Experimental Studys On The Self-Extinction Characteristics Of Tunnel Fire

To study the self-extinction characteristics of tunnel fire under natural ventilation,a series of fire tests with both methanol and propane fire ranging from 2.8k W to 16.8k W were conducted in two 1:20 reduced-scale tunnels with identical cross-section size of 0.45 m wide×0.23 m high but different length and gradient,i.e.,10 m and 20.8m long respectively,and 0%,1% and 5% respectively.Natunal ventilation was adopted for both tuunels,and the fire sources were located at the center of the tunnels.The temperature distuibution,flame geometry,gas concentrations(i.e.,O2,CO and CO2),MLR,longitudinal velocity and so on were analysed in all experiments.For both methanol and propane fires,self-extinguishing were occured in the 20.8 m long horizontal tunnel.The self-extinguishing time of propane fire is shorter and the CO production is higher than methanol fire.In the 10 m long tunnel,there was no self-extinguishing phenomenon in the fire tests of the two fuels,the intensity of combustion was higher than that in 20.8m long tunnels.The flames were less luminous blue at the flame base,and gradually lifted from the fule surface close to the self-extinction,which geometry fluctuated strongly.The phenomenon was qute similar to the ghosting flame.In the inclined tunnels,the flames were asymmetrically distributed along the center line of fire source,and which length along the upstream was significantly shorter than the downstream.In the 20.8m long horizontal tunnel,experiments with methanol and propane fires selfextinguished at around 600 s after the ignition except for 5.6k W methanol fire,which lasted for more than 20 min.The oxygen concentration dropped sharply to within 12% closed to selfextinction,which was lower than the limiting oxygen concentration((i.e.,10%-12%),the smoke layer settled to the ground at a position of around 6 m on the both sides of the fire source,and the fire self-extinguished.But in the 10 m long tunnel,self-extinction were occurd for all the experimental tests,smoke layer reached the portals easily and the procedure` of combustion could last more than 20 min.2.8k W methanol fire did not occur the self-extinction in the three inclined tunnels(i.e.,0%,1% and 5%).However,other fire tests ranging from 5.6k W to 16.8k W were self-extinguished within 600 s after ignition except for that conducted in the 5% tilted tunnel.In the 0% and 1% tilted tunnel,there were no obvious longitudinal velocities,the temperature distribution decreased rapidly with the distance from the fire source,and the smoke layer descended to the ground level before it reached the portals,leading to the oxygen concentration continually decreased to 12% to occur self-extinction.But in the 5% tilted tunnel,the strong "stack effect" produced a significant longitudinal airflow with speed of more than 0.1m/s which drived the smoke layer moving in the uphill direction while the make-up airflow reach to the fire source from the downhill side.The length of smoke layer in the uphill side decreased rapidly to around 2m,which temperature rise was lower than that in the uphill side.For the three fire tests of 5.6k W methanol fire in the 0% tilted tunnel,the experimental results were contradictory due to the different ambient temperatures.Therefore,the mbient temperatures may have some particular impacts on the self-extinction.In addition,the smoke layer’s descent process was mainly caused by the air entrainment and heat loss through wall boundary.So,these interesting pheonaminons will be further studied in the future.