature difference, enabling a predictive regression equation. (4) Through dimensional analysis, key governing dimensionless parameters are identified. Incorporating the Richardson number (Ri), which characterizes inertial-to-buoyant force competition, a predictive model for maximum ceiling temperature rise in small-radius UTLTs is ultimately established.