Study On Combustion Characteristics Of Laterally Offset Fire Source In Tunnel Under Blocking Effect

In the past ten years,our highway tunnel construction has achieved rapid development in both quality and quantity.China has moved from a"tunnel country"to a"tunnel power".With the increasing number of highway tunnels,the following safety prevention problems should also be paid attention to.Fire is a common disaster in tunnels.Due to the narrow and long structure of the tunnel,once a fire occurs,people and vehicles will be blocked,which will not only threaten the safety of people,but also damage the tunnel lining and cause economic losses.Based on the realistic background that when a tunnel fire occurs,vehicles blocking the upstream of the fire source will have an impact on the ceiling temperature field and flame combustion pattern inside the tunnel,this thesis assumes that the fire source is located at the tunnel centerline,the lane centerline(with a horizontal offset of 0.2 m),and the lane boundary line(with a horizontal offset of 0.4 m),By changing the longitudinal wind speed and the blocking distance between the obstacle and the fire source,the law of its influence on the temperature field under the ceiling and the characteristics of the flame combustion morphology was studied.The research is mainly carried out from the following aspects:1.The influence rule of the distance between obstacle and fire source on the combustion pattern of tunnel fire flame.(1)Obstacles affect the flame shape characteristics by changing the local wind speed from the longitudinal wind to the flame surface.When there is no longitudinal wind in the tunnel,the change of blocking distance has no effect on the fuel mass loss rate,flame inclination and average flame length.(2)When longitudinal ventilation is conducted inside the tunnel,a"recirculation zone"will be formed behind the obstacle.When the flame approaches,it will be sucked into the flame.The continuous"sawing"of the two opposing forces will cause the flame to tilt in different directions.When the curling force is greater than the pulling force,the flame inclines to the direction of the obstacle;When the entrainment force is less than the pull force,the flame inclines backward.With the increase of the blocking distance,the blocking effect of the obstacle becomes smaller;When the entrainment force is equal to the pull force,the flame impacts the ceiling vertically,and the corresponding blocking distance is the critical blocking distance(D_R).(3)With the increase of the lateral distance of the fire source and the continuous approach to the side wall,the critical blocking distance of the fire source is continuously reduced,and when the fire source is laterally offset,the flame will appear"bifurcation"phenomenon.The closer the fire source is to the side wall,the more obvious the"bifurcation"phenomenon is.(4)With the increase of blocking distance,the mass loss rate first increases and then decreases,and the highest point appears at the place where the distance between the obstacle and the fire source is the critical blocking distance D_R.When D>D_R:the cosine value of flame inclination decreases with the increase of D,and the average flame length increases with the increase of D,which is consistent with that without blocking.The prediction model of flame inclination angle and average flame length with barrier-fire source distance when D>D_Ris derived.2.The influence of the distance between obstacle and fire source on the flame temperature field under the tunnel fire ceiling.(1)Compared with the transverse direction(perpendicular to the tunnel direction),the high-temperature smoke generated by the flame tends to spread along the longitudinal direction(parallel to the tunnel direction).(2)When there is no longitudinal wind inside the tunnel,the change of blocking distance has no effect on the longitudinal and transverse temperature attenuation under the tunnel ceiling,which is consistent with that without blocking.(3)When the longitudinal wind is increased,the maximum ceiling temperature of the tunnel with obstacles is twice that of the tunnel without blocking.The obstacles block the longitudinal wind from reaching the surface of the fire source,making the high-temperature smoke generated by combustion gather near the fire source,and the increase of the longitudinal wind has little effect on the longitudinal and transverse temperature attenuation under the tunnel ceiling.(4)When the tunnel is ventilated longitudinally,the critical distance also has an impact on the temperature field below the ceiling.With the increasing of the barrier-fire source,the longitudinal and transverse temperatures below the ceiling first increase and then decrease.The blocking distance corresponding to the highest temperature is the critical blocking distance.On the basis of previous formulas,the maximum temperature rise prediction model under the ceiling and the transverse maximum temperature rise prediction model with barrier-fire source distance are deduced when D>D_Ris derived.