Studies On Characteristics Of Confined Fire Plumes And Mechanism Of Natural Smoke Exhaust By Shaft In Tunnel Fires

As the constantly accelerating process of urbanization in theworld, the urban population has risen dramatically along with the expansion of cities, and the pressure of urban transportation is growing to a large extent. In this context, more and more underground tunnels are under construction all over the world to alleviate the traffic pressure. However, a tunnel gives us not only traffic convenience, but also fire challenges to overcome. In recent years, serious and major tunnal fire accidents happen occasionally, causing tremendous deaths and injuries and severe damage to the tunnel structures. Statistics have shown that smoke is the most fatal factor in fires, and most of victims in fire were killed by hot and toxic smoke. Therefore, the investigation on the characteristics of confiend fire plume and smoke control method is important in theoretic and engineering. Based on the analysis of confined fire plume, ceiling jet and smoke movement in tunnel, the research topic is focused on the characteristics of confiend fire plume and the mechnism of natural smoke exhaust by vertical shaft, by adoping theoretical analysis, model scale experiments and computational fluid dynamics (CFD) simulations. The thesis mainly consists of three parts.Firstly, the influence of sidewall restriction on vertical flame height and maximum ceiling gas temperature of weak plume impinging flow was experimentally investigated. In this part, the distance between fire source and sidewall was decreased from 1.0 m (at the longitudinal centerline of tunnel) to 0 m (against sidewall). Owing to the confinement effect of sidewall, the flame height increased with the decrease of fire-sidewall distance. Especially for fire immediately against sidewall, the air entrainment process was confined to the largest extent and the flame height increased significantly. Based on the relationships between flame height and HRR (heat release rate), the Entrainment Factor (EF), which describes the ratio of air entrainment into the confined plume relative to the unconfined one, was estimated for different fire locations. For fire against sidewall, EF was 0.46, which means that only 46%of air was entrained into the flame volume compared to the fire at the longitudinal centerline of tunnel. This is the reason why flame increases significantly for wall fire. For the weak plume impinging flow, the flame was lower than ceiling. Experimetal results showed that the maximum excess temperature first changed small with the decrease of distance between fire source and the sidewall, while if the distance decreased to 0, i.e., fire was flush with sidewall, due to the increase of flame height and the influence of negatively buoyant wall jet, the maximum ceiling gas temperature would increase significantly. In this part, the changing law of ceiling excess temperture with transverse fire location and HRR was investigated from the two aspects of flame height and accumulated smoke layer, respectively. The dimensionless heat release rate, Q’, which expressed the relative size of heat release rate compared to the tunnel geometry was defined and a modified concept of virtual origin was introduced for calculating the maximum ceiling gas temperature in the presence of a hot upper layer beneath ceiling. The predicted correlations for ceiling excess temperature of weak plume impinging flow were proposed for different transverse fire locations.Secondly, the ceiling flame extension and temperature distribution of strong plume impinging flow were investigated experimentally. Experimental results showed that the flame extension under the ceiling presented a circular shape when the fire was located at the longitudinal centerline of tunnel, whereas for fire placed flush with sidewall, it became a half ellipse. With the increase of HRR, the long axis of the ellipse changes from perpendicular to the sidewall plane to parallel to it. By adopting demensinal analysis, the predicted model of the total flame extension in both the longitudianl and transverse directions were put forward and compared with previous results. Additionally, based on the comparative analysis of vertical and horisontal temperature distribution under the ceiling and the experimental video data, the ceiling excess temperature at the impingement point, △T0, of 400K and 600K can be considered as the two critical values to estimate the impinging condition. For △T0<400K,400<△T0<600K, △T0>600K, buoyant plume, intermittent flame and continuous flame impinged on the ceiling, respectively. Similar to the flame extension, the horizontal temperature profiles under the ceiling of wall fire show clear asymmetry. The correlations of the ceiling excess temperature in both transverse and longitudinal directions are proposed by using the plume radius at ceiling level as the characteristic length scale.Thirdly, a set of model scale burning experiments and CFD simulations were conducted to investigate the smoke exhaust effect and the machnism of natrual ventilation by vertical shaft in urban road tunnel fire. With the increase of shaft, two special phenomena, plug-holing and turbulent boundary-layer separation, were observed in the experiments and numerical simulations, both of which will weaken the effectiveness of smoke exhaust. When the shaft height was relatively small, the boundary layer separation was significant and an area of low smoke concentration appeared in the upstream region inside the shaft, reducing the effective volume of shaft and preventing the through flow of smoke. With increasing shaft height, boundary layer separation became inconspicuous and the effect of smoke exhaust were improved. However, if the shaft was too high, the stength of stack effect became too large, causing the occurance of plug-holing. Then, the ambient fresh air beneath smoke layer was exhausted directly, which strongly decreased the smoke exhaust effectiveness. Therefore, it was not the case that the higher the vertical shaft, the better the smoke exhaust effect. Based on the controlling inertia force acting on smoke movement, the exhausting process with different shaft heights could be divided into the horizontal inertia force control, the vertical inertia force control and the transition process between them. In addition, the critical shaft height related to better smoke exhaust effect could be determined by a new criterion of Ri’ proposed in this thesis.