Study Of The Critical Velocity In Railway Tunnel Fire

With rapid development of domestic economic,the railway modernization has been accelerating,and more and more railway tunnels have emerged,and tunnel fire safety has become an urgent issue to be solved.Statistics have shown that 85% of the casualties are caused by toxic fire smoke.Due to the characteristic of structure and that of ventilation condition,it is difficult to exhaust fire smoke out of a tunnel immediately.Study on smoke control in railway tunnels are very important,which can effectively provide a guide for evacuation and fire fighting in railway tunnels.In this paper,case studies,model experiments and numerical simulation are used to investigate the critical velocity of the tunnel and the cross-passage when the train is on fire.The main work of the paper are as follow:(1)113 fire accidents in railway tunnels are investigated.The results show that tunnel fire accidents occur most in the 1970 s and 1990 s,and since 1990 s the accidents decline,but the number of fire is still large;The number of train fire is the largest,accounting for 85.84% of all railway tunnel fire accidents.The possibility of the external fire of train carriage occurred is largest and also cause the most serious consequences of the accidents,it is the key point of fire prevention;Electrical failure,poor management and arson is a major cause of train fires,accounting for 90.27% of all fire accidents,and the consequences are also serious.(2)Experiments are conducted with a 1:20 scale to analyze the effect of blockage ratio on critical velocity.The critical velocities under the conditions of nine blockage ratios at the heat release rate of 5.88 kw are measured by model tunnel fire tests.The results show that critical velocity decreases while blockage ratio increases.The decrease rate is slightly smaller than the blockage ratio.The blockage-ratio modified coefficient obtained by experiment result is 0.548.The formula of the critical velocity and dimensionless critical velocity containing blockage ratio is proposed.After the relationship of the critical velocity decrease rate and the blockage ratio is contracted and analyzed based on this experiment and previous studies,the main factors that leadto the different relationships about the critical velocity decrease rate and the blockage ratio are brought up,which include the height of fire burning surface,the location of fire source(the relative position about the fire source and blockage vehicles and whether the fire source is in the blockage vehicles or out of it)and the lateral wall open area of vehicle.(3)The cross-passage between two tunnels are designed for evacuation during a train fire,critical velocity in a railway tunnel cross-passage is one of the important parameters of smoke control in a cross-passage.Dimensional analysis and numerical are used to investigate the critical velocity in a tunnel cross-passage.The effect of the heat release rate,ventilation velocity,fireproof door geometry,angle between the cross-passage and the tunnel are taken into account.The results show that there are two regimes of variation of critical velocity in a tunnel cross-passage against ventilation velocity.At low ventilation velocity the critical velocity in a tunnel cross-passage varies as 3/7 power of the ventilation velocity.However,at higher ventilation velocity,it varies as-3/40 power of the ventilation velocity.It is also shown clearly that the critical velocity in a tunnel cross-passage varies as 1/3 power of the heat release rate,as 6/5 power of the fireproof door height,as-3/8 power of the angle between the cross-passage and the tunnel,almost independent of the fireproof door width.A dimensionless correlation that can correlate well with the experimental data is proposed.