Study On Fire Ceiling Jet Evolution Characteristics Of Offset Fire Source In Tunnel

The development of social economy makes the tunnel become an indispensable part of the transportation system,and its safe operation is extremely important.In many tunnel disasters,fire accidents are the proudest to happen,highest risk and most harmful disaster.They will not only cause serious casualties and property losses,but also ruin the tunnel lining structure.At the same time,tunnel fire disasters involve very complicated flow mechanics and heat transfer problems.Therefore,the research of controlling tunnel fire accidents has important engineering significance and scientific value.So far,domestic and abroad researchers have made a lot of achievements,but the research on the fire dynamic characteristics and control of offset fire sources in the tunnel is still insufficient,and further research is needed.In this thesis,single offset fire source,double fire sources and cross-sectional abrupt changed tunnel fire source,were taken as the research objects.Through the combination of full-scale fire experiments,theoretical analysis and numerical simulations,the evolution process of ceiling-jet,temperature distribution,smoke reverse behavior and critical velocity in different kind of transverse distance of tunnel fire sources were studied.The main research achievements included:Firstly,the influence of thermal conductivity characteristic of tunnel surrounding on smoke ceiling jet flow during tunnel fire was analyzed.The concept of "temperature boundary layer of ceiling jet" defined by the maximum temperature rise height was proposed.The relationship between the thickness of temperature boundary layer under different thermal conductivity conditions and the heat conduction efficiency under various material conditions were obtained.A corresponding exponential prediction model for the vertical smoke temperature decay near the ceiling wall was established.The research showed that smoke horizontal movement along the ceiling was similar to the boundary layer flow through of a semi-infinite plate surface.The heat conduction process of surrounding rock through ceiling wall was equivalent to the heat conduction process of semi-infinite solid object.The thermal conductivity efficiency of the ceiling was affected by the thermal physical parameters of the material.Vertical smoke layer temperature near the ceiling wall presented a top hat distribution.Secondly,the temporal and spatial evolution of the ceiling jet from offset fire source in tunnel was studied.by observing the experimental phenomenon of full-scale offset fire source,the ceiling jet dynamic process was revealed.The asymmetric air entrainment caused by the offset fire affected the combustion rate and changes the smoke distribution in different stages.It was also found that the maximum ceiling temperature rise and longitudinal temperature attenuation law was affected by the fire source transverse deviation and the virtual point of fire source according to the burning intensity.In addition,the effect of longitudinal ventilation on smoke back-layering from different offset fire sources was analyzed,relationship between smoke backlayering length and heat release rate and ventilation speed was deduced theoretically,the corresponding prediction model was established.Thirdly,the characteristics of fire ceiling jet with asymmetric abrupt change of cross section in tunnel are clarified.Through the analysis of the obvious changes of the air and smoke in the tunnel when they passed through the abrupt cross section,it was found that the vortex influence area within a certain distance near the abrupt cross section had a direct influence on the buoyancy and air entrainment process of the fire smoke,a prediction model of longitudinal gas temperature attenuation across the abrupt change section was established.At the same time,analyzed the influence of abrupt cross section on the sudden expansion and contraction smoke flow,the prediction model of temperature attenuation law with distance from the abrupt cross section was established.The effect of ventilation direction on fire smoke control at different fire source locations was analyzed,and established the related prediction models.The prediction model of smoke backlayering length and critical velocity was established.Fourthly,the outer temperature of the flame was used to define the flame profile and characterize the flame morphology;the influence of different arrangement and spacing of double fire sources on the flame morphology,air entrainment and thermal feedback effect between fire sources was analyzed.A prediction model of maximum ceiling temperature rise under different fire source distance was established.The phenomenon of equivalent offset source caused by asymmetric air entrainment with different fire spacing was found.According to the comparison and verification of the research results,a longitudinal temperature rise attenuation prediction model suitable for the same offset distance fire source was proposed.Based on equivalent single source analysis method,the thesis also analysed the longitudinal ventilation of different double spacing tunnel fire,smoke control efficiency and the prediction formula.