Study On Ceiling Jet Behavior Induced By Wall Fire In Arched Tunnel

Highway tunnels are widely used in modern transportation structures as a way to effectively shorten the transportation distance between regions.However,while the construction of highway tunnels brings convenience,it also greatly increases the risk of fire.Currently,research on highway tunnel fires mainly focuses on horizontal-ceiling tunnels,and there is little research on fires in arched-ceiling tunnels.Based on this,this thesis combines small-scale model experiments with FDS simulations to study the ceiling jet and temperature distribution induced by wall fire in arched-ceiling highway tunnels.Using n-heptane as fuel,experiments were conducted on a small-scale archedceiling tunnel model with a length of 50 meters.Thirty-two experimental conditions were designed by changing the oil pool aspect ratio,fire source height,and relative side wall position.In addition,to verify the reliability of the unstructured geometry modeling method for studying fires in arched-ceiling tunnels,this thesis constructed a numerical model using this method that was 1:1 with the experimental model.Six simulation conditions were designed.The study found that:By analyzing the flame shape of the fire source in the stable combustion stage,it is found that when the long side of the fire source is close to the wall,the flame length has no obvious relationship with the height of the fire source;when the short side of the fire source is close to the wall,when the aspect ratio of the oil pool is large,the flame will split.The flame splitting is caused by the air entrainment induced by factors such as aspect ratio,height,relative orientation and arched ceiling.Moreover,the split flame can greatly increase the air entrainment,thereby significantly increasing the combustion rate of the fire source.This thesis quantitatively analyzes the ceiling extended flame and explains how it differs under horizontal and arched ceilings.It establishes an ideal physical model of flame based on pool fire entrainment theory and shows how aspect ratio and relative position affect flame length.It introduces an equivalent diameter calculation method to capture the geometric characteristics of fire source and develops a flame length model that considers fire source shape.This model can predict the flame length of oil pool with long side close to wall well,but it does not account for fire source height.Therefore,this thesis also develops another flame length model that considers both fire source shape and height.This model can predict the flame length of oil pool with short side close to wall at different heights well.Both models are applicable to strong and weak plume conditions.When the fire source height is large,a long ceiling extended flame is formed,which increases the fire spread risk.To calculate the ceiling extended flame length of strong plume more accurately,this thesis develops an effective heat release rate formula at the impact point based on the physical meaning of flame,and then develops a prediction model of lateral extension length of flame under arch-shaped ceiling.This model can predict the ceiling flame extension length under strong plume conditions accurately.The inclination angle of the ceiling affects the buoyancy of the plume,which in turn affects the lateral temperature distribution under the ceiling.To eliminate the influence of buoyancy change,this thesis corrects the moving distance of the plume along the arch-shaped ceiling to its horizontal projection,and then establish the lateral temperature prediction formula under the arched ceiling.This thesis compares the numerical simulation results of heat release rate,flame shape,flame length and lateral temperature distribution with the corresponding experimental data for each working condition,and find that the error is within acceptable range.Based on the reliability of simulation data,the thesis establishes a longitudinal temperature distribution model under arched ceiling.We find that the longitudinal temperature decay speed under arch-shaped ceiling is much faster than that under horizontal ceiling in the near fire source area.This thesis explains the difference of plume propagation between horizontal ceiling and arch-shaped ceiling in this area by analyzing the velocity vector slice of plume.The research results of this thesis can provide theoretical guidance for structural fire resistance design,fire detection and alarm system design and installation,fire risk assessment and so on for arched ceiling highway tunnel.There are 42 figures,7 tables,and 102 references in this thesis.