Multiscale Modelling of Tunnel Ventilation Flows and Fires
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Tunnels represent a key part of world transportation system with a role both in people and freight transport. Past events show that fire poses a severe threat to safety in tunnels. Indeed in the past decades over four hundred people worldwide have died as a result of fires in road, rail and metro tunnels. In Europe alone, fires in tunnels have brought vital parts of the road network to a standstill and have cost the European economy billions of euros. Disasters like Mont Blanc tunnel (Italy, 1999) and the more recent three Channel Tunnel fires (2008, 2006 and 1996) show that tunnel fire emergencies must be managed by a global safety system and strategies capable of integrating detection, ventilation, evacuation and fire fighting response, keeping as low as possible damage to occupants, rescue teams and structures. Within this safety strategy, the ventilation system plays a crucial role because it takes charge of maintaining tenable conditions to allow safe evacuation and rescue procedures as well as fire fighting. The response of the ventilation system during a fire is a complex problem. The resulting air flow within a tunnel is dependent on the combination of the fire-induced flows and the active ventilation devices (jet fans, axial fans), tunnel layout, atmospheric conditions at the portals and the presence of vehicles. The calculation of tunnel ventilation flows and fires is more economical and time efficient when done using numerical models but physical accuracy is an issue. Different modelling approaches can be used depending on the accuracy required and the resources available. If details of the flow field are needed, 2D or 3D computational fluid dynamics (CFD) tools can be used providing details of the flow behaviour around walls, flames, ventilation devices and obstructions. The computational cost of CFD is very high, even for medium size tunnels (few hundreds meters). If the analysis requires only bulk flow velocities, 1D models can be adopted. Their low computational cost favours large number of parametric studies involving broad range ventilation scenarios, portal conditions and fire sizes/locations.Another class of methods, called multiscale methods, adopts different levels of complexity in the numerical representation of the system. Regions of interest are described using more detailed models (i.e. CFD models), while the rest of the system XIX can be represented using a simpler approach (i.e. 1D models). Multiscale methods are characterized by low computational complexity compared to full CFD models but provide the same accuracy. The much lower computational cost is of great engineering value, especially for parametric and sensitivity studies required in the design or assessment of ventilation and fire safety systems. Multiscale techniques are used here for the first time to model tunnel ventilation flows and fires.This thesis provides in Chapter 1 a general introduction on the fundamentals of tunnel ventilation flows and fires. Chapter 2 contains a description of 1D models, and a case study on the Frejus tunnel (IT) involving some comparisons to experimental data. Chapter 3 discusses CFD techniques with an extensive review of the literature in the last 30 years. The chapter provides also two model validations for cold ventilation flows in the Norfolk Tunnels (AU) and fire induced flows in a small scale tunnel. Chapter 4 introduces multiscale methods and addresses the typical 1D-CFD coupling strategies. Chapter 5 applies multiscale modelling for cold flow steady-state scenarios in the Dartford Tunnels (UK) where a further validation against experimental data has been introduced. Chapter 6 present the calculations from coupling fire and ventilation flows in realistic modern tunnel layout and investigates the accuracy of the multiscale predictions as compared to full CFD. Chapter 7 represents application of multiscale computing techniques to transient problems involving the dynamic response of the ventilation system. The multiscale model has been demonstrated to be a valid technique for the simulation of complex tunnel ventilation systems both in steady-state and timedependent problems. It is as accurate as full CFD models and it can be successfully adopted to conduct parametric and sensitivity studies in long tunnels, to design ventilation systems, to assess system redundancy and the performance under different hazards conditions. Time-dependent simulations allow determining the evolution of hazardous zones in the tunnel domain or to determine the correct timing for the activation of fixed fire fighting systems. Another significant advantage is that it allows for full coupling of the fire and the whole tunnel domain including the ventilation devices. This allows for an accurate assessment of the fire throttling effect that is shown here to be significant and for a prediction of the minimum number of jet fans needed to cope with a certain fire size. Furthermore, it is firmly believed that the multiscale methodology represents the only feasible tool to conduct accurate simulations in tunnels longer than few kilometres, when the limitation of the computational cost becomes too restrictive.
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