from the fire center to the direction of tunnel exit are shown. One is the relationship for the 1/4 scale tunnel experiments, one for numerical results of 1/4 scale tunnel, one for numerical results of 6.9 m high hypothetical real scale tunnel mentioned below, one for Zwenberg tunnel experiments $ ^{[17]} $ , and the last for experimental results by Kitahara et al. $ ^{[16]} $

### 5. Calculations of Fires in a Hypothetical Tunnel

Fire simulations in a hypothetical tunnel were conducted. The height $ (H) $ , width $ (W) $ and length $ (L) $ of the tunnel are 6.9 m, 10 m and 200 m, respectively. The fire was located centrally as in the case of calculations of 1/4 scale model tunnel fires. As mentioned above, large fires in real scale tunnels are the magnitude of 50 MW or more. Therefore as a typical case 45 MW of convective heat in the volumetric heat source $ (5 \times 5 \times 6.9 \, \text{m}(H)) $ located in the center and 6 MW of radiant energy, heating the floor and side walls, were employed.

Calculations were conducted using the same code as mentioned above, with increased the number of cells. Similarly to the above 1/4 scale model, 1/4 region of the tunnel was calculated. Cells used for the calculations were $ 19 \times 27 \times 51(L/2 + L/7) = 26163 $ , where L/7 corresponds to the outside area adjacent to the tunnel exit.

Temperatures near the top of the ceiling along the tunnel are also shown in Figure 9 as mentioned above. The Zwenberg tunnel fire tests $ ^{[17]} $ were one of the past largest fire tests. As seen in Fig. 9 the temperature decay along the tunnel length in this numerical simulation is very similar and close to that in the Zwenberg tunnel fire tests although the tunnel size and heat release rate are different. The calculated velocities of heated gas near the ceiling and lower cool air were 7.5 m/s and 1.1 m/s, respectively. The height of smoke layer was about 3.6 m.

### 6. Discussions

Based on the reports of fire accidents, previous experiments of tunnel fires and experimental results in the 1/4 scale model tunnel, general aspects of tunnel fires were obtained. The flow patterns in the tunnel were similar both in experiments and in numerical calculations. If large fires occur in relatively short tunnels, the hot gas layer is stably stratified and a lot of air is supplied to fires, as seen above. Also the fuel surface is strongly heated by radiation from both the flame and the glowing ceiling. Present experiments showed twice the larger regression rate of fuel in tunnels than in the open. However, Sugawa et al. $ ^{37} $ reported only 20% increase of fuel regression rate in a large room. This discrepancy may be a scale effect, but it is reasonable that the glowing walls can accelerate the evaporation of the fuel. The walls in the experiments by Sugawa et al. may not be radiated fully and consequently not glowing.

Many numerical simulations of tunnel fires in two and three dimensions have already been made, but no detailed considerations about convective heat within the total heat release rate have ever been carried out. Present numerical calculations showed almost reasonable results as seen in the case of 6.9 m high tunnel fire simulation (45 MW) shown in Fig. 9, other than the area near the fire where some differences between the experiments and the calculations were left.

The flow around the burning area is extremely complex due to the turbulence and radiation. Around the fire source a sharp break upon the temperature vs. distance is seen in Fig. 9.