STUDY FOR SAFETY AT A RELATIVELY SHORT TUNNEL WHEN A TUNNEL FIRE OCCURRED

Transcription

1 STUDY FOR SAFETY AT A RELATIVELY SHORT TUNNEL WHEN A TUNNEL FIRE OCCURRED Y. Mikame 1,2, N. Kawabata 1, M. Seike 1, M. Hasegawa 1 1 Kanazawa University, Japan 2 Metropolitan Expressway Company Limited, Japan ABSTRACT Progress in vehicle exhaust control has greatly improved environments in road tunnels in recent years. Consequently, we discuss the downsizing or removal of tunnel ventilation systems formerly installed to secure the environments in road tunnels. On the other hand, tunnel ventilation systems are also used as smoke control systems in case of fires. So it is also important to discuss safety from tunnel fires. We studied factors that affect the safety of tunnel users in a relatively short road tunnel (around 500m, without a ventilation system) when a tunnel fire has occurred. The examination was analysed by a three-dimensional simulation (Large-Eddy Simulation model) to reproduce the smoke spread and by a onedimensional evacuation simulation. We evaluated the number of people requiring help (NPRH) who cannot evacuate from a tunnel fire. Among the results of the study without a ventilation system, we found some conditions related to safety when a tunnel fire has occurred. The conditions were the fire source point, road longitudinal gradient, presence or absence of wind velocity and a bus. Especially when evacuating from a bus, people need more time to evacuate. This case has an increased risk of someone being left behind in the bus. And we confirmed that many passengers cannot evacuate from a bus because of the smoke from the fire when the bus is close to the fire source point. The second factor influencing safety is the natural wind in the tunnels. Even in a small natural wind of about 1.0 m/s case, smoke catches up to tunnel users when the direction of the natural wind matches the evacuation direction. In this case, we have confirmed that many evacuees cannot evacuate, even there is no bus. Keywords: tunnel fire, short tunnels, natural ventilation, bus, evacuation simulation 1. INTRODUCTION In Japan, the scale of tunnel ventilation systems has been determined by the amount of vehicles exhausting gases, and this system is used when a tunnel fire has occurred. Tunnel ventilation systems have been installed not only in long distance tunnels, but also in short distance tunnels with heavy traffic in urban areas. The Tokyo metropolitan expressways carry heavy traffic, so its operators put tunnel ventilation systems in tunnels about 300m long. Lately, the improvement of vehicle exhaust control has greatly improved the environments of road tunnels. For this reason, this system does not ventilate vehicle exhausts, but operates when a tunnel fire has occurred. Long tunnels about 10km are designed to prepare for a tunnel fire [1], this is a dangerous condition. In contrast, in relatively short tunnels of about 500m, the safety of tunnel users has almost never been studied. For tunnel ventilation systems, the central issue is downsizing and removal. This study determined factors affecting the safety of tunnel users when removing a tunnel ventilation system from a relatively short road tunnel.

2 EXAMINATION CONDITIONS 2.1. Simulation model The spread of smoke from a tunnel fire was simulated using an original three-dimensional simulation (Fireles) [2]. This simulation is developed partly by the authors, and the turbulence model is a Large-Eddy-Simulation. Comparing this with full size tunnel experiment results confirmed the accuracy of this simulation [3]. In Japan, it is generally used to study road tunnel safety when a tunnel fire has occurred. To grasp the simulation of the evacuation of tunnel users, this study used an evacuation simulation [4]. This simulation calculated the number of people requiring help (NPRH), those who cannot evacuate for 10 minutes after the occurrence of the tunnel fire Specification of the model tunnel Tunnel length 450m Cross section of the tunnel rectangular (around 8.5 m W x m H) Longitudinal gradient -4% - 4% The computational grid sizes 0.33 m in the x direction, 0.31 m in the y direction, 0.23m in the z direction Number of divisions 1429 in the x direction, 43 in the y direction, 29 in the z direction (Include area outside the tunnel) Tunnel entrance Tunnel exit x 225m 130m Fire source point -4% 0% 4% Figure 1: Outline of the tunnel that was studied 2.3. Tunnel fire conditions When a tunnel fire has occurred, factors affecting the safety of tunnel users are fire source points, natural wind, and arrangements and configuration of vehicles. The fire source points have an impact on smoke spread by the longitudinal gradient. The natural wind, which is the pressure difference between the tunnel entrance and exit for natural ventilation (Δ), causes the smoke to spread in the tunnel. The arrangement and configuration of vehicles is related to the number of NPRH. Table 1 shows fire source points and pressure difference Δ conditions. Vehicles were arranged from the tunnel entrance to the fire source point and from the fire source point to the tunnel exit. The configuration of vehicles assumed only passenger vehicles, large size vehicles comprised 10% of total traffic and a bus was included. In this paper, it was assumed that a single large size vehicle caught fire in the tunnel. Heat release rate adopted was 30MW, and it changes over time to become constant after 480 seconds.

3 Category Fire source points Δ (Pressure difference between the tunnel entrance and exit for natural ventilation) Table 1: Tunnel fire conditions Conditions x=225m (Sections of longitudinal gradient 0% ) x=130m (Sections of longitudinal gradient -4%) 0Pa [0 m/s],5pa [0.85 m/s],10pa [1.2 m/s] Table 2: Number of passengers (per vehicle) Passenger vehicles Large size vehicles Number of passengers Average of 1.4 people Average of 1.3 people 50 people Conditions under which it is difficult for tunnel users to evacuate were defined as smoke density greater than Cs 0.4 [1/m] and smoke height less than 1.5 m from road surface. Smoke density of Cs 0.4 [1/m] reaching the ceiling triggered the start of evacuation of tunnel users; they evacuate when they see other tunnel users evacuating. The evacuation velocity distribution linearly increased from 0.9 m/s to 1.2 m/s, was constant from 1.2 m/s to 1.8 m/s, and linearly decreased from 1.8 m/s to 2.1 m/s. The direction of evacuation of tunnel users was assumed to be evacuation towards the tunnel portals. 3. TRENDS IN CONFIGURATION AND PLACEMENT OF VEHICLES 3.1. Simulation conditions Table 3 shows simulation cases, excluding cases including emergency exits. These cases are mentioned later. A bus is located 100 m from the tunnel entrance and exit. Table 3: Simulation cases F225 F225-B F130 F130-B Case Vehicle positions Passenger vehicles Number of Vehicles Large-size vehicles F225-L Left of fire source point F225-R Right of fire source point F225-B-L Left of fire source point F225-B-R Right of fire source point F130-L Left of fire source point F130-R Right of fire source point F130-B-L Left of fire source point F130-B-R Right of fire source point Simulation results Table 4 and Figure 2 shows the simulation results for case F225 and case F225-B. For case F225-L and case F225-B-L, the fire source point is 225m and the vehicles are arranged from the tunnel entrance to the fire source point, and the results show no NPRH being affected by the pressure difference Δ and configuration of vehicles. In case F225-R and case F225-B-R, vehicles are located on the right side of the fire source point, NPRH break out in these cases according to the increase of the pressure difference Δ. This result shows that tunnel users are exposed to smoke by a tunnel fire which has occurred and it flows from the fire in the

4 direction tunnel users evacuate, so as a result, tunnel users cannot evacuate. In the case of an arrangement including a bus (case F225-B-R), NPRH are drastically increased. Table 4: NPRH of fire source point 225m F225 F225-B Case Vehicle positions Δ 0 Pa [0 m/s] NPRH [people] Δ 5 Pa [0.85 m/s] Δ 10 Pa [1.2 m/s] Average numbers of people in a tunnel F225-L Left of fire source point F225-R Right of fire source point Total F225-B-L Left of fire source point F225-B-R Right of fire source point Total Figure 2: The results of fire source point 225m Figure 3 shows the simulation results at Δ 5 Pa and Figure 4 shows results at Δ10 Pa. The x-axis represents the distance from the tunnel entrance and the y-axis represents the elapsed time after the start of the simulation. In the figure, the green part indicates where smoke density is Cs 0.4 [1/m] at 1.5m from the road surface and the black-line shows the state of tunnel users evacuation. Figure 3: The results of case F225 Δ5 Pa Figure 4: The results of case F225 Δ10 Pa

5 Table 5, Figure 6 and Figure 7 shows the simulation from the tunnel entrance to the fire source point 130m from the tunnel entrance. NPRH trends differ between cases of fire source points 225m and 130m. For case F130-B-L, vehicles are arranged from the tunnel entrance to the fire source point and include a bus, and the results show there are NPRH in all cases. The factors probably are the exposure of tunnel users to smoke backlayering and the presence of a bus near the fire source point. However, owing to the smoke backlayering moving to the tunnel exit, NPRH decreased as the pressure difference Δ increased. With case F130-L and case F130-R, a trend similar to the source point 225m is indicated. Table 5: NPRH of fire source point 130m F130 F130-B Case Vehicle positions Δ 0 Pa [0 m/s] NPRH [people] Δ 5 Pa [0.85 m/s] Δ 10 Pa [1.2 m/s] Average number of people in a tunnel F130-L Left of fire source point F130-R Right of fire source point Total F130-B-L Left of fire source point F130-B-R Right of fire source point Total Figure 5: The results of fire source point 130m Figure 6: The results of case F130-B, Δ5 Pa Figure 7: The results of case F130-B, Δ10 Pa

6 VALIDITY OF TUNNEL DISASTER PREVENTION SYSTEM As mentioned above, if there is a bus in the tunnel, NPRH tend to increase. Therefore, this section determined the effect of an emergency exit. In the tunnel, a bus is leeward of the fire source point 225m; and the interval between the fire source point and the bus is 10m. The distances from the bus to an emergency exit are 50m, 75m, 100m, 125m, and 175m. Figure 8 is a schematic view of these conditions. The pressure difference Δ is 0 Pa. Tunnel entrance, -4%, 0%, 4% Tunnel exit 225m Emergency exit 10m 50m 75m 100m 125m 175m Figure 8: Arrangement of emergency exits [225m] Figure 9 shows the simulation results for smoke density distribution in the x-z plane across the tunnel model. t = 120 second t = 240 second t = 360 second t = 480 second z Height [m] t = 600 second x Distance from the tunnel entrance [m] Figure 9: Smoke density distribution in the x-z plane across the tunnel model Figure 10 indicates the simulation results for the effect of the emergency exit. For the distance of 50m and 75m, the emergency exit reduces NPRH.

7 Figure 10: Effectiveness of the emergency exit, Δ 0Pa 5. CONCLUSIONS This study quantitatively revealed factors affecting safety for tunnel users when a tunnel fire has occurred in a relatively short road tunnel. The factors affecting safety of tunnel users are as follows. A natural wind flows in the direction the tunnel users evacuate: tunnel users are on the leeward of the fire source point, and smoke catches up to tunnel users. The tunnel users are exposed to the smoke spread by backlayering. When a bus is in the tunnel, especially if the bus is close to a fire source point. Under the study conditions, with the distance of 50m and 75m, the emergency exits effectively reduce NPRH. 6. REFERENCE [1] PIARC Committee on Road Tunnels (C3.3), Operational Strategies for Emergency Ventilation, [2] Y.Kunikane, N.Kawabata, K.Takekuni, A.Shimoda, Heat Release Rate Induced by Gasoline Pool Fire in a Large-Cross-Section Tunnel, 4th International Conference, Tunnel Fires, in Basel, p , ( ) [3] N.Kawabata, Y.Kunikane, K.Takekuni, A.Shimoda, Numerical Simulation of Smoke Descent in a Tunnel Fire Accident, 4th International Conference, Tunnel Fires, in Basel, p , ( ) [4] M. Seike, N. Kawabata and M. Hasegawa, Study of Assessment of Fire Safety in a Road Tunnel by Evacuee s Behavior based on Smoke Behavior by 3-D CFD Analysis, Advanced Research Workshop Evacuation and Human Behavior in Emergency Situations, Santander, pp , 2011.