Study On Fire Plume Characteristics Of Near-wall Fire Source In Tunnel Fire Under Longitudinal Ventilation

When a vehicle fire breaks out in a tunnel,people park their cars at the side of the tunnel instead of in the middle of the tunnel.Longitudinal ventilation is a useful measure to control smoke when tunnel catches fire.But longitudinal ventilation will affect the combustion rate of the fire source,and thus cause the different ceiling maximum temperature and longitudinal temperature distribution.However,most literatures do not cover the fire caused by near-wall burner in tunnel with arched ceiling under longitudinal ventilation.Under the influence of arched side walls and longitudinal ventilation,smoke will appear "S" type transport mode which is completely different from the fire whose fire source is located in the central axis of the tunnel,and this transport mode has not been further studied by scholars.Therefore,in this thesis,the fire plume characteristics and plume "S" type transport mode of near-wall fire source in tunnel with arched ceiling under longitudinal ventilation were studied by combining the data of 81 model-scale experiments and 142 full-scale numerical simulations.Finally,two practical engineering problems,the critical velocity and the relationship between back-layering length and ventilation velocity,are studied.The main contents of this thesis are as follows:（1）According to the flame characteristics with and without longitudinal ventilation,the corresponding flame shape model was established and calculated,and the flame length-fuel mass loss rate model was obtained;According to the flame characteristics under the restriction of the side wall of arched tunnel,the corresponding flame shape model was established and calculated,and the model of flame extension length and unburned fuel mass loss rate was obtained.Combined with experimental data,it is found that under the condition of natural ventilation,the 1.5 power of flame height is proportional to the fuel mass loss rate.Under longitudinal ventilation,air entrainment rate is proportional to 0.22 power of longitudinal velocity,and the product of flame length and air entrainment rate is proportional to the rate of fuel mass loss.The above models can well describe the characteristic of flame（extension）length.Based on the experimental data,a model of the relationship between fuel mass loss rate and longitudinal ventilation velocity was established,and the applicable scope of the flame tilts angle model was broadened.It was found that the flame tilts angle was smaller when the flame attached to the wall under the same longitudinal velocity.（2）Supplemented by experimental data and dominated by numerical simulation data,the maximum temperature plume trace of smoke generated by the near-wall fire source of the arched tunnel was obtained.The plume trace temperature was compared with the longitudinal temperature of the middle axis of tunnel ceiling,and it was found that the longitudinal temperature of the middle axis of tunnel ceiling could be used to represent the plume trace temperature.On this basis,the longitudinal temperature attenuation law of tunnel ceiling is studied.Based on the previous model of longitudinal temperature attenuation of tunnel ceiling under natural ventilation,the model is further improved to make it suitable for tunnel fire of arched tunnel with different fire-wall distance under longitudinal ventilation.Based on experimental and numerical simulation data,compared with the previous model of longitudinal temperature attenuation of single exponential function,it is found that the improved longitudinal temperature attenuation model can well describe the longitudinal temperature attenuation law.（3）According to the experimental data and numerical simulation results,it is found that the critical velocity is higher in the conditions where the fire source is located in the middle of the tunnel and the fire source is attached to the wall than in the other two conditions.However,the distance between fire source and side wall has little effect on critical velocity and back-layering length,because the dimensionless critical velocity is still proportional to the 1/3 power of the dimensionless heat release rate.The relationship between the dimensionless back-layering length and the dimensionless control velocity can be divided into three stages according to the difference of slope of formulae.The first stage is that ventilation cannot restrain the length of back-layering at all;The second stage is that the length of back-layering will decrease rapidly with the increase of ventilation velocity.The third stage is that the length of back-layering slowly decreases with the increase of ventilation speed.Therefore,when the tunnel fire occurs,the longitudinal ventilation should be adjusted after the second stage.