Characteristics Of Multiple Fires In The Tunnel And Smoke Exhaustion Using Naturally Ventilated Shafts

Due to vehicle collision or fire spread,there are also multi-fire phenomena in the tunnel.When the fire number increases from one to two,the fire risk increases a lot and fire rescue is more difficult.If the flame merging phenomenon occurs,the flame height further increases,which can cause more serious damage to the tunnel structure.The fire smoke is the main reason for casualties,but the study on smoke control using shafts in the tunnel with multiple fires is relatively less.Therefore,it is of great significance to conduct study to investigate the fire flame merging behaviors,critical fire spread conditions and smoke control using shafts in the tunnel with multi-fire scenarios.In this paper,the reduced-sacle model tunnel fire experiments and numerical simulation method are used to carry out the research work from the following three aspects:1.A series of small-scale model tunnel experiments were conducted to study the flame behavior characteristics of the multiple fires in a naturally ventilated tunnel,including flame merging,flame tilt angle,considering fire spacing and heat release rate.Different from the flame tilt phenomenon of multiple fires in open space,the flame tilt of multiple fires in the tunnel still occurs when the separation distance is large.The fundamental reason for the flame tilt is the induced longitudinal airflow formed by fresh air supply in the tunnel.The tilt angle of the outside flames of the three fires and the two flames of the double fires increases first and then gradually stabilizes with the increase of the separation distance.Through theoretical analysis,it is found that the dimensionless induced wind velocity is affected by the tunnel section size,fire radius and number of fire sources.Based on the mass conservation,a prediction model of flame tilt angle is establised.When the fire spacing is larger than the critical maximum flame merging distance,there is no interaction between the flames,but the height of the outer flame of three fires and the flame of two fires are lower than that of the single fire source with the same fire size due to the flame tilt.As the fire spacing decreases,the air entrainment restraction is strengthened gradually,leading to the flame height increases gradually.When fire spacing is very small,the fire spacing will have little effect on the flame height,namely the flame behavior is consistent with that of single spaure fire source.Based on the flame tilt phenomenan,the flame heigh prediction model of double and three fires was proposed.By introducing the equivalent virtual point sources,the maximum temperature prediction model was established for multiple fire in a naturally ventilated tunnel.2.Using the small-scale tunnel fire experiment platform,the characteristics of the fire spread,critical average temperature and critical fire spread distance in the longitudinal ventilated tunnel are studied.Besides,the characteristics of large multiple fires in the tunnel are discussed and analyzed.The results show that the heat release rate of the downstream wood stack increases faster than that of the upstream wood piles due to the additional heat feedback from the upstream flame and the pre-heating from the hot smoke,which will result in a faster increase of the total heat release rate of the tunnel.The average temperature difference of 453K can be used as the critical condition for the fire spread of wood piles in the longitudinal tunnel.The calculation method of critical fire spread distance in tunnel is further developed,and the effectiveness of this method in small-scale tunnel fire and full-scale tunnel fire is verified.The influence of ventilation wind velocity,heat release rate and tunnel perimeter on critical fire spread distance are illustrated.The fire spread distance of the downstream fire source is related to the incoming gas temperature and the heat release rate of the downstream fire source,which can be expressed by a virtual fire source power(Qc,Inc+Qx).When the total heat release rate of the downstream fire source is greater than that of the upstream fire source,the fire spread distance of the downstream fire source will be farther and farther.3.Full-scale numerical simulations were carried out to study the verall smoke control effect of natural ventilation shaft in the tunnel under multiple fires by considering heat release rate,fire number,fire spacing,fire source location and shaft interval.Besides,a comparative analysis was made with single fire source in the tunnel.The results show that the mass flow rate of smoke from multiple fires is larger than that from single fire,and it tends to be stable with the increase of fire spacing.Due to the interaction of the multiple fires,the smoke layer height of the multiple fires in the tunnel is lower than that of the single fire(about 20%fore two fires,and 25%for three fires),and decreases first and then tends to be stable with the increase of fire spacing.When the heat release rate is relatively low(such as car fires),the critical distance increases with the decrease of the fire spacing,but it is not longer than the single fire scene with total heat release rate.For fires with high heat release rate(such as bus fires),the critical distance increases with the increase of shaft interval,which is more unfavorable for fire-fighting and rescue.With the decrease of the distance between the fire source and the nearst shaft,the total smoke spread length decreases.The shaft interval is the main factor affecting the smoke spread length,the heat release rate and fire spacing has little influence on the smoke spaed length.With the increase of the shaft interval,the smoke spread length increases obviously,which is adverse for evacuation.