Research On Fire Spread Characteristics Of High Altitude Highway Shaft Tunnel Fires

Against the backdrop of rapid economic development in China,the transportation industry has undergone significant improvements.The construction of a large number of highway tunnels has improved the efficiency of transportation and shortened the travel time.Due to the narrow and closed structural characteristics of tunnels,when a fire occurs,a large amount of toxic and harmful gases will be generated,causing great difficulties for the escape of trapped personnel in the tunnel.Therefore,studying the smoke spread law and smoke exhaust effect of vertical tunnels is of great significance.This article uses numerical simulation software FDS to establish a full-scale tunnel model and studies the effects of different altitudes on smoke spread,smoke temperature,smoke thermal stratification,and mass flow rate and CO concentration of shaft smoke in highway shaft tunnels.Analyzing the simulation experimental results,it was found that when the heat release power of the fire source is consistent,the speed of smoke spread increases with the increase of altitude,and the ceiling temperature,lateral temperature,and vertical temperature of the tunnel fire source and non fire source sections also increase to a certain extent;At the same altitude,the temperature of the tunnel ceiling,lateral temperature,and vertical temperature all increase with the increase of the heat release rate of the fire source;At the same altitude,the temperature of the tunnel ceiling,lateral temperature,and vertical temperature all increase with the increase of the heat release rate of the fire source;The average temperature and stratification intensity of smoke in the vertical direction of the tunnel fire source section and non fire source section show an increasing trend with the increase of altitude,indicating that the thermal stratification intensity of smoke at high altitudes is greater than that at low altitudes.A large amount of smoke accumulates above the tunnel,leading to an increase in the temperature of ceiling smoke at high altitudes;Compared to an altitude of0 m,the mass flow rate of flue gas discharged from the shaft at an altitude of 4200 m has decreased by 36.1%.As the altitude increases,the CO concentration of flue gas in the shaft also increases,and the higher the altitude,the faster the rate of change in CO concentration.The influence of altitude on the critical wind speed of tunnels and the smoke exhaust effect of vertical shafts was explored through simulation experiments.The research results indicate that under the same fire scale,there is a positive proportional relationship between altitude and critical wind speed.The critical wind speed increases with the increase of altitude.At the same altitude,when the heat release power of the fire source increases,the critical wind speed also increases;The research results indicate that under the same fire scale,there is a positive proportional relationship between altitude and critical wind speed.The critical wind speed increases with the increase of altitude.At the same altitude,when the heat release power of the fire source increases,the critical wind speed also increases;When there is no longitudinal wind or when the longitudinal wind is small,the vertical inertia force of the flue gas is greater,and the chimney effect of the vertical shaft is stronger,but it is prone to suction through phenomenon.When the longitudinal wind is large,the horizontal inertia force of the flue gas is greater,which will weaken the chimney effect of the vertical shaft and affect the smoke exhaust effect of the vertical shaft.The optimal longitudinal wind speed is conducive to the smoke exhaust efficiency of the vertical shaft.Under the action of longitudinal wind,five different blocking ratios of 0%,15%,20%,25%,and 30% were set up to study the effects of these five blocking ratios on the thermal stratification,characteristic height flue gas temperature,and CO concentration of flue gas downstream of the fire source at altitudes of 0m and 4200 m.The results show that when the heat release power of the fire source is 20 MW,the increase in blockage ratio exacerbates the turbulence of the smoke downstream of the fire source,and the height of the smoke decreases.At the same time,the stratification and thermal stratification strength of the smoke in the tunnel fire source and non fire source sections show a decreasing trend with the increase of the resistance to plug ratio;Under different altitudes and blockage ratios,the characteristic height flue gas temperature exhibits the same variation pattern.The downstream characteristic height flue gas temperature and CO concentration increase with the increase of blockage ratio,indicating that an increase in blockage ratio is not conducive to personnel escape.