Studies On The Characteristics Of Fire-induced Smoke And Smoke Control In Underground Interconnected Infrastructure

Underground interconnected infrastructures are a new and rapidly developing type of transportation infrastructure that has been commonly employed for the interconnection of busy roads in urban areas,which effectively solves the traffic bottleneck in these areas.Underground interconnected infrastructures usually include numerous branches of tunnels.In such an infrastructure,different tunnel branches are connected,forming a complex underground transportation network.When a fire occurs in underground interconnected infrastructures,fire-induced smoke will spread not only in the tunnel where the source of the fire is located but also to adjacent connected tunnels.Models of smoke exhaustion are complicated,and research on smoke characteristics and smoke control in underground interconnected infrastructures is still insufficient.More research on theoretical models is needed to provide theoretical support for the formulation of smoke ventilation strategies in underground interconnected infrastructures.This paper presents research concerning on the characteristics of fireinduced smoke and the control of smoke flow in underground interconnected infrastructure with different smoke exhaustion modes.The temperature distribution of smoke flow is revealed,and the characteristics of smoke spread are analyzed.The results are as follows.(1)The effects of different turning radii on the maximum temperature rise and temperature decay beneath the ceiling are analyzed in a tunnel with natural ventilation.A series of experiments were carried out on through the model tunnels with different turning radii.The result shows that,when the tunnel fire is in a condition with a strong plume-driven ceiling jet,different turning radii have little impact on the maximum temperature increase and decay beneath the tunnel ceiling.The measured maximum temperature rise beneath the tunnel ceiling is larger than the predicted value of the previous correlation based on the assumption of a weak plume-driven ceiling jet.According to the classical correlation for predicting the axial temperature rise in the fire plume,the maximum temperature rise beneath the tunnel ceiling is predicted.The correlation of the temperature decay beneath the tunnel ceiling is proposed,which can describe the trend of temperature decay.(2)A correlation predicting the rise time of the smoke plume front is established based on a theoretical analysis,which can be applied for a tunnel with longitudinal wind,as well as that with a windless environment.A numerical simulation was conducted to study the rising smoke plume,in which different heat release rates,different source–ceiling heights,and different longitudinal wind speeds were taken into account.The results show that the time that the smoke plume front rises in the tunnel under the influence of longitudinal wind is larger than that in a windless environment.The deduced correlation is verified through numerical results,which can predict the rise time of the smoke plume front in tunnels with windy or windless environments.(3)Based on the theory of the ventilation network,a mathematical method is proposed to predict the confluence ratio of the wind flow of different tunnel branches under the uniform flow field(no fire source is set)for the underground interconnected infrastructure with longitudinal ventilation.The influences of different jet fans’ pressure rise,different locations of the jet fans,and the resistance on the distribution of the wind flow in an underground interconnected infrastructure are studied.The predicted correlation of the confluence ratio can adequately describe the variation in the wind flow distribution with different locations of jet fans and different jet fans’ pressure rise.(4)The characteristics of the temperature distribution and the spread of the smoke flow are analyzed for the scenario where the fire source is located at the junction point of the underground interconnected infrastructure with longitudinal ventilation.The results show that the temperature decay coefficient in the downstream direction of the junction point is affected by both the longitudinal ventilation speed upstream of the junction point and the longitudinal ventilation speed of the connected tunnel.A correlation is proposed to predict the temperature decay for the scenarios of different heat release rates and different longitudinal ventilation speeds of tunnel sections.The spread of smoke flow in the upstream direction of the junction is mainly affected by the heat release rate and the upstream longitudinal ventilation speed,while the spread of the smoke flow in the connected tunnel is also affected by the longitudinal ventilation speed of the connected tunnel.As the upstream longitudinal ventilation speed increases,the smoke tends to flow downstream of the junction point.Therefore,the increase in the upstream longitudinal ventilation speed is conducive to preventing the spread of smoke in the connected tunnel.(5)The characteristics of the velocity and the temperature distribution of the exhaust vents are studied in the underground interconnected infrastructure with centralized smoke exhaust.The temperature distribution beneath the tunnel ceiling and the spread of the smoke flow are also analyzed.The temperature decay beneath the tunnel ceiling with centralized smoke exhaust is consistent with the exponential decay,in which the decay coefficient is affected by the heat release rate of the fire source and the volumetric flow rate of the centralized smoke exhaust system.Based on the predicted model of back-layering smoke length that was proposed for a longitudinally ventilated tunnel fire,a correlation is proposed for predicting the smoke-layering length in the underground interconnected infrastructure with centralized smoke exhaust.The characteristics of fire-induced smoke,the distribution of the temperature of smoke flow,and the characteristics of the smoke spread in the underground interconnected infrastructure are studied.The obtained results may provide a reference for the formulation of smoke-control strategies for underground interconnected infrastructure,and they may also serve as a reference for the design of ventilation systems in similar underground interconnected infrastructure projects.