Study On Smoke Movement Characteristics And Smoke Exhaust Strategy Of Bifurcated Tunnel Fires

With the rapid development of China’s transportation industry,the transportation network in cities has become more and more complicated.When urban underground roads are connected to the ground road network,underground transportation hub or other underground roads,the bifurcated tunnel is formed.The ventilation mode of tunnels in China basically adopts the longitudinal ventilation and smoke exhaust method.The tunnel ventilation and smoke exhaust system is designed based on the critical velocity and the pressure balance in the tunnel.However,due to the special structure of the different tunnel cross sections and the expanding section in the bifurcated tunnel,smoke flow and pressure changes are particularly complex.Therefore,it is urgent to systematically analyze the key parameters of ventilation,smoke exhaust and strategy in the bifurcated tunnel.This paper first studied the effect of tunnel width and tunnel expansion on the ventilation system in the tunnel,and used Fluent to simulate the tunnel fire conditions.Introduced the concept of effective wind speed to analyze the influence of tunnel width and tunnel expansion on critical velocity under different heat release rates,then compared with the results of small-scale experiment.The effects of different heat release rate,different ventilation velocity,tunnel width and tunnel expanding section on fire pressure were analyzed.The formula of fire wind pressure on tunnel width,heat release rate and ventilation velocity was derived.Secondly,using Fluent to simulate the fire on the expanding section of the bifurcated tunnel and the fire after the tunnel bifurcation.The different tunnel bifurcation angles were analyzed for the critical velocity and fire pressure.The design requirements for the smoke exhaust system of the tunnel under various fire conditions were obtained.Finally,for the bifurcated tunnel on the west line of the Zijingang,according to the studies of critical velocity and fire pressure,the ventilation pressure coupling calculation in the three sections of the tunnel is considered with the fire pressure and the critical velocity,the fans requirements under operation ventilation and fire conditions were determined,the design of the ventilation and smoke exhaust system for the bifurcated tunnel was completed.And the impact of the location of the fire source on the best smoke exhaust strategy was analyzed,the best smoke exhaust strategy was determined.The main conclusions are as follows:(1)In the constant-section tunnel,the two factors: flow resistance around the fire zone and the increase in smoke friction resistance,interact with the increase of the wind speed.As a result,the fire pressure first increases with the increase of wind speed,then decreases with the increase of wind speed after reaching a peak value,and finally stabilize when the wind speed increases to the critical velocity;and as the tunnel width increases,the influence of ventilation velocity on fire pressure is gradually weakened;the change of fire pressure in expanding tunnel is the same as that of tunnel of equal section,but due to the backlayering speed is faster in the tunnel’s expanding section,the greater the inertia item of the smoke,the greater the fire pressure.(2)In the case of small-angle bifurcation of tunnel(0-15°),whether there is bifurcation or different tunnel bifurcated angles has little effect on the critical velocity,but the larger the tunnel bifurcation angle,the smaller increase in smoke friction resistance,the smaller the fire wind pressure value;comparing with the expanding tunnel,the bifurcated tunnel due to the existence of the bifurcation,the suddenly expansion causes the increase in smoke friction resistance,requires greater wind pressure;the fire pressure at the expanding section is not large.Therefore,when the bifurcated tunnel’s expanding section is in fire,it should pay more attention to the effect caused by the decrease of effective wind speed in the expanding section.When the main and branch roads are fired after the bifurcation,because the fire wind pressure exists in the roads after the bifurcation,the fire wind pressure will obviously affect the diversion of smoke and change the pressure equilibrium state in the bifurcated tunnel.(3)In the case of fire conditions,the influence of critical velocity,fire pressure,and location of the fire source on the coupling calculation of pressure balance in the tunnel needs to be fully considered;when fire occurs on the main road before the bifurcation,as long as the fans on the main road before the bifurcation are turned on,the critical velocity of the fire in the expanding section of the main road before the bifurcation can effectively suppress the backlayering;when the fire occurs in the main and branch roads after the bifurcation,based on the smoke exhaust of the main road fans before the bifurcation,and then the fire section fans are turned on to jointly exhaust smoke can effectively suppress the backlayering.