Study On The Influence Of Asymmetric V-Shaped Tunnel Structure On Smoke Propagation Characteristics And Flame Characteristics

With the rapid development of our country’s social economy and the improvement of the level of urbanization,the traffic volume continues to increase,traffic congestion and environmental pollution have become the stubborn problems of urban traffic.Building urban underground highway tunnels is an important method to solve urban traffic congestion and improve urban traffic environment.However,in recent years,with the continuous development and utilization of urban underground space,the available underground space in urban has become increasingly tight.New urban underground tunnels need to across existing underground facilities from above or below,such as subways,utility tunnel,and underground transportation hubs.And the vertical structure is becoming more and more complex.Urban underground highway tunnels have the structural characteristics of variable slopes.When a fire accidient occurs near the slope change point of the tunnel,the buoyancy generated by the high temperature smoke will push the smoke flowing upward along the slope direction.If the slope and length of the tunnel on both sides of the V-shaped slope are different,an asymmetric driving pressure difference will be generated on both sides of the slope change point,causing the smoke flow to trend to the larger slope side,and the smoke on the other side backflow to the fire source.This asymmetric pressure difference will gradually increase as the temperature in the tunnel increases.It has a great influence on the smoke plume and the flow of the ceiling jet at the fire source,which makes the smoke flow show obvious temporal and spatial variation characteristics.Therefore,it is one of the urgent research topics to study the smoke propagation characteristics in asymmetric V-shaped slope tunnels.Based on a highway underground tunnel in Beijing,the 1/20 scale V-shaped slope tunnel with adjustable slopes on both sides are conducted to carried out the experiments.The smoke propagation and flame shape in V-shaped slope tunnel is investigated by theoretical analysis,experiment and numerical simulation in this thesis.The research content is the influence of the asymmetric V-shaped tunnel structure on the flame shape,temperature distribution,somke back-layering length,longitudinal induced velocity and critical tunnel stucture conditions for suppressing smoke backflow when the fire source is located at the slope change point.The research results show the following:Firstly,the flame inclination angle will increase with the increase of the largeslope side tunnel slope,difference in slopes and heat release rate.The predicted model for flame inclination angle is proposed with taking the slope of the tunnel on both sides and heat release rate into consideration based on the experimental test.The maximum temperature beneath the ceiling decreases with the increase of the large-slope side slope and the difference in slopes.And the maximum temperature position beneath the ceiling is biased towards the large-slope side tunnel.Comparison and verification with the predecessor model are carried out.Compare the experimental results with previous prediction models of the maximum temperature.Secondly,the influence of the asymmetric V-shaped tunnel structure on the temperature longitudinal distribution along the tunnel ceiling is studied.Results show that the temperature beneath the ceiling and temperature distribution range on the smallslope side decrease with the increase of the tunnel slope on the large-slope side,and the decay rate of the temperature along the large-slope side tunnel decreases with the increase of its slope.When the slope of the large-slope side is 9%,the temperature along the entire small-slope side tunnel is almost close to the ambient temperature.The predicte model for dimensionless longitudinal temperature-rise decay beneath the celing of large-slope side tunnels and small-slope side tunnels are respectively proposed,which was verified by comparison with experimental results.Thirdly,the smoke in the two sides of the tunnel spreads to the entire tunnel when the difference in slopes is small and the base slope is small,and while the difference in slopes or the base slope is large,the smoke backflow in the small-slope side tunnel.The smoke back-layering length decreases with the increase of the difference in slopes and the base slope,and the smoke back-layering lengths of the same V-shaped tunnel structure with different heat release rate are very close.When the difference in slopes is large,the longitudinal induced ventilation velocity is mainly affected by the slope of the large-slope tunnel,and is less affected by the slope of the small-slope tunnel.Based on the causes and mechanisms of the smoke propagation characteristics and longitudinally induced ventilation,the predicte model for the smoke back-layering length on the small-slope tunnel and the longitudinal induced ventilation velocity are respectively proposed,and the simulation results are compared with the prediction model.A critical gradient conditional expression that can inhibit the diffusion of smoke to the side with small gradients is proposed,and the numerical model is tested.The critical tunnel stucture conditions for suppressing smoke backflow to small-slope tunnel is prposed,which was verified by supplementary numerical simulations.