Effect Of UTLT Portal And Exit Bifurcation Angle On Smoke Flow

In order to alleviate the congestion of ground traffic,more and more cities have built or planned to build Urban Traffic Link Tunnel(UTLT).UTLT is composed of a main tunnel and branch tunnels.The main tunnel is located under the city’s ground,and is connected to the ground road and underground parking garage system through the branch tunnels.In actual engineering,in order to meet the planning and construction needs,there are often many different bifurcation angles between the main and branch tunnels.For UTLT with longitudinal ventilation,once a fire occurs,the smoke will spread and flow in the main and branch tunnels.Whether the bifurcation angle affects the flow characteristics of smoke between the main and branch tunnels and the critical velocity are worthy of attention and discussion.Therefore,based on the previous research on the UTLT smoke flow characteristics,this paper focuses on the influence of the bifurcation angle on the UTLT fire smoke flow characteristics and the critical velocity,and further improves the design of the UTLT project smoke exhaust system.This paper investigates the bifurcation angles of UTLT in Chongqing,and divides the bifurcation angles of UTLT into 6 types according to the survey results.Choose the bifurcation angles of 90° to establish a small-scale model test bench containing the main and a branch tunnel,and carry out small-scale experiments.At the same time,use fire dynamics simulator(FDS)to conduct numerical simulations corresponding to the smallscale experiments.The comparison between the experimental data and the FDS numerical simulation results verifies the rationality of setting the boundary conditions of the FDS model.Taking an entrance branch tunnel and a part of the main tunnel connected to it as a UTLT Phase II project in Yuzhong District of Chongqing as the research object,the FDS numerical simulation was used to study the influence of bifurcation angle on the UTLT ceiling temperature when the UTLT mechanical ventilation and smoke exhaust system failed.According to the Origin data fitting,the prediction model of dimensionless temperature distribution under the tunnel ceiling is proposed.The results show that the dimensionless ceiling temperature distribution of the main tunnel is less affected by the bifurcation angle,and the dimensionless ceiling temperature distribution of the branch tunnel is more affected by the bifurcation angle.Through FDS numerical simulation,the influence of 6 typical branch tunnels with different bifurcation angles on the critical velocity of the main tunnel was explored.At the same time,it was compared with the predecessor critical velocity prediction model,and it was found that the branch tunnel had a certain influence on the critical velocity of the main tunnel.If a suitable prediction model is selected,it can be used to calculate the critical longitudinal ventilation velocity of the UTLT main tunnel.