| Urban Traffic Link Tunnel(UTLT for short)is a new type of urban public transportation system.Compared with the conventional highway tunnel with one entrance and one exit,it includes the main tunnel,the entrance and exit branch tunnel,and the connecting branch tunnel that communicates with other garages.Its main tunnel is ring-shape and its branch tunnel’ number is large,which leads to greater fire hazard.The current research on UTLT fire smoke control is mainly based on the main tunnel fire conditions,but due to the characteristics of the entrance and exit tunnels,such as large slope,short mileage,and one port connected to the main tunnel,once the fire occurs in the entrance and exit tunnels,the fire smoke control also faces huge challenges.This paper mainly relies on a UTLT constructed in Jiangbei District of Chongqing to study the flow characteristics of fire smoke in the tunnel when the UTLT entrance and exit tunnel is on fire.The slope of the UTLT entrance and exit tunnel is generally larger than that of conventional road tunnels.When the methods of the longitudinal ventilation and smoke exhaust are used,the critical velocity is an important basis for the design of longitudinal ventilation systems,and the larger slope will affect the magnitude of critical velocity.Based on the relevant design data of a UTLT in Jiangbei District,Chongqing,this paper selects a UTLT entrance and exit tunnel with a "T" connection as the object,and conducts small-scale experiments to verify the FDS results in the case of a fire in a tunnel with a "T" structure.Furthermore,the FDS numerical simulation method was used to study the critical velocity of the entrance and exit tunnel under a large slope range(-9%～9%).The results show that in uphill tunnels,the critical velocity decreases with increasing slope,and in downhill tunnels,the critical velocity increases with increasing slope.Through the data fitting process,the slope correction formula for critical velocity of the UTLT entrance and exit tunnel in the form of "T" connection within a large slope range is obtained,and it is compared with the slope correction formulas proposed by the predecessors in conventional road tunnels.It is found that the critical velocity in uphill tunnels is smaller than conventional road tunnels,and in downhill tunnels the critical velocity is larger than conventional road tunnels.Therefore,when using longitudinal ventilation and smoke exhaust design in the UTLT entrance and exit tunnel,it is necessary to carefully select the appropriate critical velocity correction formula.When a fire occurs in a tilted tunnel,a certain amount of longitudinal velocity is induced due to the slope,that is,the induced longitudinal velocity.When the wind velocity is just equal to the critical velocity,it is defined as the critical induced longitudinal velocity.Due to the characteristics of large slope and short mileage of the UTLT entrance and exit tunnel,it is possible to make full use of induced longitudinal velocity for natural smoke exhaust.This paper explores the feasibility of using natural smoke exhaust in the UTLT entrance and exit tunnel.When natural smoke exhaust is used for the entrance and exit tunnel,the relationship between induced longitudinal velocity and the tunnel slope,the fire source location,and the main tunnel pressure is theoretically analyzed,and FDS numerical simulation is used to analyze the effects of these three factors on natural smoke exhaust for the entrance and exit tunnel.The results show that the induced longitudinal velocity increases with the increase of the tunnel slope,the distance from the fire source to the tunnel exit,and the main tunnel pressure,and the induced longitudinal velocity is greatly affected by the main tunnel pressure.When the natural smoke extraction effect is not good,the natural smoke extraction capability of the UTLT entrance and exit tunnel can be improved by adjusting the main tunnel pressure. |
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