Halcrow employed Computational Fluid Dynamics in order to assess the effectiveness of the over-track exhaust (OTE) system, designed to clear smoke from a train fire in the station area. The system concept is based on continuous extraction via a single smoke extract duct located at high level alongside the track on the tunnel wall opposite the platform.

The results of the time-dependent CFD simulations highlighted a potential problem with the original design; in particular, it was shown that, for some design fire scenarios, there was a risk of ‘smoke-spill’ below the level of the proposed platform ceiling downstand and a consequent risk of smoke spreading into the concourse areas via buoyant dispersion.

The CFD model was based on very conservative assumptions; nevertheless it was apparent from the simulations that the smoke reservoir formed by the ceiling downstand was too small to effectively contain the energetic buoyant fire plume following its deflection towards the platform following its impingement on the ceiling above the fire source (upper image - see left).

Given the dimensional constraints of the project, the dimensions of the smoke reservoir could not be increased and neither was it practicable to increase the smoke extraction rate through the duct.

Instead a second, perforated, ceiling downstand was introduced along the line of the platform edge in order to absorb some of the initial kinetic energy of the fire plume ‘ceiling layer’. The perforations were intended to provide an energy absorption function by virtue of their blocking effect on the ceiling layer and also to allow the subsequent lateral passage of the decelerated smoke back towards the OTE duct. Subsequent CFD runs confirmed that these design intentions had been met and that smoke spill was now negligible (lower image - see left).