Experimental Study And Analysis Of Fire Smoke Transport Characteristics In Construction Tunnels

With the rapid development of the social economy and transportation industry,the scale of planning,construction and opening of various tunnels have become larger and larger.Once the fire is caused by the construction machinery and equipment,welding or transportation vehicles stored in the process of tunnel construction,it is not only difficult to extinguish,but also the high temperature and toxic environment caused by the fierce combustion of the fire will often cause serious damage to the structure in the tunnel,difficult evacuation and even suffocation and death.Over the years,experts and scholars at home and abroad have conducted sufficient research on tunnel fire and smoke control,which has important guiding value for ventilation system design,smoke control strategies and personnel rescue and evacuation.Most of the previous studies were based on operating tunnels,and there were relatively few studies on the fire smoke diffusion law and smoke control in construction tunnels.In this thesis,the Caojiadi construction section of Qianshiyan Tunnel is used as the research prototype,and according to the Froude similarity criterion,a 1:20 scale model test bench for construction tunnel fire is built.For different fire intensity,fire source location,and fire source height,45 sets of comparative experimental conditions were completed to conduct in-depth research on the fire smoke diffusion characteristics of construction tunnels.The main research results are as follows:The variation of flame shape near the sealing end was analyzed,and the results showed that with the increase of fire intensity,the direction of fire plume toward the sealing end would tilt faster and the angle would be larger,resulting in the phenomenon of flame "sticking to the wall".The phenomenon of "sticking to the wall" can be weakened if the fire source is far away from the sealing face or its height is raised.The dynamic diffusion law of flue gas in the main tunnel was observed through the laser irradiation of the chip source:the flue gas under the ceiling hit the end face of the downstream block and returned inertia,resulting in a backflow layer of flue gas.The thickness of flue gas layer increased rapidly,thus accelerating the flue gas settlement.Based on the visual observation of the smoke layer height variation,combined with the N-percentage method to predict the smoke layer temperature stratification height,verified the feasibility of N value of 10 to describe the law of smoke diffusion and settlement.The time of the smoke settling to the height of 0.1m in the downstream of the fire was summarized,and the influence of fire source location,fire source height and fire intensity on the smoke settling time was analyzed,so as to provide reference for evacuation in the corresponding situation.Based on the experimental results of scale model,the maximum flue gas temperature rise and longitudinal attenuation characteristics of flue gas temperature rise under the ceiling in quasi-steady state were studied.Comparing and analyzing the measured values obtained in the experiment in this thesis and the previous research results of fires based on tunnels open at both ends,blocked at one end and blocked at both ends,the differences and characteristics of fires in construction tunnels compared with traditional tunnels are concluded.The construction tunnel with both ends blocked and long inclined shaft structure has a more obvious heat accumulation effect after a fire,which makes the maximum smoke temperature rise ?Tmax under the roof higher,causing greater harm.The dimensionless distance d*is introduced to further describe the influence of the longitudinal position and height of the fire source on the?Tmax and the dimensionless correlation between ?Tmax and the dimensionless heat release rate Q*in the experiment of this thesis is established,and the overall prediction error is about 10%.According to the conclusion that the vertical distribution of the flue gas under the ceiling attenuates exponentially,the temperature distribution of the flue gas under the ceiling in this thesis is represented dimensionless.The experimental values were compared with the values of the longitudinal attenuation model of flue gas proposed by the former.The tunnel structure and ventilation conditions lead to different flue gas attenuation laws in different situations.The prediction models for the longitudinal attenuation of flue gas in upstream,downstream and inclined shaft of tunnel fire in this thesis are proposed,and the overall prediction error is about 25%.Through the study of fire in construction tunnel,the dynamic diffusion process of smoke and the characteristics of maximum temperature and longitudinal distribution of temperature are deeply understood and studied,which is beneficial to provide guidance for safe evacuation of personnel,protection and control of fire in construction tunnel and design and layout of fire protection system.