Tunnel fire size and temperature variations, smoke expansion losses: Fid-Bau Portal

Tunnel fire size and temperature variations, smoke expansion losses

Borresen, B.A. / Dahle, G.A.

Some standard design procedures used in Norwegian tunnel fires are presented. The main points are suggestion of specific standard vehicle fires, introduction added tunnel length to account for fire gas expansion flow losses and introduction of a fixed buoyancy force when downgrade blowing of fire gases. Tunnel fires are often described by maximum size. However, time to reach maximum size and fire duration are of no less importance. The fire load and duration are often more critical than the effect with respect to survival or escape. Furthermore, the fire does not develop instantaneously. 3 to 5 minutes after fire start, the fire size is moderate, even when dealing with big fire sizes. This period is important for escape. The fires are categorized into car, bus/truck, trailer, passenger train or subway coach in order to simulate effect time variation. Fixed values for each category (increase and decrease constants, maximum size and fire load) are used for calculation of fire growth, time to reach maximum size, duration and decay. The values are based on modifying international tests. With fixed fire load, high size involves shorter duration time. Fan dimensioning must include the probability of fan destruction or force reduction with increasing temperatures. The temperature calculations showed that the significant thermal capacity in the mountain mass around the tunnel causes absorption of most of the fire gas even when the fire size was approaching 100 MW. Heating and expanding of tunnel air cause higher flow velocities. This involves increasing frictional forces and hence increasing pressure losses. These can be included by adding an equivalent length to the tunnel. Due to strong reduction of downwind temperature, there is just a slight increase in pressure loss. For long tunnels the added pressure loss effect is negligible.