Ceiling temperature distribution and decay in tunnel fires: Effect of longitudinal velocity, bifurcated shaft exhaust and fire location

Abstract

This paper establishes a model tunnel to investigate the impact of longitudinal velocity (u), bifurcated shaft exhaust velocity (BSEV) and fire location on ceiling temperature and decay. The experimental results show that a longitudinal velocity of 0.6 m/s can control the upstream high temperature within 2.5 m when the distance between fire and shaft (D) is 1.0 m, and further increase in longitudinal velocity has little effect on upstream temperature distribution. Downstream temperature profile should be divided into two cases according to the magnitude of longitudinal velocity: the difference between the temperature decay model in low-speed region (u ≤ 0.5 m/s) and that in the high-speed region (u > 0.5 m/s) is particularly obvious with D at 1.0 m, and the downstream temperature decay rate in the low-speed region is the slowest compared to all the working conditions in this paper. For D more than 1.0 m, the range of high temperature distribution increases with D for certain longitudinal velocities (0.6-0.7 m/s); however, at particularly large longitudinal velocity (0.8 m/s), D has almost no effect on the upstream temperature distribution. The effect of longitudinal velocity on upstream temperature is stronger than that of BSEV. The downstream ceiling temperature decay model is little affected by longitudinal velocity and BSEV with D more than 1.0 m. The temperature decay rate first decreases, then increases, and finally decreases again as the D increases. Existing temperature attenuation models cannot predict the temperature profile in longitudinally ventilated tunnels with BSEV, but the temperature decay model considering fire location proposed in this paper can provide a reference value for tunnels with synergistic ventilation of longitudinal ventilation and BSEV.