Experimental Technology and Management

[Objective] Urban traffic link tunnels, mainly composed of bifurcated tunnels, have been rapidly constructed. A fire in this complex tunnel would cause more serious facility damage and casualties due to multipath smoke propagation and a more uneven temperature distribution compared with an ordinary single tunnel. Furthermore, a bifurcated tunnel contains different tunnel slopes and bifurcation angles to connect surface and underground transportation systems. However, previous research on tunnel fires has mainly focused on a single ordinary tunnel or a horizontal bifurcated tunnel; fires in an inclined bifurcated tunnel have rarely been studied. To clarify the mechanism of smoke propagation and the temperature profile in a bifurcated tunnel, the present study conducted a series of small-scale experiments to investigate the maximum ceiling temperature in a bifurcated tunnel with an inclined mainline. [Methods] Froude's similarity criterion was used to guide the design of the small-scale experimental bench. A 1/20 scale bifurcated tunnel platform was constructed, consisting of a 10 m mainline and a 4 m ramp, with a cross-section of 0.25 m × 0.5 m. Three bifurcation angles(10°, 20°, and 30°), five mainline tunnel slopes(0%, 1%, 3%, 5%, and 7%), and three heat release rates(1.12, 1.64, and 2.8 kW) were considered. Different longitudinal ventilation velocities supplied from the mainline before shunting were used for analyzing their effects on smoke propagation and temperature distribution. The temperature at the tunnel ceiling and along the tunnel centerline was detected and analyzed. The effects of the bifurcation angle and the mainline slope on the maximum ceiling temperature were investigated, and an empirical model was developed to predict it in a bifurcated tunnel. [Results and Conclusions] Experimental results showed that the heat release rate significantly affected the maximum ceiling temperature, with higher rates resulting in higher maximum ceiling temperatures. The larger bifurcation angle resulted in a higher maximum ceiling temperature at relatively low longitudinal ventilation; however, its effect on the maximum ceiling temperature was limited when the longitudinal ventilation velocity exceeded 0.2 m/s. In particular, the maximum ceiling temperature was more sensitive to the bifurcation angles at a relatively low heat release rate. The maximum ceiling temperature decreased with increasing longitudinal ventilation because of the cooling effect and the flame tilting effect. The maximum ceiling temperature decreased with increasing mainline slope as the stronger stack effect improved the induced airflow velocity. The effect of the mainline slope on the maximum ceiling temperature was more pronounced when the slope was <3%, but this effect weakened when the slope was >3%. The maximum ceiling temperature in the bifurcated tunnel could not be accurately predicted using previous empirical models, as these models were developed based on tests conducted for ordinary single-line or horizontally branched tunnels. Therefore, a predictive model for the maximum ceiling temperature in a branched tunnel with a mainline slope was developed by accounting for the mainline slope, heat release rate, bifurcation angle, and longitudinal ventilation velocity. This study contributes to understanding smoke propagation and provides a validated tool for evaluating maximum temperature in a bifurcated tunnel.