| The 14th Five-Year Plan of our China establishes the specific goals of building a modern comprehensive transportation system,promoting the integrated development of various modes of transportation,and improving network effect and operational efficiency of the public transportation.However,due to China’s landform is mountainous,that’s about two-thirds of the country.In the process of expressways and railways construction,in order to reduce mileage and conserve energy,it is inevitable to build an enormous number of tunnels.The tunnel construction undoubtedly brings much convenience to people travelling,but at the same time,it brings tunnel safety problems,especially the tunnel fire problem,which should be taken seriously.In the past studies on smoke transport and control of traffic tunnel fire,it was generally assumed that one fire would occur at the same time in the tunnel.However,due to ignition or collision and other factors,sometimes two or more fires would occur in the tunnel.The fire characteristics and smoke exhaust control mechanism of the tunnel need to be further studied.Therefore,this work is mainly aimed at safety issues of multiple fire sources in traffic tunnels,though the established reduced scale tunnel and theoretical analysis.The combustion interaction,combustion characteristics,and smoke spreading and exhaust control method between adjacent double fire sources in the tunnel are analyzed,the results provide a theoretical reference for the design of smoke prevention and exhaust of multiple fire sources in traffic tunnels.The main research contents of this work are as follows:In this paper,the flame morphology and combustion characteristics of adjacent double fire sources with different separation distance were firstly carried out through experiments,the variables such as distance between different fire sources,wind speed and fire source shape(length-width ratio)are mainly considered.The results show that the fuel mass combustion rate is a non-monotonic evolution behavior for the dual fire source fire in the traffic tunnel under the longitudinal wind.With the increase of the separation distance between adjacent fire sources,the fuel mass loss rate decreases with the separation distance after reaching the critical point.Based on the static layer theory of combustion,a new model is proposed to characterize the evolution of the mass loss rate of adjacent fire sources under longitudinal wind,and the model can also describe the fuel mass loss law of the previous single fire source and adjacent fire sources without longitudinal wind.With the increase of the distance between adjacent fire sources and longitudinal wind,the flame merging characteristics can be divided into three states.Through dimensional analysis,the piecewise linear function of flame merging probability is established.For the flame,we obtain a new model to characterize the flame height of adjacent two fire sources without wind based on the air entrainment theory.At the same time,combining the new model with Froude number,a characterization model of flame height and fire distance between adjacent fire sources with longitudinal ventilation and longitudinal wind is proposed.Secondly,through experimental and theoretical analysis,this paper revealed the smoke spread law under the tunnel ceiling induced by two adjacent fire sources with different heat release rate in the condition of natural ventilation.It is found that when the ratio of the heat release rate of the upstream fire source to the total fire source is the same,the maximum temperature rise under the traffic tunnel ceiling decreases with the increase of the distance between adjacent fire sources.Under the condition of the same distance between adjacent fire sources,the maximum temperature rise of the tunnel ceiling shows a non-monotonic evolution trend,which decreases first and then increases with the increase of the ratio of the heat release rate of upstream fire sources to the total fire sources in the tunnel.Based on the dimensional analysis,this paper proposes a model for the evolution of maximum temperature rise of tunnel ceiling under adjacent fire sources.The model includes parameters such as dimensionless fire heat release rate,ratio of adjacent single fire heat release rate to total fire source,and separation distance between adjacent fire sources.Finally,this paper studied the fire smoke exhaust control behavior in the case of adjacent fire sources in the tunnel.It was found that at,the critical ventilation velocity for smoke exhaust control in traffic tunnels decreases with the increase of the separation distance between adjacent fire sources,and the effect of the separation distance between adjacent fire sources on the critical ventilation velocity decreases with the increase of the fire heat release rates at a given fire heat release rate.On the basis of dimensional analysis,a dimensionless characterization model of critical ventilation velocities for tunnel smoke exhaust control under the condition of considering the separation distance between adjacent fire sources is proposed The model can well described the experimental results and predicted the experimental data and FDS simulation results of previous tunnel fires study,and the model has high reliability. |
