Study On Ceiling Jet And Induced Heat Feedback Mechanism Of Representative Fire Scenarios In Shield Utility Tunnel

With the release of The Guiding Opinions of The General Office of the State Council on Promoting the Construction of Urban Utility Tunnel(2015,No.61),the power cable cabin in the utility tunnel has gradually become the mainstream way to place "power lifeline" in urban.However,wild application and lagged theoretical research have provided great challenging for the fire protection design and maintenance of utility tunnel,especially shield utility tunnel with curved geometry.The lack of theoretical research could be concluded by several points: evolution and mechanism of transverse extension boundary of special fire behavior(e.g.,extension flame beneath ceiling)influence by fire source location and curved ceiling;distribution and evolution mechanism of ceiling jet temperature influence by fire source location and curved ceiling from multi-dimensional perspective;heat feedback evolution and internal component competition mechanism influenced by fire source location and curved ceiling.Thus,this work employed combustion experiments and FDS numerical simulation to study the scientifical points mentioned above.The main works and conclusions are shown as follows:According to the reduced scale and full-scale experiments,evolution of extension flame length beneath curved ceiling produced by axisymmetric fire source was studied.It was found that extension flame length after impingement point is determined by the unburnt fuel mass flow and buoyancy component along flame path.Moreover,the influence on effective heat release rate under curved ceiling provided by unburnt fuel mass flow and buoyancy component along flame path was revealed.Based on the momentum conservation equation,the model predicting total jet velocity along curved ceiling was put forward.With the linear relationship between entrained air velocity and ceiling jet velocity,the effective heat release rate was described by entrained air mass flow and unburnt mass flow,respectively.On the basic of two expressions mentioned above,a new global model was established to predict extension flame length under curved ceiling.After multi validation,the new model could predict data within 15%relative error.Based on the reduced scale and full-scale experiments,temperature distribution of ceiling jet under curved ceiling produced by axisymmetric fire source was studied.In terms of vertical temperature distribution,a characteristic inclined angle was defined to be equivalent to the influence of buoyancy component along the curved ceiling.With the definition,dimensionless formula was put forward to unify the vertical temperature distribution influenced by fire source location and curved ceiling.In terms of longitudinal maximum temperature attenuation,ceiling jet under curved ceiling centerline was divided into two parts,which are radial region and one-dimensional region.According to the combination of dimensionless analysis and control governing equation,influence brought by curved ceiling and fire source location on evolution of longitudinal maximum temperature attenuation was revealed.Then,the predicting models corresponding to the different regions were established.After validation,the relative error of predicting outputs was within 20%.According to the work on vertical temperature distribution and longitudinal maximum temperature attenuation,method was proposed to make two-dimensional prediction for temperature distribution under curved ceiling centerline.With validation,uncertainty of prediction is within 20%.In terms of transverse temperature,this work defined characteristic inclined angle to describe transverse jet movement under curved ceiling.Based on the definition,a prediction model of transverse maximum temperature distribution in the impingement region under the coupling effect of curved ceiling and fire source location was established,and three specific expressions were evolved according to the types of ceiling jet(continuous flame,intermittent flame and hot smoke).With employment of reduce scale experiment and FDS numerical simulation,evolution of heat feedback under curved ceiling was studied in this work.First of all,work was conducted to make validation to the application of unstructured geometry in utility tunnel fire.It was found that FDS outputs with unstructured geometry could predict data in impingement region with acceptable accuracy.Based on the simulation and experiments,the attenuation model of incident heat feedback provided by flame body before impingement and extended flame body as well as thermal environment under curved ceiling were established by combining the theory of flame blackbody radiation and dimensionless analysis.Based on the prediction expression of each component in the heat feedback,coupling effect of geometric characteristics and fire source location on the evolution of each component in the feedback heat flow under curved ceiling was analyzed.Moreover,the evolution and competition mechanism of each component in the heat feedback under the curved ceiling were revealed in terms of variety fire source locations and target points.The main conclusions obtained from this work could provide theoretical basis for the selection and design of fire alarm and automatic fire extinguishing system in shield tunnel as well as the development of fire alarm and fire location technology.In addition,this work established theoretical foundation for the risk assessment of fire spread and analysis of thermal feedback mechanism in utility tunnel.There are 73 pictures,20 tables and 164 references in this thesis.