Sisir Chakraborty, Director, C.Doctor & Co Pvt. Ltd.
A fire that develops in a tunnel interacts with the ventilation airflow and generates complicated air flow patterns and turbulence in the vicinity. The heat generated warms up the surrounding air, and in case of a slope inside the tunnel, buoyancy forces are created along the tunnel, which could govern the movement of the air flow inside the tunnel. If the resulting longitudinal flow velocity is not high enough, a reverse flow of hot gases in the ceiling will be created. This phenomenon is known as back-layering. In order to prevent any type of back-layering, the longitudinal velocity inside the tunnel has to be higher than a critical value.
Sincere efforts should be made to avail the natural ventilation available in the tunnel. The main problem with natural ventilation in tunnels is that not only the tunnel geometry, but the size and location of the fire govern the flow of hot gases in the tunnel, but also winds and atmospheric conditions outside the portals may have a strong influence on the ventilation system.