Numerical Study On Smoke Temperature And Back-layering Length In Tunnel Fires With Vehicle Blockage

With the development of society economic and the enhancing utilization of urban spacings in domestic, an increasing number of road tunnels have been built in recent years. Upon a fire occurred in a tunnel, due to the its long and narrow characteristics, the induced smoke will spread rapidly along the tunnel ceiling, resulting an increasing inner temperature of tunnel. The accumulated smoke and toxic gases within the tunnel will lead to heavy casualties and serious property losses and bring negative effect on occupants evacuation and fire fighting.In recent years, several disastrous tunnel fire accidents have occurred at home and abroad. As a result, the social attention is being extensively focused on the fire protection of tunnels. The investigating on tunnel fires has great significant on preventing the occurrence of fire and reducing the losses of accidents. A great number of researches have been carried out for tunnel fires in the literature. Among them most of attentions were paid on studying the effects of tunnel configuration, tunnel slope, fire energy release rate, fire location and longitudinal ventilation velocity on crucial parameters including flame inclination angle, flame height, longitudinal temperature distribution and critical velocity. And many valuable empirical conclusions have obtained. It can be found that the study on the effect of vehicular blockage is quiet limited. Previous studies have investigated the effect of the cross section area of blockage, the distance between the blockage and fire source and the array ways of blockages on maximum smoke temperature and critical velocity. However, in actual tunnel fires, both the array ways of vehicles and the fire source locations are changeable. And the study of varying the height and transverse location of fire source under the condition of existing blockages is not involved in previous studies. Therefore, the current study aims to bridge the knowledge gap. Numerical simulation using FDS was conducted to systematically study the effects of height of fire source, transverse fire source location and heat release rate on longitudinal smoke temperature distribution and smoke back-layering length under a horizontal tunnel ceiling. Two scenarios were considered:one vehicle blocked and two vehicles positioned along the longitudinal tunnel blocked. The main results are as follows:When ambient air and/or smoke flow a cross section with smaller area in a tunnel with a vehicular blockage, the local velocity will be enlarged thus the gas flow will be changed. In cases with two blockages, the ceiling smoke temperature increases with the height of fire source. And the ceiling smoke temperature in cases with two blockages is lower than that in cases with one blockage. The main reason is that the local velocity in cases with two blockages is higher, the more ambient air can be entrained into the smoke layer, leading to a lower ceiling gas temperature.In a ventilated tunnel, the smoke back-layering length is mainly affected by two forces. That is the horizontal inertia force and forced ventilation. In case with one blockage burning the lane of tunnel, the smoke back-layering length increases with the height of fire source. In this regime, the horizontal inertia force is dominated in the smoke back flow. While in case with two blockages located along the longitudinal of lane and the downstream of blockage burns, the smoke back-layering length decreases with increasing the height of fire source. In this regime, the forced ventilation plays a leading role in smoke back flow.When the fire source gets close to the sidewall of tunnel, the average ceiling gas temperature will increase. This is due to that the enhanced restriction of air entrainment leads to the unburnt fuel travel a longer distance to be fully combusted.The smoke back-layering length decreases with increasing the longitudinal ventilation velocity of tunnel. When the smoke back-layering length is large, a relatively smaller longitudinal ventilation velocity can make the smoke back-layering length decrease rapidly. While with a small smoke back-layering length, a bigger longitudinal ventilation is required to reduce the back-layering length.