Turbulent Combustion Modelling On Long Tunnel Fires And Structure Fire Safety

The disaster caused by long tunnel fires has aroused extensive attention. With the rapid growth of economy and the build of longer and longer tunnels, the risk of tunnel fires will increase consequentially. A mass of high-temperature toxic gas released by tunnel fires not only causes great harm to persons, but also damages the tunnel's structure which will reduce the overall stability of tunnel. Therefore, studies on the movement characteristics of smoke and the structure fire-resistant security of long tunnels are of great significance.Based on conclusion of tunnel fires researches, applying multi-discipline integration analysis, such as multiphase turbulent reacting fluid dynamics and thermal analysis kinetics, the processes of long tunnel fires and the structure fireproofing were studied through theoretical analysis, numerical simulation, scale model test, thermogravimetric experiment and infrared-spectrum analysis. A series of valuable achievements were got.(1) The three-dimensional mathematical model of tunnel fires based on the theory of turbulent combustion can make up for the traditional VHS model which ignores combustion effect and only simulate tunnel fires under low ventilation velocity. Based on the current model, the effect of combustion on the flow structure in tunnel fires was revealed for the first time, and the pulsation of smoke was quantitatively analyzed. LES can capture three zones of forced flames - continuous flame, intermittent flame and plume zone, and portray the large fluctuations of temperature and the detail pulse of the smoke flow at the top of the fire source. Therefore, LES has many advantages than k -εmodel for complex turbulent problem.(2) Three-dimensional movement characteristics of smoke were analyzed by numerical simulations. Under the longitudinal exhaust, the movement of smoke plume influenced by both buoyancy and forced convection can be divided into three special sections: 3D spiral vortex flow section, 2D stratified flow section and 1D longitudinal spreading section. While under the central exhaust, two-way asymmetric, two-way symmetric and one-way manner are existed. (3) It was the first time that the thermogravimetric experiment and infrared-spectrum analysis on combustion of asphalt and its cement were carried out, in order to reveal the mechanism and characteristics of samples combustion, and analyze the component and releasing law of its gaseous products, which showed as following:1) The combustion process of asphalt cement has four phases including heating, oxidation of primary volatiles, oxidation of secondary volatiles, and oxidation of residual carbon. The combustion process of asphalt typically has three phases, including heating, oxidation of primary volatiles, and oxidation of secondary volatiles and residual carbon.2) Main gaseous products of asphalt and its cement combustion are CO₂, CO, NO, NO₂ and SO₂. Among these species, CO is mainly produced during the oxidation of volatiles, and the emission of SO₂ is mostly related with the sulfur content in the sample. The active volatile content and the air flux have significant impacts on the releasing law of gaseous products. Increasing air flux could reduce CO emission, otherwise it would increase soot.Kinetic analysis shows that the most probable kinetic function describing the last three phases of asphalt cement combustion are Z-L-T equation, Jander equation of 3D diffusion model and Mampel Power law based on 1D surface reaction model respectively, and the model F3 and R1 are considered as the most suitable kinetic mechanisms respectively for the second and the third phases of asphalt combustion.(4) The security of structure in tunnel fires was analyzed through numerical simulations and experiments: 1) Thermal analysis tests found that the glass transition temperature of adhesive was 106.2℃. based on this the numerical calculations showed that applying adhesive to tunnel engineering was safe when the thickness of concrete between planting-bar and concrete surface facing fire was over 22 cm. 2) when the fire load is lower than 10 MW Asphalt pavement will almost not to be ignited which can also be basically insured when the fire load is 20 or 50 MW, as long as ventilation velocity is kept at 2 or 4 m·s^-1 respectively. The influence of asphalt concrete pavement combustion is limited on tunnel fires and its heat release rate won't exceed 1.7 percent of fire load. 3) Under the central exhaust mode, the areas where could burst on the top surface of head clapboard are much larger than which on the bottom surface, consequently the difference between them would increase along with the increasing fire loads. Affected by weak conduction of concrete, the large temperature difference would be always existed between the top and bottom surface of head clapboard, thus the head clipboard may be destroyed in tunnel fires by the resulting huge temperature stress.