Research On Characteristics Of Smoke Stratification And Temperature Distribution In Tunnel Fires Under Canyon Cross Wind

As tunnels are buildings with long and narrow structure,its fire hazard is much greater than that of an open space.Therefore,the development characteristics and prevention and control technology of tunnel fires have received extensive attention from many scholars.In recent years,with the acceleration of urbanization,the scale of mountain tunnels has expanded year by year,and the probability of tunnel fires has also increased.Since the bridge-tunnel connecting section in mountainous areas usually spans canyons,there is a canyon cross wind that is mainly90 degrees to the longitudinal direction of the tunnel near tunnel portals induced by the canyon effect,which may affect the fire dynamics behavior in tunnels.In this paper,based on theoretical analysis and numerical simulation results,a series of small-scale experiments were used to study the smoke horizontal spread and vertical stratification characteristics in tunnel fires under canyon cross wind.The main contents include the following:Firstly,based on the experimental method,the characteristics of flow field in the tunnel induced by canyon cross wind were studied.It is found that the airflow velocity inside tunnels increases with increasing canyon cross wind speed.The flow field in the tunnel induced by canyon cross wind can be divided into negative pressure zone,transition zone and unidirectional flow zone.In the negative pressure zone and the transition zone where vortexes exist,the velocity and direction of airflow are unstable,while stable in the unidirectional flow zone.And the airflow direction in the negative pressure zone is from the inside of tunnel to the portal near canyon cross wind,while the airflow in the transition zone and the unidirectional flow zone is from the inside of tunnel to the other portal away from canyon cross wind.Secondly,the smoke horizontal flow characteristics and temperature distribution of smoke layer in tunnel fires under the action of canyon cross wind were investigated.It is found that there are two special smoke movement behaviors in tunnels induced by canyon cross wind.When the fire source is positioned at the negative pressure zone,there is always hot smoke upstream the fire with increasing canyon cross wind speed.However,when the fire source is positioned at the transition zone and the unidirectional flow zone,there is no hot smoke appearing upstream the fire with a certain canyon cross wind speed.And the fire source power,the airflow barrier formed by the canyon cross wind and the air entrainment are the main factors that affect the smoke temperature.The dominance of the above three factors on the influence of smoke temperature was analyzed when the fire source is located in different flow field zones.In addition,the longitudinal decay rate of the smoke layer temperature rise upstream and downstream the fire was analyzed.And the prediction model of smoke layer temperature rise along the longitudinal direction of the tunnel was established,which considers canyon cross wind speed,fire location and fire size.Thirdly,based on reduced-scale experiments,the effect of canyon cross wind on smoke stratification characteristics in tunnel fires was studied.It is found that the dimensionless smoke layer thickness downstream the fire increases with increasing canyon cross wind speed,and decreases with increasing fire source area.When the canyon cross wind speed is higher,the closer the fire source is to the canyon cross wind,the greater the smoke layer thickness downstream the fire.However,the dimensionless smoke layer thickness upstream the fire(fire source is located in the transition zone and the unidirectional flow zone)is always between 0.2 and 0.3.Moreover,the experimental data of vertical temperature rise is in good agreement with the relationship proposed by Newman for judging the smoke stratification state.As the canyon cross wind speed increases and the distance from the fire source to the canyon cross wind decreases,the smoke stratification downstream the fire gradually changes from obvious to not obvious.However,the smoke stratification upstream the fire(fire source is located in the transition zone the unidirectional flow zone)is always obvious.Finally,the dimensionless critical criterion for predicting the smoke stratification stability at different fire positions under the canyon cross wind was proposed.The purpose of this study is to clarify the characteristics of smoke stratification and temperature distribution in tunnel fires under the action of canyon cross wind,and to provide reference for smoke control and personnel evacuation rescue strategies in mountain tunnel fires.There are 31 figures,5 tables and 86 references in this dissertation.