Influence Of Upper Vents On Ventilation And Smoke Extraction In Urban Tunnel

In order to relieve the increasing traffic pressure in urban area,the number of road tunnels is increasing.Most tunnels reduce the concentration of pollutants through supply and exhaust shafts or longitudinal ventilation with jet fans.But some urban tunnels can not be set up with high wind shaft,thus the pollutant concentration at the entrance of these tunnels will exceed the standard concentration.Thus ventilation method in urban tunnels with upper vents has been better applied.The main tunnel is always connected with in and out ramps that are connected to the road.Air flow of the upper vents and ramps affects the air flow of the main tunnel which forms a more complex ventilation network.And the smoke extraction process is more complex than usual.Therefore,the influence of upper vents on ventilation and smoke extraction in urban underground tunnels is studied here.(1)Under normal operating conditions,SES numerical simulation method was used to study the effect of upper vents arrangement on the natural ventilation in urban tunnels with multiple ramps.The results show that when the area of the vents remains the same,vents should be arranged at the bifurcation point,which can effectively reduce the concentration of pollutants to achieve better ventilation effect inside tunnels and at the tunnel outlet.When the tunnel section between the two bifurcation points exceeds 1000 meters,it is necessary to add vents above the tunnel section downstream.(2)Under fire conditions,FDS numerical simulation was used to analyze ventilation and smoke extraction in urban tunnels with upper vents.The results show that smoke is mainly discharged through ventilation which can exceeds more than 50 percent of the total mass flow.The smoke distribution ratio of the main tunnel,ramp and upper vent is independent from the fire source power.And the angle has no effect on the smoke characteristics of the main tunnel.(3)Under fire conditions,FDS numerical simulation and experiment methods were used.The critical velocity dimensionless of ramp correlation that correlates well with the experimental data is proposed.Because of the upper vents,the critical velocity is independent from the ventilation velocity in the main tunnel.And its value and growth rate is lower than without upper vents.