A typical fire safety objective for train fires in tunnels is to create a longitudinal flow and control smoke movement to prevent or limit backlayering to assist occupant evacuation and fire brigade intervention. The tunnel ventilation flowrate required for achieving this critical velocity is a function of the tunnel geometry, train geometry, heat release rate and fire characteristics. The train geometry is usually considered in terms of blockage ratio against the full tunnel area and the tunnel flow is usually considered to be restricted to the annulus area alongside the train. This might be an accurate approximation in traditional train carriages, where each wagon is separated from others by intercarriage doors however trains with an open gangway with few or no intercarriage doors are becoming increasingly common. This study assesses whether the annulus assumption for the calculation and assessment of the critical velocity is still valid with open gangway trains. The assessment is completed using computational fluid dynamics (CFD) based on a validated fire and ventilation model.
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