Study On Effect Of Fire Source Location And Blockage On Smoke Spread And Control In Urban Underground Bifurcated Tunnel

The urban underground road becomes the key link of urban traffic construction because it can effectively relieve traffic jams.It usually consists of a main tunnel and multiple branch tunnels connected by a bifurcated structure to join the surface road network or underground transportation hub.Hence,the urban underground road has a complex spatial system different from a mountain tunnel with a single access point.In addition,the connectivity leads to a large traffic volume in an urban underground road.When vehicles drive in and out of the main tunnel,they must change lanes in the bifurcation area.Hence,the possibility of fire due to vehicle collisions would increase.In an urban underground road fire scenario,smoke spreads in the complex structure space of the tunnel,and smoke control would be more challenging.As a result,it could cause more casualties and adverse social impacts.The fire safety of urban underground roads has received extensive attention.Therefore,it is of great practical significance to research the smoke spread and control of bifurcated tunnel fire to ensure the stability of tunnel structures and the safety of personnel.Currently,researchers on bifurcated tunnel fire mainly focus on the influence of the bifurcation angle.The main problem is that those previous studies primarily use a single fire source with a fixed location.The effects of fire source location and vehicle blockage upstream of the fire source under longitudinal ventilation have not been systematically considered.Moreover,the current technical specifications and standards for underground roads only provide some guiding articles.They do not consider the effects of fire location and vehicle blocking on smoke spread in detail and cannot directly guide smoke control of bifurcated tunnel fire.Therefore,temperature distribution and smoke spread in bifurcated tunnels will be studied by the combination of theoretical analysis,model experiments,and numerical simulation.In addition,the scenarios where fire source location changes in each section of the tunnel connected by the bifurcated structure are considered.The results of this thesis can provide a theoretical basis and technical support for the fire safety design of bifurcated tunnels.The 1:20 bifurcated model tunnel has been upgraded to promote its functions for fire research.We redesigned and created the fire source section and reserved different fire source holes at the bottom of the tunnel for meeting the research needs.In addition,two vehicle models were made according to the actual vehicle size and the similarity ratio of the tunnel.These vehicle models were applied to study the fire scenarios of a blockage located upstream of the fire source under longitudinal ventilation.Since the bifurcated tunnel consists of the main and branch tunnels,the main road tunnel fires on both sides of the bifurcation point were studied in this paper.Then the main tunnel was more defined explicitly,taking the bifurcation point as a reference.Define the part on the same side of the branch tunnel as the main tunnel after the bifurcation point and the rest as the main tunnel before the bifurcation point.The main research work is introduced as follows:First,the characteristics of ceiling temperature distribution in different areas of the bifurcated tunnel are clarified when fires occur under natural and longitudinal ventilation,respectively.These fire scenarios focus on fire source location changes.For natural ventilation,fire sources located at the main tunnel and the branch tunnel were studied.The maximum ceiling temperature and the longitudinal attenuation law of the ceiling temperature in each tunnel were analyzed.For longitudinal ventilation,fire sources located at the branch tunnel and the main tunnel under diverging and located at the branch tunnel under converging were studied.The maximum ceiling temperature and the longitudinal attenuation law of the ceiling temperature downstream of the fire source were analyzed.Finally,based on the research results,the ceiling temperature distribution models of the bifurcation tunnel fire were established.These models reflect the effect of the fire source location under two ventilation modes.Secondly,the effect of longitudinal ventilation on smoke control in bifurcated tunnels focuses on the vehicle diverging situation.In this situation,longitudinal ventilation is implemented at the main tunnel before the bifurcation point.Then,the fire source located at the branch tunnel and the main tunnel before the bifurcation point were studied by model experiments.The fire sources located at the main tunnel after the bifurcation point were researched by numerical simulation.In addition,the experiments where fire source located at the branch tunnel were also conducted for considering a vehicle converging in an actual bifurcated tunnel.According to the experiments and simulation data,prediction models of the smoke back-layering length of bifurcation tunnel fire were established,considering the influence of fire source location.There is vehicle blockage in an actual bifurcated tunnel fire.Hence,the fire source located at the main tunnel before the bifurcation point under diverging and the branch tunnel under converging was studied by model experiments.In these experiments,there is a blockage upstream of the fire source.A smoke back-layering length prediction model that comprehensively considers the fire source location and the influence of blockage was proposed.Then,based on the study of smoke back-layering length,the critical velocity required to suppress smoke back-layering under different fire scenarios(fire source location,vehicle blocking upstream of the fire source)was determined.By exploring the critical velocity of traditional tunnel fire and analyzing the experiments and numerical simulation data,prediction models of the critical velocity of bifurcated tunnel fire were proposed.These models consider the influence of the distance between the fire source location and the bifurcation point.In addition,this paper also focused on the coupling effect of bifurcated structure and vehicle blockage on the critical velocity and established the corresponding prediction models.Finally,the improvement strategy of fire smoke control of an actual urban underground bifurcated tunnel was analyzed.The strategy comprehensively considers the effect of bifurcation and blockage.According to the results of this paper,when considering the impact of fire source location and vehicle blockage upstream fire source.The critical velocity selected according to the engineering design cannot meet the requirements of smoke control.The results show that designing smoke exhausts of bifurcated tunnel fire needs to increase the critical velocity.In addition,to control the smoke spreads of a bifurcated tunnel fire,it is necessary to reduce the installation distance of ceiling jet fans near the bifurcated area.