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

Due to vehicle collision or fire spread,there are also multiple-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.The flame height further increases if the flame merging phenomenon occurs,which can cause more serious damage to the tunnel structure.The fire smoke is the main reason for the casualties,but the studies on smoke control using naturally ventilated shafts in the tunnel with multiple fires is relatively less.Therefore,it is of great significance to conduct study to investigate the characterics of flame merging behaviors,critical fire spread conditions and smoke control performance using shafts in the tunnel under multi-fire scenarios.In this paper,the reduced-sacle model tunnel fire tests 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 fame behaviors of the multiple fires in a naturally ventilated tunnel,including flame merging,flame tilt angle,considering fire number,fire spacing and heat release rate.The results show that the tilt angle of the outside flames of the three fires and the flames of the two fires increases first and then gradually stabilizes with the increase of the distance between fire sources.And the flame tilt angle still stays large when the distance between fire sources is relatively long,which is different from the flame tilt phenomenon of symmetrical multiple fires in free space.The fundamental reason for the flame tilt is the longitudinal induced airflow formed by fresh air supply in the tunnel.It is found through theoretical analysis that the dimensionless induced wind speed is affected by the tunnel section size,fire radius and number of fire sources.Based on the principle of mass conservation,a prediction model of flame tilt angle of the multiple fires in the tunnel is established.There is no interaction between the flames when the fire spacing is longer than the critical maximum flame merging distance,but the height of the outer flames of three fires and the flame of two fires are lower than that of the single fire source with the same fire size because of the phenomenon of the flame tilt.As the fire spacing decreases,the air entrainment restraction is strengthened gradually,leading to the flame height increases gradually.The flame is merging competely with the further decrease of fire spacing,and the fire spacing will have little effect on the flame height,namely the flame behavior is consistent with that of single spacre fire source.Based on the flame tilt phenomenan,the flame height 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.The characteristics of the fire spread,critical average temperature and critical fire spread distance in the longitudinal ventilated tunnel are studied by the model tunnel fire experiments.Besides,the characteristics of multiple fires in large tunnel fires are discussed and analyzed.The results show that the heat release rate of the downstream wood piles increases faster than the upstream wood piles due to the additional heat feedback from the upstream flame and the pre-heating by high-temperature fire smoke from the upstream fires,which will lead to a faster increase of the total heat release rate in the tunnel.A model was proposed to calculated the average temperature difference across the tunnel section,and it is found that its value of453 K can be used as the critical condition for the fire spread of wood piles.A simplified calculation method of the critical fire spread distance is further developed,and the accuracy of predicted value is verified by the results in the previous small-scale tunnel fire tests and full-scale tunnel fire tests.The effect of ventilation velocity,heat release rate and tunnel perimeter on critical fire spread distance are illustrated based on the simplified temperature calculation model.For the fire scenarios with multiple fires,the critical 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 tital heat release rate(Qc,Inc+Qx).The fire spread distance of the downstream fire source will be farther and farther when the total heat release rate of the downstream fire source is greater than that of the upstream fire source.3.Full-scale numerical simulations were carried out to study the overall smoke control effect using natural ventilation shafts in a tunnel under multiple fires,considering heat release rate,fire spacing,fire source location and shpaft sacing.Besides,a comparative analysis was made with single fire source in the tunnel.The results show that the total smoke mass flow rate of multiple fires increases with the the increase of fire spacing,and it tends to be stable when the fire spacing is relatively long.The the total smoke mass flow rate of the multiple fires is larger than that of the single fire with same total heat release rate.Due to the interaction between the multiple fires,the smoke layer height of the multiple fires in the tunnel it decreases first and then tends to be stable with the increase of fire spacing.The smoke layer height of the multiple fires is lower than that of the single fire with same total heat release rate(about 20%for two fires and about 25%for three fires)when the fire spacing is releavely long.The critical safe distance fore firemen from the fire source increases with the reduction of the fire spacing when the heat release rate is relatively low(such as car fires),but it is not longer than the single fire with same total heat release rate.For fires with large heat release rate(such as bus fires),the critical distance increases with the increase of shaft spacing,which is more unfavorable for fire-fighting and rescue.The smoke spread length decreases with the decrease of the distance between the fire source and the nearst shaft,and the smoke spread length is shorter than that on the other side.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.The smoke spread length increases obviously with the increase of the shaft interval,which is adverse for evacuation.