cross sectional area(S) were 1.6 m, 1.5 m, 21 m and 2.01 m², respectively. Hence the large scale tunnel corresponding to the 1/4 scale model tunnel was assumed to be H=6.4 m, W=6 m and L=84 m. The wall with the thickness of 0.1 m was made of highly heat-proof (up to 1500°C) material with the heat conductivity of 0.65 kcal/m/hr/°C at 400°C.

Weather conditions with no or weak wind and without precipitation were selected. N-heptane (about 18 $ \Omega $ ) was burned in a 0.5 m square steel tray located centrally on the tunnel floor. The combustion of fuel continued about 10–15 minutes. Temperatures of the hot gas traveling towards the tunnel exit were measured in the vertical central plane in the north half area (1.6 × 10.5 m) of the tunnel, since the tunnel and flames at no wind were almost symmetric. Chromel-Alumel thermocouples (0.6 mm in diameter) were installed at totally 6 × 11 grid points in the vertical central plane (in north half length). Each grid is 1 m long and 0.2 m high. No thermocouples were installed in the area lower than 0.5 m above the floor.

Further, burning tests of fuel in a 0.5 m square tray both in the tunnel and in the open were conducted to examine the regression rate of fuel surface and the smoke density in the flame. Details of the experiments, including the method to measure the fuel regression rate, has already been reported $ ^{19} $ .

### 3.2 Experimental Results

#### 3.2.1 The Regression Rate of Fuel Surface and Observation of Combustion

The regression rate of surface $ (0.5 \times 0.5 \, \text{m}) $ of the n-heptane burning in the open was about 4 mm/min, close to the conventional value $ ^{20} $ . However, the regression rate of fuel burning in the 1/4 scale model tunnel was about 8 mm/min, namely twice the larger than in the open.

The top of the flame reaching the tunnel ceiling was pressed like a mushroom. After the measurement, the ceiling surface above the fire was slightly damaged and turned white, whereas the ceiling except directly above the flame was turned black due to the dense soot layer.

Figure 2 shows the flame of n-heptane burning centrally in the tunnel. The flame was highly turbulent and large amount of soot was flowing out of the tunnel exit, but it was too dense to measure the smoke concentration optically. Contrary, in the open the soot was relatively little.

#### 3.2.2 Temperatures and Air velocities

Figure 3 shows the isothermal patterns measured at 2, 4 and 6 minutes. The maximum temperature near the ceiling was about $ 900^{\circ} $ C. The temperature at the exit of the tunnel was nearly $ 350^{\circ} $ C. The velocities of outflowing hot air and inflowing cool air at 3 m inside the tunnel exit were 3 m/s and 0.5 m/s respectively.

### 4. Numerical Calculations

#### 4.1 Estimation of Convective Heat Release Rate in the 0.5 m Square Burning Flame

Since the regression rate of the fuel surface was about 8 mm/min, the mass flow rate of the evaporated n-heptane was about 22.6 g/s and so the heat release rate $ (Q) $ becomes nearly 1070 kW, if the combustion was perfect. However, in reality the combustion was imperfect because the smoke, the concentration of which was difficult to measure, was very dense in the tunnel. Hence here was attempted an estimation of the smoke concentration. The outflow rate