Numerical Simulation Study On The Fire Of Super Large Section Highway Tunnel Under Water Mist System

At present,the application of water mist system in highway tunnels has attracted more attention,but most of the previous studies have focused on general cross-section tunnels,and there is insufficient research on large section tunnels,especially super large section tunnels with a cross-sectional area greater than 100 m2.In addition,the various technical specifications in our country have not made clear provisions on the arrangement of water mist in the tunnel.Therefore,it is necessary to study the reasonable arrangement of water mist nozzles in large section and super large section highway tunnels.This article takes two types of large section tunnels(cross-sectional area 58m2,79m2)and four types of super large section tunnels(cross-sectional area 104m2,131m2,161m2,194m2)as the research objects.Choose the top or side layout of water mist nozzle.consider the fire source power,location,and whether blocked or not.Take the critical temperature of the fire resistance limit of the tunnel ceiling and the critical heat radiation intensity of the adjacent vehicle being ignited as the criterion.Using FDS to analyze the tunnel fire characteristics under the action of water mist,then obtain the water mist nozzle arrangement scheme suitable for the highway tunnel.The results shows:(1)When a fire occurs in a large-section and super-large section tunnel,the effect of water mist is greatly affected by the cross-sectional area of the tunnel.For super-large section tunnels with a cross-sectional area greater than 100m2,the single-row arrangement of water mist cannot effectively suppress the 30MW large truck fire.The roof structure of the tunnel will be damaged by high temperature and adjacent cars will be ignited by heat radiation.Therefore,the top or side spray nozzle layout should be added.For super-large section tunnels with a cross-sectional area greater than 131m2,the double row top spray arrangement cannot effectively suppress the 30MW large truck fire.In this case,the double row side spray arrangement should be adopted.For super-large section tunnels with a cross-sectional area greater than 161m2,the double row side spray arrangement cannot effectively suppress the 30MW large truck fire.In this case,the nozzle arrangement combining top spray and side spray should be adopted.(2)Under the optimized water mist nozzle arrangement scheme,when the longitudinal wind speed in the tunnel is not greater than 3.5m/s,it will not affect the suppression effect of the water mist system.(3)When the cross-sectional area of the tunnel is 58m2～161m2,and the fire source power is in the range of 5MW～30MW,the water mist double-row top spray is arranged,the max temperature of the ceiling directly above the offset fire source meets a linear relationship with the cross-sectional area of the tunnel.This article can provide reference and technical support for the application of water mist system in large cross section and super large cross section highway tunnels.