Experimental Study on the Influence of Different Transverse Fire Locations on the Critical Longitudinal Ventilation Velocity in Tunnel Fires

A set of fire experiments were conducted in a small-scale tunnel of length 8 m, width 1 m and height 0.5 m, to investigate the influence of different transverse fire locations on the critical longitudinal ventilation velocity for suppressing the smoke backflow in tunnel fire. Six pool sizes were separately used in the experiment, and the burning rate of these pool fires were in the range of 0.05–0.5 g/s. Experimental results showed that for ethanol pool fire, the mass burning rate of fire source kept almost unchanged in the process of the fire moving from the central tunnel to the sidewall, while the maximum temperature rise under the ceiling increased exponentially with the decrease of the distance between the fire and the sidewall, and reached about 1–1.7 times of the fire at the longitudinal centerline. Because critical longitudinal ventilation velocity is associated with the temperature in the impingement area, the critical velocity was investigated in the experiments as well. The dimensionless critical velocity varied with the burning rate, and located in the range of 0.1–0.24 in the experiment. When fires occurred at the center of the tunnel, there was a good agreement between the critical longitudinal ventilation velocity and the model of Wu. The critical longitudinal ventilation velocity would increase exponentially with the decrease of the distance between the fire and the sidewall. When fires neighbored sidewall, the critical longitudinal ventilation velocity was agreed well with the value calculated using the “mirror” method based on Wu’s model.