- 277 - NUMERICAL INVESTIGATION OF THE FLOW BEHAVIOUR IN A MODERN TRAFFIC TUNNEL IN CASE OF FIRE INCIDENT Iseler J., Heiser W. EAS GmbH, Karlsruhe, Germany ABSTRACT A numerical study of the flow behaviour in the Limerick Tunnel in case of a fire incident was undertaken. Due to the construction requirements, a 100MW fire in one of the two tubes was considered. The analyses were focused on the smoke removal in the incident tube and on the present pressure difference at the passage doors for a predefined massflow of the ventilation system. The numerical study was meant to judge the risk of a smoke migration from the incident tube to the other one. With the obtained pressure distribution at the passage doors, it was also possible to investigate, whether the resulting pressure at the fire exit doors prevents an opening of them. Keywords: Numerical simulation, Saccardo nozzle, tunnel flow, fire incident 1. INTRODUCTION The Limerick tunnel in Ireland represents a state-of-the-art city tunnel. The installed ventilation fulfils modern standards and guarantees low emission values even for high traffic rates. The ventilation system consists of five jet fans and a Saccardo nozzle, installed next to the tunnel entry and exit. Furthermore, the ventilation performs quite well in case of a fire incident: A nozzle exit speed of more than 30 m/s permits a fast removal of the toxic gases. Figure 1: Tunnel building scheme with aerodynamic installations
- 278 - Beside a fast smoke removal, a moderate pressure difference at the passage doors has to be maintained in order to permit an escape from the incident tube to the smoke-free tube. However, the setting of a reasonable pressure difference is quite challenging: One the one hand, the pressure difference should kept below a distinct level, permitting an opening of the door at any rate. On the other hand, the positive pressure gradient (directed from the incident to the non-incident tube) should be high enough to prevent the smoke from entering the nonincident tube, when the passage doors are open. Therefore an exact thrust setting is needed: A positive pressure gradient is realised by running only the fans in the incident tube at design speed. The rotational speed in the non-incident is then derived by the definition of a threshold pressure difference value. In the described case, the rotational speed was calculated analytically by considering global parameters of the tunnel and the ventilation. Aerodynamic losses were considered by defining distinct loss coefficients. In most cases, this represents a reasonable and fast method. However, local influences on the tunnel flow (like traffic sings) can be incorporated only in a rough way in such a study. So, if the pressure differences at the passage doors are influenced significantly by local flow phenomena, an analytical method would fail. Therefore, a complementary unsteady numerical study were undertaken to confirm the derived settings, using a state-of-the-art CFD method. 2. COMPUTATIONAL GRID AND BOUNDARY CONDITIONS The computational model of the Limerick tunnel was based on the CAD-geometry, which includes the Saccardo nozzle, all traffic signs, the cable ducts mounted on the ceiling, and the fire exit doors. In order to predict the smoke movement and the pressure distribution on the escape doors accurately, a grid with more than 1e+06 elements was constructed. The burning truck was placed in the middle of the incident tube and therefore in the lowest part of the Tunnel. Thus, a smoke migration in both directions was possible. This corresponds to a worst case scenario. In accordance to older investigations, the wind speed was 3m/s. The maximum volume flow rate of the ventilation system was set to 258 m 3 /s in the non incident tube and to 252.84 m 3 /s in the remaining tube. The maximum heat release at the burning truck was 100MW. In order to predict the heat transfer accurately, a high grid resolution was defined around the truck (fig.2). Figure 2: Computational grid nearby the burning truck
- 279 - A high element number was also required at the passed through Saccardo nozzles, since high velocity gradient were assumed in this region. Additionally, the high grid resolution in this area permitted a quantitative evaluation concerning the influence of the electric ducts on the flow behaviour (fig.3). The ventilation system is run up 30 seconds after the fire inception. Since the run-up time of the fans was expected to be 60s, the maximum volume flow rate is attained after 1.5 minutes. According to the experimental data of the used fans, a linear increase of the volume flow rate was assumed. The increase of the heat flow rate at the burning truck was also considered to be linear with 40 MW per minute. The simulations were performed under the assumption, that the burning truck is the only vehicle in the whole tunnel. Figure 3: Computational grid nearby Saccardo nozzle 3. NUMERICAL RESULTS Figure 4 shows the time dependent emission of smoke at the burning truck and its gradual removal. The smoke is visualized by isosurfaces for (concentration)-values between 100% and 30%. In order to get an impression of the changing flow behaviour, streamlines are also plotted into the figure. The streamlines start from the tube inlet and from the Saccardo nozzle. Additionally, they are coloured as a function of the flow velocity. In the very beginning, the flow behaviour is dominated by the wind contribution. According to the drag at the truck, a distinct curvature of the streamlines can be observed there. 20 seconds after the fire inception, high smoke concentration values are present next to the truck. Now, a significant streamline curvature can be seen in this region. The smoke acts as barrier, which forces the flow to go around. Thus, higher pressure losses are located here. Furthermore, the blockage leads to an increase of the static pressure in the upstream direction. After 100s, the ventilation system ensures an effective removal of the toxic gases. Downstream of the fire, higher velocity values can be seen which is a result of the increasing specific volume of the fluid due to the heat transfer. The fourth picture shows the flow behaviour after nearly 3 minutes. Now, the maximum heat flow rate of 100MW is present. As a consequence, a stronger flowacceleration downstream of the truck is visible. Despite the high flow blockage next to the truck, the ventilation system guarantees a controlled smoke-removal.
- 280 - Figure 4: Time dependent smoke migration Figure 5 shows the streamlines in both tubes next to the traffic signs (coloured in blue) near the end of the electric duct/light duct (left figure) and in front of the burning truck (right). The two pictures confirm the stated observation: The traffic signs influences the flow path only locally. A change of the global velocity field caused by the traffic signs can not be stated. However, the ceiling lights and the electric ducts cause a bigger change of the velocity field nearby the Saccardo nozzle in the incident tube: The short distance between nozzle exit and the crosswise running part of the electric duct (see fig.3) results in a vertical directed displacement of the nozzle flow. As a consequence, the maximum velocity migrates also to lower heights. In the non-incident tube, the distance between nozzle outlet and duct is far greater. A distortion of the ventilation flow can not be observed. Thus the maximum velocity remains next to the ceiling in the smoke free tube.
- 281 - Figure 5: Flow behaviour at traffic signs Figure 6: Streamlines and velocity distribution in both tubes near Saccardo nozzle Beside an effective removal of the toxic gases, a moderate pressure difference at the passage doors is also in the field of interests. The following figures show the distribution of the pressure difference for all passage doors for the last timestep (170s). Positive values mean a positive pressure gradient with higher pressure values in the non-incident tube. The numbering of the doors is increasing from the southern to the northern portal. Door no.1 is positioned upstream of the passed through Saccardo nozzle, door no.2 is located about 30m downstream of the nozzle. Door no. 7 is located less than 10m upstream of the burning truck and door no.8 25m downstream of the vehicle.
- 282 - Pressure difference- different ducts 70,0000 50,0000 Pressure difference [Pa] 30,0000 10,0000-10,0000 0 2 4 6 8 10 12 14-30,0000-50,0000 Different ducts + fire -70,0000 Passage doors Figure 7: Distribution of pressure difference at passage doors The pressure difference distribution results from three effects: Vertical displacement of the nozzle flow in the incident tube due to the crosswise running duct (door no.2) Upstream acting drag effect of the burning truck (door no.3 no.7) Total pressure loss at the burning truck (incident tube) combined with gas expansion behind the truck The vertical displacement of the nozzle flow nearby the cross running duct leads to a migration of the total pressure maximum to lower heights. As a result, the static pressure values are significant higher at ground level, than in the non-incident tube. This leads to a static pressure difference of Δp = -65.4Pa at passage door no.2 (fig.7). According to turbulent diffusion, this effect shrinks after a distinct distance from the nozzle. Therefore, a lower pressure difference is given between door no.3 and door no.7. The negative Δp between no.3 and door no.7 is mainly driven by the drag effect of the burning truck, whereas the smoke contribution is significant higher than the contribution by the truck itself. This in upstream direction acting stagnation leads to a decrease of the flow velocity and instantly to high static pressure values. Therefore, the static pressure differences remain negative up to the location of the truck. According to the drag of the smoke and the truck, higher total pressure losses appear nearby the fire. As shown in figure 8, a higher portion of the fluid is convected to the sidewalls nearby the truck. This leads to a vortex creation and finally to flow losses. At the same time, the heat release at the truck results in an increase of the specific volume and in a rising flow velocity. According to a shrinking total pressure and a flow-acceleration, the static pressure is also shrinking. Since the pressure values in the non-incident tube vary only slightly, the pressure difference changes its sign downstream of the fire.
- 283 - Figure 8: Influence of burning truck on flow behaviour The highest values are obtained at door no.8, which means about 25m downstream of the burning truck. Here, the pressure difference is 55Pa. Downstream of that location, a steady decrease of the pressure difference can be stated. At the last passage door, Δp is 12.4Pa. Since the smoke reveals a movement to the northern portal, a migration of the smoke from the incident to the non-incident tube is unlikely. At the same time, the pressure difference remains mostly at an acceptable level ensuring an opening of the doors in the smoke-filled part. Only nearby the burning truck, a difference slightly beyond the threshold value of 50Pa exists. Figure 9: Flow simulation for similar ducts In order to get an idea of the individual effects like the electric duct or drag of the burning truck, further investigations were undertaken. First, the influence of the duct was analysed. For that reason, the duct in the incident tube was constructed in a similar way to the duct in the non-incident tube. As a result, the distance between crosswise duct and nozzle changed from 0.6m to 3m. The data for the ventilation system and the heat release were defined identically. According to the right picture of figure 9, the adaption of the ducts leads to a similar flow behaviour in both tubes nearby the nozzle.
- 284 - Figure 10 shows the distribution of the static pressure difference for the original geometry (magenta line) and for similar electric ducts. The plot reveals that the differences between the two lines remain below 3 Pa from passage door no.3 to no.13. Downstream of the fire position (door no.8-13) the values of both lines are nearly identical. The actual impact on the pressure difference appears at door no.2. Here the delta is nearly up to 30Pa. In other word, the influence of the duct on the flow behaviour is limited on the passage door next to the Saccardo nozzle. Pressure difference - fire 70,0000 50,0000 Pressure difference [Pa] 30,0000 10,0000-10,0000 0 2 4 6 8 10 12 14-30,0000-50,0000 Similar ducts - fire Different ducts - fire -70,0000 Passage doors Figure 10: Influence of installed ducts on the pressure difference The last figure shows the distribution of the pressure difference for a burning and a notburning truck (blue line). In both cases, one can clearly identify the local influence of the different duct geometries on the flow behaviour (minimum at door no.2). The figure also clarifies the enormous impact of the fire on the tunnel flow: At all passage doors, significant higher pressure differences (absolute values) are given. Actually, the fire influence seems to scale the pressure difference. The biggest deviation appears downstream of the truck at door no.8. Here, the pressure difference has increased from 2.8Pa to 55Pa. Pressure difference- different ducts 70,0000 50,0000 Pressure difference [Pa] 30,0000 10,0000-10,0000 0 2 4 6 8 10 12 14-30,0000-50,0000 Different ducts + no fire Different ducts + fire -70,0000 Passage doors Figure 11: Impact of the fire on pressure difference
- 285-4. SUMMARY A numerical study of the flow behaviour in the Limerick tunnel was undertaken. All geometric details like traffic signs and electric ducts were integrated in the numeric model. Furthermore, a burning truck with a heat release of 100MW located in the middle of the tunnel was considered. The numerical results proved that the toxic gases are removed under the given conditions. Furthermore the results reveal a minor impact on the flow behaviour by the installed traffic signs. Changes in the flow field are only visible nearby the signs. However, the electric duct in the west tube influences the flow field significantly since the distance between nozzle and the crosswise running part of the duct is less than 1m. The short distance leads to a forced displacement of the nozzle flow to lower height and to high static pressure values near the ground. As a result, the pressure difference at the passage door no.2 nearby the nozzle is up to Δp = -65.4Pa. The pressure difference at the passage doors is driven by overall three effects. A shown in the last chapter, the pressure difference is mainly influenced by the drag of the smoke and the heat release at the burning truck. According to the obtained distribution of the pressure difference, a migration of the smoke from the incident to the non-incident tube is unlikely. The maximum pressure difference of +55Pa exceeds the threshold value of 50Pa only by little. 5. AKNOWLEDGEMENTS The simulations were carried out during a project work commissioned by STRABAG.