Study On The Fire Smoke Movement Characteristics And Smoke Backlayering Control In Urban Branched Tunnel

Fire accident is one of the main threats to safety of underground road tunnel.Controlling the smoke movement in road tunnel is significant for safety of the tunnel structure,facilities and persons.The air flow in this road tunnel is more complicated than single-point tunnel,and the smoke movement is difficult to control due to the special branched structure.Therefore,the fire safety in this type tunnel is paid much attention and awaiting to solve.This paper investigates the characteristic of smoke propagation and smoke control in branched tunnel under different conditions.This paper analyzes the research meaning and necessity of fire safety in branched tunnel.The 1/20 scale branched tunnel are conducted to carried out the experiments of smoke propagation with different heat release rate.The smoke propagation in branched tunnel is investigated by experiments and theoretical analysis,as well as the numerical simulation are carried out to analyze the smoke controlling strategy.The research content in this paper include temperature profile in branched tunnel,smoke back-layering flow behavior,critical ventilation velocity and required dynamic to prevent smoke backflow.In addition,the smoke controlling strategy in multiple-point tunnel is also investigated.Firstly,focusing on the temperature distribution beneath the ceiling,the maximum temperature and temperature longitudinal decay along the tunnel ceiling under natural ventilation and forced longitudinal ventilation are expressed by conservation equation.The experiments are carried out to detect the temperature in branched tunnel with different bifurcation angle under varied heat release rate and ventilation velocity,the effect of bifurcation angle on temperature profile are analyzed.Results show that the effect of bifurcation angle on maximum temperature rise under natural ventilation is negligible.On the contrary,the maximum temperature rise beneath the tunnel ceiling under forced ventilation is affected by bifurcation angle.The temperature longitudinal decay in traffic converging flow and traffic diverging flow are investigated,and the effect of bifurcation angle on temperature longitudinal decay are studied.The predicted model for maximum temperature rise and temperature longitudinal decay beneath the ceiling are proposed with taking the bifurcation angle into consideration based on the theoretical analysis and experimental test.Secondly,focusing on the smoke back-layering length in tunnel,the smoke back-layering length in mainline tunnel under wind supply from mainline before shunting and after shunting are researched,respectively.The force equilibrium of smoke back-layer upstream of the fire source are expressed based on the conservation of energy,then,the expression of smoke back-layering length is deduced.The volume flow of smoke and the air entrainment in branched tunnel are influenced by bifurcation angle that result in the varied driving force and resistance of smoke back-layer upstream of the fires source.Thus,the smoke back-layering length upstream of the fire source in the branched tunnel are affected by the bifurcation angle.This paper take the bifurcation angle into consideration to investigate the smoke back-layering length under different bifurcation angle,experimentally.Results show that the effect of bifurcation angle and ventilation velocity on smoke back-layering length is significantly.The smoke back-layering length decrease with the increasing of ventilation velocity,and increase with the lager heat release rate.Moreover,the effect of bifurcation angle on smoke back-layering length is nonlinearly.The two-piecewise predicted model for smoke back-layering length is proposed combined the measured results and the theoretical equation.In addition,the predicted model takes the the effect of bifurcation angle on smoke back-layering length into consideration.Thirdly,focusing on the critical longitudinal ventilation velocity and required driving force for prevent smoke back-layering,the effect of bifurcation angle on critical velocity is studied.When the smoke front plane is over the fire source,in other words the smoke back-layering length is zero,the corresponding ventilation velocity is critical velocity.The Bernoulli equation are carried out to analysis the force condition in branched tunnel,while the frictional resistance,the local resistance induced by branched structure and the resistance induced by fire source are considered.Furtherly,the predicted model for critical velocity in branched tunnel under traffic converging flow and diverging flow are proposed.The theoretical model shows that the critical velocity affected not only by bifurcation angle and heat release rate but also the geometry of tunnel.The specific expression for critical velocity and driving force for preventing the smoke back-layering in branched tunnel are proposed combing the theoretical analysis and experimental test.The predictions by proposed model are compared with the experimental results,they are in good agreement.The predicted model for critical velocity,which takes the bifurcation angle into consideration,could be applied to calculating the critical velocity in branched tunnel.The driving force for preventing smoke back-layering equals to the pressure loss induced by fire source and wind during the none smoke back-layering.Thus,the driving force for preventing smoke back-layering is calculated and used as initial condition for numerical simulation The simulated results show that the smoke back-layering could be prevented by predicted driving force.Finally,the calculated method of ventilation velocity and pressure loss in multiple-point tunnel are analyzed.The local resistance coefficient in multiple-point tunnel under different split ratio of ramp are determine by combined the simulation and mathematical equation.Furtherly,the required air volume flow rate in different ramp and mainline tunnel upstream of the fire source are calculated,which used as initial condition of air flow for numerical simulation for preventing smoke back-layering.The simulated results show that the smoke back-layering could be prevented under different split ratio of ramp,and the ventilation velocity at the fire source could reach critical value.The optimal ventilation scheme is chosen by considering the driving force and ventilation volume flow.