Study On Fire Smoke Movement Characteristics In Tunnel With Large Aspect Ratio Cross-section

Fire disaster is one of the major potential safety hazard during tunnel operational stage.The high temperature and toxic smoke released by combustion can not only cause a large number of casualties and property losses,but also lead to bad public opinion and long-term impact.Meanwhile,the demand of transportation grows dramatically in recent years along with the increase of the urbanization level and the economic development in China.The cross-section of newly constructed tunnels are tending to become larger to contain more lanes.The multi-dimensional dynamic effect will become remarkable in a tunnel with wide-shallow cross-section.Therefore,it has significant meaning to focus on the smoke movement characteristics of smoke flow in the wide-shallow tunnel fire.In this paper,based on multi-disciplinary approached including engineering science,thermodynamics and computational fluid dynamics,and combination of theoretical derivation,physical experiment and numerical simulation,the smoke kinetic characteristics,ceiling temperature decay law and key indicator of smoke control in wide-shallow tunnel fire are systemically investigated.The results are as follows:(1)The back-layering layering length near the side wall is larger than that in central position.The inhomogeneity phenomenon of smoke front is caused by the inhomogeneity of air flow in low ventilation velocity condition,while it is influenced by both the inhomogeneity of air flow and smoke multi-dimensional effect in in high ventilation velocity condition.Moreover,due to the inhomogeneity of smoke front,uc,t,which is the longitudinal ventilation velocity to avoid smoke back flow of total cross section is larger than uc,c,which is the longitudinal ventilation velocity to avoid smoke back flow in fire source position.The ratio of uc,t to uc,c becomes larger following the increase of aspect ratio,and gets peak value 1.17 when AR approaches 7.8.(2)Smoke flow has remarkable bifurcation characteristics when longitudinal velocity is relatively large,and results in a low-temperature region in downstream the fire source.Following the increase of AR,the range of low-temperature region increases and the temperature of low-temperature region decreases.(3)The temperature decay of buoyancy-driven smoke flow obeys an exponential function law,the decay coefficient k gets larger as AR increases,HRR(heat release rate)decreases and ventilation velocity increases.A theoretical correlation between k and tunnel width W,mass flow rate of back-layering smoke msup is established,and the model to predict ceiling temperature decay is derived based on scaled-model test data and numerical simulation results.Comparison results shown that there is a good agreement between the model prediction and larger scale test value,for the maximum error is 0.046.(4)The increase of the tunnel width will strengthen the heat transfer between the smoke and the tunnel ceiling,reduce the smoke layer thickness,and weaken the deflected effect of smokes in sidewalls and the transformation strength from radical to one-dimensional spreading,which results in the declining of backlayering length with the increase of the aspect ratio.A new prediction model for backlayering length and critical velocity with more complete dynamic mechanism was proposed through improving the correction coefficient of the aspect ratio.The prediction results of the model are in good agreement with the experimental data.The new model shows that under small fire conditions,uc*gradually decreases as the aspect ratio increases,and eventually approaches a limiting value,which is the critical velocity without side wall constraints.When Q*>0.34,the determination of fire scale is independent of aspect ratio,the fire is a large one,while when Q*?0.34,the determination of fire scale relies on the aspect ratio.