An experimental study on self-extinction of tunnel fire under natural ventilation condition

Fire tests with both methanol and propane fires ranging from 5.6 to 16.8 kW were conducted in two 1:20 reduced-scale tunnels with identical cross-section size of 0.45 m wide x 0.23 m high but different lengths, i.e., 20.8 m and 10 m long respectively. Natural smoke control was adopted for both tunnels. Fires were located at the center of the tunnels. Temperatures and gas concentrations (i.e., O-2, CO and CO2) were measured in the experiments. The experimental results showed that both methanol fires and propane fires self-extinguished in the 20.8 m-long tunnel within around 600 s, except the 5.6 kW methanol fire. The larger the heat release rates were, the quicker the self-extinction of fires were. The oxygen concentrations in the 20.8 m-long tunnel declined monotonously until they approached around 12% except for the 5.6 kW methanol fire, which kept well above the limiting oxygen concentration. For fires close to the self-extinction, the flames lifted from the burner surface and fluctuated strongly. Self-extinction was not observed with both methanol fires and propane fires of the same heat release rates in the 10 m-long tunnel. The oxygen concentrations in the 10 m-long tunnel decreased temporarily to 17% and rose up to around 20%. In the long tunnel with large fire sizes, the smoke layer descended to ground level at a certain distance away from the fire as the temperature of smoke layer dropped to ambient temperature. The descended smoke layer stopped the supply of fresh air reaching the fire seat. The backflow of air toward the fire was highly vitiated due to the mixing of combustion products with fresh air. The fires self-extinguished when the concentration of oxygen close to fire seat approached the limit of flammability. The self-extinguishment of fires in tunnel provides us new insight on the strategy of firefighting in tunnel fires.