Evaluating the impact of mainline slope on temperature distribution in a branched tunnel fire: An experimental study and theoretical model analysis

Smoke and heat transport in branched tunnel fires become more complex when an inclined mainline is subjected to longitudinal ventilation. This study conducted 1:20 reduced-scale experiments to investigate the maximum ceiling temperature rise and temperature longitudinal decay in a branched tunnel with a uniformly inclined mainline. Five mainline slopes from 0 % to 7 % and longitudinal ventilation velocities up to 0.45 m/s were considered. Results show that the maximum ceiling temperature increases with the heat release rate but decreases with the longitudinal ventilation velocity. The maximum ceiling temperature rise decreases with increasing slope under relatively low ventilation velocities. However, this trend diminishes under relatively high ventilation velocities, indicating that the slope effect is modulated by longitudinal ventilation. The longitudinal ceiling temperature exhibits an exponential decay pattern. Increasing the mainline slope weakens the upstream temperature decay but accelerates the downstream decay. To characterize the coupled effects of mainline slope and longitudinal ventilation, an effective dimensionless velocity V* eff and the slope correction coefficient c s were introduced. The coefficient c s was determined as 0.026, and V* eff = 0.19 was adopted as the transition between buoyancy- and ventilation-dominated regimes. A piecewise prediction model for maximum ceiling temperature rise was developed and the temperature longitudinal decay along downstream was correlated. These results provide a quantitative basis for ceiling temperature prediction and smoke control design in branched tunnel with uniformly inclined mainlines.