In 1949 a huge fire happened in the Holland Tunnel between New York and New Jersey in U.S.A. It is believed $ ^{2)} $ the heat released in the “Holland Tunnel Fire” was 50–100 MW since vehicles ignited 100 m away from the original vehicle fire. The average temperature rise of gases in layer above the fire was estimated $ 1000^{\circ}C $ , assuming the area of $ 4 \times 12 $ m for the spilt petrol and heat release rates of cars, vans and lorries to be 3, 10 and 20 MW respectively, whereas the real fire was due to 80 drum cans of carbon disulfide (totally $ 80 \times 180 \Omega $ ) and 10 burned vehicles.

It was reported $ ^{3)} $ that approximately 400 MW heat was released in the “Caldecott Tunnel Fire” where 33 k $ \Omega $ (760 $ \Omega $ debouched out of the tunnel) of petrol spilt from a side-rolled trailer tank truck and, after all, 32.24 k $ \Omega $ of petrol burned within 1 hour while 3 hours were needed for fire fighters to control the fire. The maximum temperature near the tunnel ceiling was estimated 1050°C. However, we believe the value of 400 MW is overestimated. Since the fire continued for about 1 hour, the burning rate of fuel becomes nearly 9 $ \Omega $ /sec and so the average heat release rate becomes about 300 MW, assuming the perfect combustion. However, it is common in large tunnel fires that soot and CO are extremely created. Further, some of them may recirculate into the flame. This indicates the combustion reaction in an enclosed area is imperfect, differently from an open fire. Taking imperfectness into account, the heat release rate may be reduced.

There were 189 vehicles burned in the “Nihonzaka Tunnel” $ ^{7} $ . The fire brigades needed about 8 days to control the fire, but at the early stage (1 hour after the ignition) there were only 6 burning vehicles including trucks transporting plastic materials or drum cans of resin. Assuming the average heat release rate of vehicles to be 10 MW (for middle class vehicles like vans mentioned above), the total heat release rate at 1 hour after the ignition is estimated to be 60 MW. The maximum temperature near the ceiling in the tunnel is estimated about 1000°C, based on the observations of the fire fighters watching walls glowing as if in a blast furnace and feeling impossible to approach there. It is imagined that most of the 189 vehicles burned could have been ignited by radiation from the ceiling and the smoke layer including combustible gas and flame tip.

On the other hand, 10 vehicles were burned within 3 hours $ ^{8)} $ in the “Sakai Tunnel Fire”. Assuming the same average heat release rate of single van, the whole heat release rate becomes 100 MW. The maximum temperature near the tunnel ceiling is estimated about $ 1000^{\circ} $ C, based on the same reason mentioned above.

Thus the past four large tunnel fires indicated that (1) about 5–10 vehicles were simultaneously burning, (2) the heat release rate of 50–100 MW will cause large fires in tunnels and (3) the maximum temperature near the ceiling may rise to about $ 1000^{\circ} $ C.

### 2.2 Previous Experimental Studies of Tunnel Fires

There have already been made many studies of fires in tunnels. Here is attempted an analysis of aspects of the tunnel fires examined experimentally.

Japan Highway Public Corporation has carried out many tunnel fire tests $ ^{10-12} $ , using newly built highway tunnels, but their main object was to test suppression and ventilation control systems in tunnels. For example in the “East Kakei Tunnel (6.9 m high)” experiments, the