Study On Smoke Flow Characteristics And Personnel Evacuation In Long Highway Tunnel Fire

With the flourishing development of China’s infrastructure,the transportation industry,represented by road tunnels,has made great progress.China adds more than1100 km of new road tunnels each year.As the number of road tunnels increases,tunnel fires have become a serious problem that affects the safety of tunnel operations.Tunnel fires generate high temperatures,dense smoke,toxic and harmful substances,which severely hinder the evacuation of trapped personnel.Most of China’s road tunnels adopt longitudinal ventilation,where the upper space of the tunnel is used as the smoke exhaust duct during smoke extraction.This method occupies a certain amount of space that could be used for personnel evacuation.Therefore,it is important to reveal the influence law of longitudinal wind speed on the evolution characteristics of tunnel fire smoke flow and explore the reasonable longitudinal wind speed during the evacuation stage of tunnel fires.This is crucial to ensure the safety of tunnel operations and provide guidance for tunnel ventilation during disasters.Based on the combustion wind tunnel experiment platform of the Urban Underground Space Fire Laboratory in Jiangsu Province,this thesis explores the influence of longitudinal wind speed on the length of near fire zone.By analyzing the temperature difference gradient in different cross-sections,a criterion for the transition from three-dimensional flow to one-dimensional longitudinal flow of high-temperature smoke is proposed.Non-dimensional analysis is used to describe the relationship between longitudinal wind speed and the length of the near-source area.The results show that under a no-wind condition,the influence of the heat source power on the length of the near fire zone is small.However,as the longitudinal wind speed increases,the length of the near fire zone gradually increases.The non-dimensional length of the near fire zone and the non-dimensional longitudinal wind speed show a linear relationship with a growth rate that is negatively correlated with the heat source power.Based on the construction background of the Chengkai Expressway tunnel,a numerical model of a 900m tunnel fire was established.Fifteen fire scenarios with different fire source powers,fire source locations,and longitudinal wind speeds were simulated to analyze the characteristics of smoke flow evolution under different longitudinal wind speeds.The results showed that when the longitudinal wind speed is constant,the spread rate of the smoke front remains constant,and the upstream smoke front velocity is negatively correlated with the longitudinal wind speed,while the downstream velocity is positively correlated.Excessive longitudinal wind speeds can cause the smoke front to overflow the smoke exhaust shaft.The ceiling jet temperature decreases along the longitudinal direction of the tunnel from the center of the fire source.With the increase of the longitudinal wind speed,the maximum temperature of the ceiling and the high-temperature area upstream of the fire source gradually decrease,and the temperature distribution of the ceiling jet downstream of the fire source basically follows the equation y=A₁×exp（-x/t₁）+y₀,which conforms to a single exponential decay.Longitudinal wind speeds of 1.2m/s and above can maintain the smoke layer height above the personnel characteristic height in each transverse section,but excessive longitudinal wind speeds can decrease the smoke layer height in downstream transverse sections,especially in transverse sections farther away from the fire source.A safety evaluation method for tunnel fire scenarios based on safety indices is proposed.The location of the evacuation bottleneck in the tunnel is determined,and environmental temperature,CO concentration,and smoke layer height are taken as factors affecting personnel evacuation.Corresponding safe evacuation critical values are determined.Based on the simulation results of tunnel fires in Chapter 4,the ASET at the evacuation bottleneck is determined.The tunnel fire model is imported into the Pathfinder software to establish a tunnel personnel evacuation model.Parameters such as personnel evacuation speed and response time are analyzed and set.Two evacuation scenarios are simulated according to different fire source locations,and RSET is calculated at each evacuation bottleneck.Based on the total safety index,the favorable longitudinal wind speed for personnel evacuation is determined.The results showed that when the fire source is located between two transverse passages,a longitudinal wind speed of 1.2m/s is more favorable for personnel evacuation,and the total safety index is about 1.6.When the fire source is directly facing the entrance of the transverse passage,a longitudinal wind speed of 1.8m/s is more favorable for personnel evacuation,and the total safety index is about 1.39.