A model experimental study on backdraught in tunnel fires

Backdraught is a special fire phenomenon in a limited-ventilation space. Research on the occurrence of backdraught in compartment fires has been extensive, but backdraught in subway tunnel fires, which occur in underground spaces with a large slenderness ratio, has received insufficient attention. In the present study, 31 cases, divided into two groups under conditions of natural ventilation and mechanical ventilation, were examined using a 1/8 reduced-scale model tunnel to investigate the critical conditions and characteristics of backdraught occurrence, as well as the differences between tunnel backdraught and compartment backdraught. The existing critical values of the mass fraction for different ventilation conditions are also discussed. The results indicated that the key parameter determining the occurrence of backdraught in subway tunnel fires is the mass fraction of the volatilized unburned fuel in the tunnel. The critical values of the mass fraction in natural ventilation and mechanical ventilation were 8.78% and 11.71%, respectively, with a humidity of 15% in fresh air. With natural ventilation, backdraught occurrence in a single tube tunnel configuration was similar to that in a compartment, and the ignition delay depended on the velocity of the gravity current; with mechanical ventilation, backdraught occurrence in a twin-tube tunnel configuration was different from that in a compartment, and its ignition delay was determined mainly by the ignition source delay. In addition, as the velocity of air flowing into the tunnel increased, so did the intensity of the backdraught. The experimental data were qualitatively validated and analyzed based on the flammability diagram of the fuel, and these results were similar to those obtained in previous experiments. Also, the humidity of the fresh air flowing into the tunnel affected the occurrence of backdraught; it would not occur under conditions of higher humidity even if the mass fraction mentioned above was higher. Furthermore, it may be economical and feasible to install a humidification device in the tunnel ventilation system to humidify the fresh air flowing into the tunnel to inhibit backdraught.