A centralized smoke exhaust method using branch tunnel in bifurcated tunnel fires: Impact on maximum ceiling temperature in main tunnel

Bifurcated tunnels are widely used in urban road and metro systems, yet their branching geometry complicates smoke control compared with single-line tunnels. This study numerically investigated the maximum ceiling temperature (MCT) in the main tunnel when a centralized exhaust vent is arranged in the branch tunnel, considering different fire locations, exhaust velocities ( ), and heat release rates. Results showed that as increases from 0 to 8 m/s, the MCT decreases by approximately 50%. The influence of fire location depends on the exhaust intensity: under weak exhaust ( ), the MCT decreases as the fire moves farther from the junction due to reduced smoke accumulation and recirculation near the bifurcation, whereas under strong exhaust ( , satisfying ) the MCT becomes weakly dependent on fire location when the smoke layer has developed into the one-dimensional flow before reaching the junction. The flame exhibits a longitudinal tilt toward the junction for non-junction fires and lateral guidance into the branch for junction fires. Based on the flame tilting characteristics, an empirical correlation incorporating exhaust volume, tunnel geometry and fire parameters was developed to predict the MCT under strong exhaust. A comparison indicates that centralized smoke exhaust better protects the downstream region, while longitudinal ventilation is more effective in reducing smoke accumulation near the fire source.