SMOKE EXTRACTION IN TUNNELS WITH CONSIDERABLE SLOPE

In many tunnels with transverse ventilation, the smoke-extraction capacity is determined on the base of the estimated smoke production rate of the design fire. This design method implicitly assumes a stable stratification of the smoke layer. As the assumption of a stratified smoke layer is not justified when higher flow velocities are expected, the entire air volume that flows in the tunnel must be removed. In tunnels with gradients exceeding three percent, the chimney effect due to the rising temperature in the tunnel may cause flow velocities of 4 m/s and more. Therefore, the smoke-extraction capacity has to be designed for an anticipated longitudinal flow at the fire location. In this paper, three models for the calculation of the effective pressure difference due to the chimney effect are compared. With these models, the expected flow velocity in the tunnel can be calculated based on buoyancy and external forces. Examples for smoke extraction systems are given for existing and planned tunnels. In the tunnel Gotschna, the smoke extraction capacity is designed for an expected maximum flow velocity during the design fire. In the tunnel Vue-des-Alpes, the extraction capacity is not sufficient and the longitud inal flow during the fire is limited by means of several jet fans. NOMENCLATURE A [m2] area cp [W/kgK] thermal capacity g [m/s2] gravity constant i [-] inclination L [m] length Dp [Pa] pressure difference Q [W] heat flux T [K] temperature t [s] time U [m] perimeter u [m/s] velocity x [m] axial distance DH [m] elevation difference DT [K] temperature difference Dx [m] element length a [W/m2K] heat conduction coefficient e [-] emissivity h [-] reduction coefficient r [kg/m3] density of air s [W/m2K4]Stefan-Boltzmann constant fire related to the fire stack related to chimney effect T tunnel wall tunnel wall 0 initial conditio