| More and more constructions of tunnel bring convenience to people’s travel and life,however,it also greatly increases the probability of fire accidents.Once a fire occurs in the tunnel,it will have a major threat to the safety of people’s life,property and the tunnel structure.Many scholars at home and abroad have studied the tunnel fire under well-ventilation condition,nevertheless,the research on the fire self-extinction characteristics in long tunnel with unrestricted exit boundary is less.In this paper,a series of experiments were carried out in a 1:20 reduced-scale tunnel model with length of 20.8m,width of 0.45 m and height of0.23 m,and the self-extinction mechanism and factors influencing the fire self-extinction characteristics were explored in long tunnel.Liquid methanol was used as fuel and symmetrical blocks with sealing ratio of 65% and distances of 3,4,5,6,8,10 meters from sealing to fire site were set up in the tunnel to study the effect of sealing on the characteristics of tunnel fire.And relevant experiments were carried out in tunnels with different wall to explore the impact of tunnel wall boundaries on the fire self-extinction characteristics.The results showed that under natural ventilation condition,the methanol self-extinguished in the tunnel,and the larger the heat release rate is,the shorter the self-extinguishment time is,which is consistent with the previous research results.When the tunnel is sealed symmetrically,the self-extinguishment time of methanol is less than that of the tunnel without sealing;except that the blocks are located 8m and 10 m away from the fire source,the closer the blocks is to fire site,the faster the oxygen concentration drops at the fire source,the shorter the selfextinction time is,and the better the control effect on the fire and smoke is.In addition,the self-extinction time of down-ward blocks are shorter than that of the upward blocks,that is,the barrier hanging under the ceiling is more conducive to the self-extinction of tunnel fire.Therefore,in the fire protection design of tunnel,it can be considered to install the blocking components similar to the fire shutter in advance at the tunnel ceiling in order to control and put out the fire in time in the future when there is a fire,and appropriate blocking distances and sealing ratios can be used to block the tunnel and put out the fire according to the size of the fire.It was found that the fire self-extinction times of tunnel with different wall boundaries are basically the same,that is to say,the influence of the tunnel wall boundaries on the selfextinguishment time can be ignored;however,the fire proof board will increase the mass loss rate of methanol combustion,accelerate the decline rate of oxygen concentration and the growth rate of carbon monoxide and carbon dioxide at the fire source,and improve the maximum temperature at the fire site and the temperature rise along tunnel.High temperature is easy to pose a threat to the tunnel structure,so it is necessary to pay attention to the selection of tunnel lining materials in the design of tunnel fire safety.The existence of makeups in the tunnel floor makes the sufficient oxygen for the fire source to burn,and then the movement speed of the smoke under the ceiling is faster,and smoke continuously flows out of the tunnel.However,the smoke made of methanol in the selfextinguished tunnel rapidly sank on the ground due to the temperature drop under the inertia drag of the flowing air.At the same time,the flue gas which is about 8.5m away from the fire source sinks and mixes with the air and flows back to the fire source,hindering the entry of the external air,which results in the insufficient air supply in the tunnel and the oxygen concentration at the fire source falling to the combustion limit of the fuel and makes the flame self-extinguish.The research on the self-extinction mechanism of tunnel fire provides a new idea for the prevention and control of tunnel fire in the future,which is of great significance to the fire rescue. |
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