| High temperature smoke will directly lead to the tunnel structure damage,resulting in inestimable consequences.Natural shaft smoke extraction is an effective way to control tunnel smoke.The shaft can discharge fire smoke out of the tunnel through the driving force of stack effect and buy time for people to escape.Therefore,it is necessary to study the natural shaft ventilation in the tunnel and its related influencing factors.In this paper,the effect of shaft structure and blockage on the smoke extraction efficiency and characteristics of tunnel shaft is analyzed by numerical simulation,in order to optimize the actual tunnel shaft structure to achieve a better effect of fire smoke control.The main research work is as follows:(1)The smoke extraction efficiency under different shaft structure optimization was compared and analyzed,and the influence mechanism of shaft structure on smoke extraction efficiency was revealed.Firstly,the smoke spreading process of 6 kinds of shaft structures is simulated by the Fire Dynamicss Simulator(FDS6.6.0).Then,the distribution of smoke temperature and CO concentration in the tunnel and the shaft under different shaft structures is compared and analyzed,and the calculation formula of natural smoke extraction efficiency of the tunnel shaft is given.Finally,the effect mechanism of shaft structure on smoke exhaust efficiency is analyzed based on smoke extraction efficiency.The results show that the optimized shaft can further improve the smoke extraction efficiency of the shaft compared with the common shaft,of which is about 43%,and the BCS shaft has the most significant effect of 70% in improving the smoke extraction efficiency.(2)The effect of the height of the shaft on the smoke extraction under the structure of the BCS shaft is studied,and the factors affecting the plug-holing are revealed.Firstly,aiming at the BCS shaft structure whose smoke exhaust efficiency is well optimized,further numerical simulation is conducted by changing the height of the shaft to discuss the plug-holing in the BCS shaft area and the influencing reasons.Then,combining with the calculated smoke extraction efficiency,the mechanism of inhibiting plug-holing of BCS shaft structure was analyzed.Finally,the suitable plug-holing criterion is given for BCS shaft.The results show that the board structure and with the increase of the height of the shaft,the smoke extraction efficiency of the shaft increases first and then decreases,and the critical height of the shaft with better suction and plug-holing is determined to be 7m to 8 m.(3)The characteristics of natural smoke extraction of shaft under different vehicle blockage were studied,and the influence mechanism of that was revealed.Firstly,the influence of blocking ratio on the smoke extraction process of common shaft and BCS shaft was studied by numerical simulation experiment under the condition of blockage.Then,based on the influence of blocking ratio on the smoke extraction process,the changes of temperature and CO volume fraction in tunnels and shafts were discussed.Finally,the variation law of smoke extraction efficiency with blockage and its effect factors were analyzed.The results show that with the increase of the blocking ratio,the smoke extraction efficiency of the shaft decreases first,then increases,and finally decreases slightly,and the optimal smoke extraction efficiency can be achieved when the blocking ratio is 0.48. |
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