| In the environment of economic development,urbanization is carried out smoothly and orderly,the exploitation of underground space has become an important direction for urban development in order to alleviate the traffic pressure caused by the increase of population.In recent years,cities over the country have built a large number of urban road tunnel and subway tunnel that brings great convenience to people’s transportation.However,the fire hazards and the fire accidents have brought great challenges to the tunnel firefighting.In fire accidents,the proportion of deaths caused by toxic and harmful gas attains 85%,the study of smoke movement and the selection of high-efficiency smoke exhausting model are of great significance to reduce the loss and rescue work for fire accidents.Even though many researches have conducted,the law of smoke movement,flow field and temperature field of the shaft areas,smoke exhausting efficiency will be different with the effect of longitudinal ventilation.Since then,theoretical analysis,scaled experiments and full scale simulation experiments will be carried out to research smoke stratification and influence of boundary conditions to the shafts exhausting with ventilation in this paper.The main contents are the following:1.Smoke stratification and determination methods of smoke layer.Theoretical analysis and scaled experiments have been carried out to study smoke stratification with the influence of longitudinal ventilation in tunnel fires.The stability of smoke layer began to be destroyed if the ventilation attained to 0.0961 m/s when fire source HRR(heat release rate)equal to 5.73 kW.Smoke layer will be disturbed completely when the ventilation attained to 0.226 m/s.The smoke layer height descend and rise again with ventilation inflection point of 0.485 m/s,0.629 m/s,0.629 m/s for fire sources equal to 2.30 kW,5.74 kW,9.18 kW respectively.The conclusions of different thermal layer determination methods are compared and analyzed:As for N-percentage rule,it’s recommended that the value of N to be 30 in tunnel fire;Dimensionless temperature method could be chosen if velocity ventilation is low;The results of the determination by buoyancy frequency are lower for all experiments.2.Analyzing the air entrainment of shafts exhausting with the influence of longitudinal ventilation,scaled experiments were conducted to study the smoke stratification in the shafts area and plug-holing.The air will be entrained into the shaft from the top if shaft height is low or longitudinal ventilation is too large which will affect the exhausting.When it comes to the determination of plug-holing,non-dimensional value Fcritical = 1.5 could be suitable when no ventilation applied,but it will be inappropriate for ventilation conditions.Nonetheless,Riciritical=1.4 is appropriate.3.The influence of longitudinal ventilation,shaft height and arrangement to shaft exhausting efficiency was investigated by numerical simulation experiments.Smoke exhaust rate will decrease as ventilation increase if shaft height lower than 3 m.The amount of smoke exhausting will decrease firstly and then aggrandize with the increase of ventilation if shaft height higher than 3 m.The exhaust rate will attain the maximum value when ventilation velocity within the limits of 1.5-2 m/s.The inhibitory effect of new arrangement of shafts to the plug-holing is evident when no ventilation applied.Nevertheless,the increase of smoke exhausting of the shaft by new arrangement is limited if exhaust rate attain the maximum affected by ventilation. |
