Study On The Smoke Characeristics And Critical Smoke Exhaust For Long-distance Subway Tunnel With Double-point Extraction Ventilation

With the acceleration of the urbanization process,subways have played an irreplaceable role in urban public transportation due to its advantages such as large passenger capacity,fast speed and good safety.With the continuous increase of subway mileage,accidents occurring in the subways are also increasing.Among them,subway fires are more harmful.Due to the characteristics of closed space in the subway tunnel and the large amount of smoke emitted by the subway tunnel fire,the damage to the safety and property of the subway tunnel fire is much greater than that of ordinary fires.Therefore,the smoke emission of subway tunnel fires has become a key question of research subway tunnel fire.Point extraction ventilation is a potential technical measure for fire ventilation in long-distance subway tunnels.However,the current research has only conducted a preliminary analysis of smoke control,and lacks a systematic study of smoke migration characteristics.The research in this paper can be improved The theoretical system of tunnel fires provides a basis and reference for the engineering application of point fire smoke extraction systems for long subway tunnel fires.Based on the summary of the research experience and achievements of domestic and foreign tunnel fire point extraction ventilation,this paper relies on the National Natural Science Foundation of China(51708454)and builds a 1:20 scale model experimental bench based on Froude model.A series of following experiments were performed:(1)By observing the smoke in the model tunnel,the diffusion characteristics of the smoke in the long-distance subway tunnel are obtained: the smoke in the fire section is well stratified,the smoke is mostly concentrated on the ceiling of the tunnel,and is not affected by the volumetric flow rate.When the volumetric flow rate is small,the smoke spreads to the non-fire section through the smoke exhaust vent,and the smoke layering is poor,the flue gas sinks to the bottom of the tunnel and returns to the fire section,and finally the smoke fills the entire tunnel.(2)The experiment researched the temperature distribution characteristics of the ceiling smoke temperature of a double-point smoke exhaust in a long-distance subway tunnel fire,and the influence of the heat release rate of the fire source,the size of the exhaust vent,exhaust vent spacing and the volumetric flow rate of the smoke to the ceiling smoke temperature.The results show that the ceiling smoke temperature of the tunnel decreases with the distance from the fire source,and the the ceiling smoke temperature decreases suddenly and gradually approaches room temperature;the ceiling smoke temperature of the long-distance subway tunnel fire increases with the increase of the heat release rate of fire sources,the size of the smoke exhaust vent,exhaust vent spacing and the volumetric flow rate have almost no effect on the ceiling smoke temperature.Through derivation,the calculation model of the ceiling temperature of the long-distance subway tunnel is obtained.(3)According to the smoke diffusion characteristics in the tunnel and the ceiling temperature distribution,combined with the smoke control standards,a smoke control strategy for a long-distance subway tunnel is proposed.:All smoke must be exhausted,and is controlled in the fire section and does not spread to the non-fire section.(4)The concept of the critical exhaust volumetric flow rate of long-distance subway tunnel is proposed,which can ensure that the smoke is exactly discharged from the exhaust vents and does not spread to the non-fire section.The effects of fire source heat release rate,smoke exhaust vent length,and exhaust vent spacing on critical exhaust volumetric flow rate are studied,the results show that the critical exhaust volumetric flow rate gradually increases with the increase of the fire source heat release rate;as the smoke exhaust vent length increases,the critical exhaust volumetric flow rate first shows a downward trend,but when the smoke exhaust vent length reaches about 0.1m,the critical exhaust volumetric flow rate increases with the increase of the smoke exhaust vent length;the critical exhaust volumetric flow rate decreases as the distance between the smoke exhaust vent increases.The study provides a reference for determining the reasonable smoke volume of point smoke exhaust.(5)The relationship between the critical exhaust volumetric flow rate and the critical Froude number is studied,and the calculation model of the the critical exhaust volumetric flow rate of the long-distance subway tunnel with double-point exhaust extraction is obtained by fitting.