The Simulation Of Backdraft Phenomena In Subway Tunnel Fire

Subway fire is one of the most serious accidents with fatalness. The process of it takes on remarkably complicated nonlinear character. The theory of nonlinear science and experiments must be applied in research on the rule of burning in subway. The potential backdraft in subway tunnel fire is a typical phenomena studied in four aspects as follows in this thesis.First, the occurrence possibility of backdraft in subway tunnel fire is analyzed with theoretical analyses. Through the analyses of the occurrence conditions of compartment backdraft, the resemblance between the compartment fire and subway tunnel fire has been found. So, some research conclusions of compartment backdraft are used for reference to study the subway tunnel fire backdraft though there are some differences between them. But they are not completely suit for the subway tunnel backdraft, the structural features of subway tunnel and the characters of subway tunnel fire should be considered. Fortunately, it's assured that the backdraft might occur under some definite conditions by means of theory analyses and compare studies. So we believe that the research of backdraft in subway tunnel fire is very necessary.Secondly, those critical conditions determining the occurrence of the fire backdraft are discussed, and the ignition delay and mitigation methods are studied too. The existing 1:8 scale model experiment device for subway tunnel fire was changed to be suitable for the subway tunnel fire backdraft experiments. The apparatus is fitted with the Exhaust Ventilation System. The ventilator with continuously variable frequency regulation can supply different airflow conditions. The backdraft phenomena is reproduced during the model experiment processes by using liquid fuel-heptane as fuel, and the conclusions of theoretical analyses are confirmed. A great number of instruments, such as thermocouples, species concentration analyzer and so on, are adopted to measure the corresponding parameters in the model, which are studied for the effects of the occurrence of the subway tunnel backdraft. Experiment results show that the key parameter determining the occurrence of the subway tunnel backdraft is the mass fraction of fuel volatile in the model. When it reaches a critical value, the backdraft will occur. With natural ventilation, the ignition delay is mainly caused by the required time of flammable gas mixture forming and arriving at the location of the fire source. With mechanical ventilation, the ignition delay is decided mainly by the temperature of ignition source and the time reaching the ignition temperature of flammable gas mixture. Fire backdraft can be mitigated and restrained by increasing the humidity of the fresh air which flows into the model after the valve opened. A optimized combination value between the humidity of fresh air and the operation frequency of fan has been found to inhibit backdraft occurrence.Thirdly, the catastrophe of the fire backdraft is analyzed. By detailed calculation of above model experiments data and analyses of the number of control parameters and state parameters of the fire burning system, it is confirmed that the subway tunnel fire backdraft is a typical catastrophe behavior which belongs to coattail catastrophe. And then the working condition districts and no working condition districts have been found in the bifurcation set of coattail catastrophe. Of course, some internal and oversea conclusions of compartment backdraft are introduced here.Fourthly, the smoke distribution prior to fire backdraft in the model tunnel is simulated with CFD software PHOENICS, in which the standard k-s model based on RANS is used to simulate the smoke distribution prior to backdraft. The results show the distribution of the concentration and the temperature of the smoke, and the region of the smoke distribution simulated by PHOENICS accords with the situation of the model experiments. CFD software can simulate the smoke distribution prior to backdraft accurately, but it's very difficult to simulate the process of the fire backdraft. The numerical simulation of the fire backdraft is an important work in the future.