Definition and optimisation of the efficiency of smoke extraction in a road tunnel
Résumé
Transverse ventilation is commonly used in long tunnels, tunnels with two-way traffic, or in tunnels which are frequently congested. This strategy consists of extracting the hot gases near the ceiling in order to prevent the smoke from propagating throughout the tunnel and to preserve the natural stratification of smoke. The aim of the design process is to obtain a high extraction efficiency at an optimal cost. Thus far, the regulations related to the design of such systems have been based mostly on empirical considerations, for example the number of dampers or their size and shape. The aim of this work is to determine the most influential parameters leading to an increase in efficiency. As a first step, a universal expression of the efficiency is given, avoiding in particular the need to define the interface between fresh air and smoke, which is often unclear. To achieve this, only integral values over the whole section of the tunnel, i.e. fluxes, are used. The efficiency of the system can then be defined as the ratio of the buoyancy flux extracted through the damper to the buoyancy flux released by the fire. The yield is defined as the efficiency related to the cost of extraction. A numerical study using the CFD code Fluent is carried out to investigate the influence of various parameters on the efficiency: the layout and shape of the dampers in the ceiling, the distance to the fire, the speed of the air flow in the tunnel, the flow rate in the extraction duct and the heat release rate. The modelling hypotheses are validated by comparative tests. The study of the behaviour of a single damper is used to characterise the local phenomena. A full setup with several dampers is then studied. It appears that the air flow in the tunnel is the most important parameter, whereas the shape of damper has little influence on the efficiency.
Domaines
Milieux fluides et réactifs
Origine : Fichiers produits par l'(les) auteur(s)