2.1 Background

Longitudinal ventilation systems in road tunnels are designed for operation during a design fire at a minimum flow velocity. Usually a one-dimensional, steady-state flow model is applied for several predefined scenarios. This procedure is described in detail in national standards, eg in Switzerland [1], Austria [3] and Germany [4].

For the ventilation design it is necessary to determine the critical fire location. The model takes into account a required flow towards the fire, flow resistances of the tunnel structure, barometric pressure differences and wind pressure at the portals, temperature-related buoyancy as well as the flow resistance of the vehicles. The resulting pressure losses indicate the number of jet fans required. In Austria, the temperature-related expansion of the air must also be taken into account [3]. In these models it is assumed that the fire itself does not appear as flow resistance. The influence of the fire on the longitudinal flow remains negligible.