Study On The Influence Of The Ceiling Jet Characteristics Of The Fire Flue Gas On Tunnel Mutation Section

With the rapid development of China's transportation industry,various structural forms of traffic tunnel projects continue to emerge,which also brings more tunnel fire risks.Because tunnel fire is easy to cause catastrophic consequences such as tunnel structure damage and casualties,tunnel fire prevention and ventilation and smoke exhaust are very important,the core of which is to master the characteristics of tunnel fire ceiling jet.The current related research is mainly aimed at the equal section tunnel or simplified to the equal section tunnel.But in the actual engineering practice,there is often a change of section shape or size,which directly affects the smoke flow characteristics of fire and fire rescue design.Therefore,it is of great theoretical significance and engineering application value to study the jet characteristics of flue gas ceiling in mutation section tunnel.In this paper,theoretical analysis and numerical simulation are used to study the influence of tunnel mutation section on the ceiling jet of fire smoke.The main research contents include:(1)the influence of tunnel mutation section on fire smoke spreading process;(2)the temperature characteristics of fire smoke at tunnel mutation section and the longitudinal attenuation law of the highest temperature of ceiling jet;(3)the influence of tunnel mutation section on fire smoke back-layering and critical ventilation velocity.The results show that:The distance from the fire source and the ratio of sudden expansion of the mutation section of the tunnel significantly affect the fire smoke spread.The fire source is located at the mutation section,the velocity of flue gas spreading to the low ceiling area is less than that to the high ceiling area;when the sudden expansion ratio increases,the velocity of flue gas spreading to the high ceiling area increases,while the velocity of smoke spreading to the low roof area decreases.The fire source is located in the low ceiling of the tunnel,the flue gas forms a sudden expansion flow when it spreads from the low ceiling to the high ceiling area,the smoke velocity increases,and the increase decreases with the increase of the distance between the fire source and the mutation section.The fire source is located in the high ceiling of the tunnel,the shrinkage flow occurs when the flue gas spreads vertically from high ceiling to low ceiling,the velocity of flue gas decreases,and the decrease decreases with the increase of the distance between the fire source and the abrupt section.The distance from the fire source and the ratio of sudden expansion affect the longitudinal distribution of flue gas temperature.The fire source is located at t the mutation section,at the same distance from the fire source,the temperature of the high ceiling area is larger than that of the low ceiling;and the sudden expansion ratio increases,the temperature change of the low ceiling area is not obvious,but the temperature of the high ceiling area is obviously reduced.The fire source is located in the low ceiling or high ceiling area,and the temperature of the flue gas flowing through the mutation section will drop obviously,all of which decrease with the increase of the distance from the fire source to the abrupt cross section.At the same time,the longitudinal attenuation relation of the highest temperature is given.The mutation section of tunnel directly affects back-layering and critical ventilation velocity of fire smoke.The situation of the smoke back-layering and critical ventilation velocity in low ceiling area are different from those in high ceiling area.The critical ventilation velocity of fire source in low ceiling area is less than that of fire source in high ceiling area.When the fire source is located at the mutation section,it only needs the longitudinal wind speed of the low ceiling side to reach the critical ventilation velocity value,and the flue gas will no longer have back-layering.The research work of this paper is supported by the national key research and development program project(YFC0802206 2016)and the Shenzhen Banyin channel project and other projects.The research results enrich the research work of tunnel fire and have certain scientific significance.At the same time,it also provides guidance for fire safety design and emergency rescue of variable section tunnel,and has good engineering application value.