Study On Ventilation Resistance Characteristics And Smoke Extraction Strategies Of Urban Complex Tunnels

Urban complex tunnels have the characteristics of multiple branches,high traffic flow,and complex internal structures.The longer the airflow organization,the more complex the straight tunnel,which brings higher difficulty to the ventilation and smoke exhaust design of the tunnel.Based on a complex tunnel project in a certain city,the resistance characteristics and ventilation and smoke exhaust strategies were studied through theoretical analysis and numerical simulation.The main research results are as follows:(1)A local resistance coefficient model for complex tunnel structures was established considering wind speed,curvature radius,bending angle,branch angle,and air volume ratio.It was found that the ventilation wind speed,bending angle,and curvature radius all have an impact on the local resistance coefficient of the bending structure,while only the branch angle and air volume ratio have a significant impact on the separation and merging flow structure.The optimal plan to reduce local resistance loss was proposed.(2)A model of driving speed and the number of jet fans opened based on the ventilation strategy for urban complex tunnel operation is constructed.The minimum number of fans opened is negatively correlated with driving speed.At the same time,by analyzing the impact of internal air flow organization and make-up air openings on the number of fans opened,it is found that adding a make-up air shaft can reduce the number of fans opened.If there are few confluence ramps that are difficult to provide sufficient make-up air,it is recommended to adopt the operational ventilation strategy of opening a make-up air shaft in the sections with more jet fans opened in the tunnel or using a smoke exhaust working shaft as both a supply air shaft to reduce mechanical energy consumption.(3)An evaluation model for the effectiveness of smoke extraction strategies for complex tunnels was established,and two smoke extraction outlet arrangements were compared.When one smoke extraction outlet was installed at the end of the ramp,the smoke extraction volume reached 130 m3/s the smoke can be effectively controlled,while the scheme of setting a single smoke outlet at the confluence has a large amount of entrainment of the surrounding air,requiring a smoke exhaust volume of 170 m3/s can effectively control smoke.Considering the cost of civil engineering and the difficulty of construction,and considering that fires are extremely unlikely events,it is recommended to arrange a single smoke exhaust outlet at the confluence.