Study On Small-scale Experiment And Numerical Simulation For Fires In Urban Tunnel With Vertical Shafts

The current ground traffic system in China is relatively developed,but it can't meet the requirements of the development of city,the traffic jams and accidents are restricting the healthy development of economy.Urban tunnel with vertical shafts is a new-style tunnel which is economical and environmental.However,it remains necessary to make serious evaluation on whether it can meet the demands of the smoke control and extraction through persuasive experiments.A series of small-scale experiments are conducted to provide theoretical support for the vertical shafts design and prediction consequences of tunnel fires.The application of natural ventilation technology in tunnel engineering is increasing,but the smoke exhaust effectiveness research of natural ventilation with vertical shafts is less than the mechanical ventilation.Based on Froude number,the similarity demonstration of tunnel fire experiment is conducted;through theoretical analysis,the model rate is determined which should be more than 1:15.Experimental conditions and operability etc are considered and the similarity ratio is determined as 1:10 through many experiments and demonstration.In addition,experimental platform is built.On the basis of previous studies,the development process of smoke movement,smoke back-layering and smoke plug-holing phenomena are analyzed in theory.The theoretical models,which are suitable for predicting the maximum temperature rise under tunnel ceiling and smoke back-layering length of full enclosed tunnel,are modified to be suitable for the urban tunnel with vertical shafts.A car fire accident scene is determined,and the fire power is calcaulated 3.16 MW corresponding to the actual situation.The effects of shaft dimension parameters such as shft height,shaft group distance,shaft quantity,single shaft interval and shaft width,on the natural ventilation smoke exhaust are studied,the law of smoke movement when shaft dimension changes and design reference data of shaft dimension are determined.Full-scale urban tunnel fire model with vertical shafts is built by the FDS numerical analysis software,the validity of the numerical model is verifed compared with the results of small-sacle experiments,and the effect of smoke exhaust of vertical shafts with natural ventilation are further to analyze according to the exhaust rate of pure smoke,the results show that plug-holing is an important factor to reduce smoke exhaust efficiency.In addition,the exhaust rate of pure smoke increases with the increase of shaft height,but decreases firstly and then increases with the increase of shaft group distance and the exhaust rate of pure smoke per square meter decreases with the increase of shaft width.The shaft combination scheme in term of decreasing the maximum temperature under tunnel ceiling,reducing smoke diffusion rate and decreasing smoke sedimentation rate are analyzed based on orthgonal experiments..The importance degree of factors affected smoke exhaust is determined,and the optimum combination of scheme is finally determined.At the same time,the theoretical models of predicting the highest temperature rise under tunnel and smoke back-layering length are verified according experiment and numerical simulation results.