Studies On Fire Smoke Development And Control Effect In Natural Ventilation Tunnel Fires

With the rapid development of China’s economy and the deepening of urbanization process,the increase of urban scale and population has brought huge traffic pressure.In order to reduce the traffic pressure brought by urban population growth,the development of underground space is accelerating,and the number of tunnels is increasing year by year.However,with the increasing number and length of tunnels,the frequency of tunnel fire accidents is increasing.Once a fire accident occurs,it is difficult to effectively discharge the high temperature fire smoke,which will pose a serious threat to people’s lives and tunnel structure.Catastrophic tunnel fire accidents have occurred more often in recent years.Considering at some problems existing in past studies in tunnel fires,this paper studies the combustion characteristics of rectangular fire source in tunnel fire,smoke flow in inclined tunnel and natural smoke exhaust in multiple shafts of tunnel by combining small-scale experiments and numerical simulations.In some of the actual tunnel fire cases,the fire surface can not be considered as axisymmetric fire source.In these cases,the spreading process of tunnel fire can not be explained by the results of axisymmetric fire experiments.In this paper,a series of experiments were carried out on a small-scale tunnel model to study the combustion characteristics of rectangular gas fire and the characteristics of ceiling jets.By changing the aspect ratio and arrangement orientation of fire source,the influence of these factors on the flame shape and flame length is analyzed.Based on the theoretical analysis,the dimensionless relationship of flame length of rectangular gas fire in tunnel considering the of heat release rate,the aspect ratio of fire source and the arrangement orientation is established.The dependence of the maximum temperature and the maximum temperature distribution under the ceiling of wall fire on those factors are also analyzed.Inclined road or railway tunnels are widespread in the world,especially in mountainous areas.The fire risk of inclined long tunnels has also attracted extensive attention.In this paper,the numerical model of inclined tunnel are built to study the fire smoke flow characteristics in inclined tunnels with different lengths and slopes.The effects of tunnel length and slope on the maximum temperature and its position under the ceiling and the length of back-flow were quantitatively studied by correlating the longitudinal ventilation rate of horizontal tunnel and the inlet wind speed at the entrance of inclined tunnel under natural ventilation.Along with the rapid development of the city transportation,many shallow urban road tunnels with multiple openings or shafts have been built.Considering its fire safety,the shaft natural smoke extraction system must provide enough smoke exhaust performance to ensure the safety of tunnel structure and personnel in the tunnel in case of fire.In this paper,the numerical model of natural ventilation tunnel is established.The fire smoke control effect of exhaust section is studied by changing the size of shaft(opening ratio)and heat release rate.The effects of different shaft sizes on the smoke mass flow rate,heat flow rate in the tunnel and smoke exhaust rate in tunnel shafts are analyzed.Through the calculation of Ri number of tunnel shafts,the applicability of Ri number criterion in smoke control effect in multi shaft tunnel is verified.