numerical simulation of runehamar tunnel fire teststunnel2016/history/tunnel_2012... · 2012. 4....

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6thInternational Conference ‘Tunnel Safety and Ventilation’ 2012, Graz

NUMERICAL SIMULATION OF RUNEHAMAR TUNNEL FIRE TESTS

Li Y.Z., Ingason H., Lönnermark, A. SP Technical Research Institute of Sweden

ABSTRACT

This paper focuses on numerical simulation of a Runehamar tunnel fire test, i.e. a large tunnel fire with heat release rate around 200 MW. The widely used CFD tool Fire Dynamics Simulator (FDS) is employed in the simulations. The Runehamar tunnel fire tests performed by SP were well designed and valuable data were obtained. This makes it possible to assess CFD modelling of large tunnel fires. The paper further explores the fire dynamics in large tunnel fires. Different parameters are investigated and compared, including gas temperatures and heat fluxes in the vicinity of the fire, gas temperatures and velocities downstream of the fire.

Keywords: tunnel fire, large fires, CFD, fire dynamics

1. INTRODUCTION

Tunnel fire safety is a key issue while designing a new tunnel or rebuilding a tunnel. The computational fluid dynamics (CFD) technology is widely used to aid the design of fire safety systems in tunnels. Note that any CFD tool is built on many assumptions and simplifications. The users need to have knowledge of its credibility and limitations.

Especially in a large tunnel fire, the fire dynamics is difficult to simulate since the combustion is complex including both vertical flame and horizontal flame regions. However, the regions close to the fire source are in focus for many design aspects. Examples of such focus are the design of the ventilation system and the fire protection of the tunnel structures.

In order to fully understand the mechanisms of fire dynamics in a catastrophic tunnel fire, SP initiated, planned and performed the Runehamar tunnel fire tests in 2003 [1-3] (Ingason and Lönnermark 2005; Lönnermark and Ingason, 2006; Lönnermark et al., 2006). Recently, the comprehensive Runehamar tunnel fire tests report has been published with all the test data available [4] (Ingason et al., 2011).

The Runehamar tunnel fire tests were well designed and valuable data were obtained, which makes it possible to assess CFD modelling of large tunnel fires. The paper further explores the fire dynamics in large tunnel fires. Different parameters are investigated and compared, including gas temperatures and heat fluxes in the vicinity of the fire, gas temperatures and velocities downstream of the fire.

2. SHORT DESCRIPTION OF THE RUNEHAMAR FIRE TESTS

The Runehamar tunnel is situated about 5 km from Åndalsnes, 40 km south of Molde in Norway and is a two-way asphalted road tunnel that was taken out of use in the late 1980s. It is approximately 1600 m long, 6 m high and 9 m wide with a cross-section of about 47 m2. The tunnel has an average uphill slope of 0.5 % up to about 500 m from the east portal (where the fans were located) to the west portal, followed by a 200 m long plateau and then a 900 long downhill section with an average slope of 1 % towards the west portal. The longitudinal flow inside the tunnel was created using two mobile fan units.

The fire was mainly located about 1037 m from the east portal, i.e. on the downhill section of the tunnel. The commodities were placed on particleboards on a rack storage system to simulate a HGV measuring 10.45 m by 2.9 m. The total height was 4.5 m and a 0.5 mm thick

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polyester tarpaulin covered the cargo. The height of the platform floor was 1100 mm above the road surface.

Before the mock-up tests, a pool fire test (T0) was carried out. This test was carried out to check the instrumentation and calibrate the measurements of the heat release rate measurements. The fire source consisted of diesel loaded in a pan with a diameter of 2.27 m. The total volume of the fuel was 200 L. No data or information about this test has been published previously.

For the safety of the personnel, the tunnel was protected by Promatect T fire protection boards near the position of the fire. The boards were attached to a steel frame system from GERCO consisting of crossbar steel beams and pipes over a length of 75 m. The steel frame system was in a straight line and equal in geometry over the entire 75 m length. The ceiling consisted of boards covering the entire length of the steel framework (75 m). The walls were 39 m long and consisted of vertical boards (30 mm thick) attached to the steel framework (see Figure 1).

Figure 1: Tunnel cross section at the fire site.

Temperatures were measured at several positions along the tunnel, from -100 m upstream of the fire to a measurement station +458 m downstream of the fire, i.e. 105 m from the west entrance. Upstream of the fire the thermocouples were located at -15 m, -25 m, -40 m, -70 m and -100 m and 0.3 m beneath the ceiling. Downstream of the fire the thermocouples were located at 0 m, +10 m, +20 m, +40 m, +70 m, +100 m, +150 m, +250 m, +350 m and +458 m. The majority of the temperatures were measured using unsheathed thermocouples, 0.25 mm type K. Most of the gas temperatures downstream of the fire were measured 0.3 m below the tunnel ceiling, which means 4.8 m above the road surface in the region with fire protection boards and about 5.7 m above the road elsewhere. At the measurement station at + 458 m, thermocouples were placed at five different heights; 0.7 m, 1.8 m, 2.9 m, 4.1 m, and 5.1 m. Gas concentrations were also measured in the corresponding locations.

Five bi-directional pressure difference probes were used at the measurement station at +458 m, together with one located upstream at -50 m and 3 m above the road surface. The gas velocity was determined with aid of the measured pressure difference for each probe and the corresponding gas temperature.

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Four plate thermometers were placed on the ceiling at 0 m, 10 m, 20 m and 40 m downstream of the fire centre respectively, and a plate thermometer was placed under the target and towards the fire load at 20 m downstream.

3. NUMERICAL SIMULATION METHOD AND VERIFICATION

3.1. Numerical method

The simulations were conducted using Fire Dynamics Simulation (FDS 5.5.3) developed by NIST [5-6] (McGrattan et al., 2010). FDS is widely used in fire community and in tunnel fire engineering application.

Large eddy simulation (LES) mode is chosen. Large eddies are modelled directly and small ones are simulated using Smagorinsky sub-models.

The default combustion model is mixture fraction model. Note that the diffusion flame immersed in a vitiated atmosphere could extinguish before consuming all the available oxygen. A simple model of flame extinction is embedded in FDS 5. The critical oxygen concentration, YO2,lim, below which the combustion doesn’t occur is directly correlated with the gas temperature in the mesh. Virtual sparks are assumed in every mesh which fulfils the above equation. This method should be paid much attention to since a large tunnel fire produces really vitiated atmosphere in the vicinity of the fire and this equation will have strong influence on location of the combustion zone.

The radiation from the fire is one of the most important parts while simulating a large tunnel fire. Since the radiation from the fire is a function of the flame temperature and chemical composition, neither of which are reliably calculated in a large fire simulation, FDS use a very simple way to solve this problem mainly by introducing the radiative fraction of the heat release rate. It is known that the radiative fraction of an open fire ranges from 20 % to 40 % for most common fuel sources. By default it is 0.35 for LES. However, note that the flame extends along the tunnel ceiling, and the flame length in a large tunnel fire is significantly longer than the flame height of an open fire, the radiative fraction could be higher. In the present paper, 0.45 is used. Note that this fraction doesn’t include the reabsorption of the thermal radiation.

The convective heat transfer is solved simply by semi-empirical equations for natural and forced convection. The heat conduction through the tunnel walls is solved using one-dimensional conduction equations and 20 meshes are used by default. Wall functions are introduced to predict the velocity close to the surface and the viscous stress.

Clearly the numerical method is still quite simple and some of these parameters need to be verified in large tunnel fires.

3.2. Verification of modelling

Data from one of our earlier model scale tests [7-8] (Li et al., 2010; Li et al., 2011) was used to verify the modeling. The model scale tests were carried out in a model tunnel with a dimension of 12 m (L) × 250 mm (W) × 250 mm (H). The propane fire burner was set at the floor level and 6 m away from the downstream exit. In the simulation, the tunnel is closely the same as that used in the model-scale tests except a length of 8 m rather than 12 m. The ambient pressure is 95590 Pa and the ambient temperature is 23.8 oC. The heat release rate is 9.45 kW and the longitudinal ventilation velocity is 0.51 m/s. The fire source was placed in the centre of the tunnel. Only half of tunnel was simulated due to fully symmetry. The mesh size used in the simulation is 0.075D*, which has been proved to be a reasonable mesh size according to the above analysis.

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The simulated vertical temperature distribution at different places was compared to the tests data, as shown in Figure 2. It is shown that the simulated results correlate well with the tests data measured upstream and downstream of the fire. The relative error is within 20 %.

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Figure 2: A comparison of gas temperatures between the simulations and the tests.

3.3. Simulations of Runehamar tunnel fire tests

A total tunnel length of 700 m was simulated, including 100 m upstream and 600 m downstream of the fire. The fire source has a width of 3 m and a length of 10 m. Propane was used as fuel. The default wall surface was granite and the fire protection board promatect T close to the fire was simulated. The boundary of inlet was velocity boundary and the outlet was ambient. To reduce computation time, half of the tunnel was simulated. The mesh size was approximately 20 cm.

Runehamar test T1 was simulated. The maximum heat release rate was 202 MW and the average longitudinal velocity ranged from 2 m/s to 2.5 m/s. The ambient temperature was approximately 11 oC. The estimated average heat of combustion is 18.5 MJ/kg. The fraction of soot production and CO production in the fuel is 2.2 %, and 0.88 % respectively.

4. RESULTS AND DISCUSSION

4.1. Ceiling gas temperatures

The ceiling gas temperatures from 40 m upstream (−40 m) to 350 m downstream of the fire are presented and compared in Figure 3. All the thermocouples were placed at the centre line of the tunnel and 0.3 m below the ceiling. Generally, the simulation results correlate well with the measured values, especially from 10 m to 150 m downstream of the fire. However, the ceiling gas temperatures at 250 m and 350 m downstream were underestimated approximately 30 % in the peak period. Further, the maximum temperature beneath the tunnel ceiling is approximately 1350 oC. This confirms the reliability of the measurement. It should be noted that the ceiling gas temperature is overestimated upstream of the fire. This may indicate that the backlayering length of a large tunnel fire could be overestimated using FDS 5. Also, the gas temperatures above the fire are overestimated in the peak period. This may arise from the difference in the fuel configuration, that is, the fuel consisted of solid fuels in full scale test but gas burners in the simulation. Despite this, well agreement can be found.

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Figure 3: Gas temperature below the ceiling in the vicinity of the fire.

4.2. Radiation

The incident heat fluxes measured by plate thermometers and the numerical results are presented and compared in Figure 4. Note that the figures correspond to 4 plate thermometers on the ceiling and 1 plate thermometer toward the fire at 1.6 m above the floor. Comparing the measured and simulated results shows that a relatively good agreement can be found, however, not as good as in ceiling gas temperatures. The reason is that the heat flux is quite sensitive to the temperature, that is, 4th power of the temperature (Kelvin). Clearly, it is shown in Figure 4 that the simulated incident heat fluxes were correlated well with the measured values at 10 m. However, the incident heat fluxes were overestimated at fire site, slightly underestimated at 20 m and significantly underestimated at 40 m downstream of the fire.

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Figure 5 shows the simulated net heat flux at the tunnel ceilings. The data plotted here corresponds to 30 s average values. Note that the net heat fluxes are much lower than the incident heat fluxes presented in Figure 4. The maximum net heat flux of about 40 kW/m2 was found at around 10 m from the fire. In the growth period of the fire, the ceiling close to the fire has higher net heat flux. However, after around 10 min when the fire grows up to 100 MW, the heat fluxes found at these four locations are approximately the same and decreases gradually with time. The reason is that in this period the thermal resistance of the tunnel structures dominates the heat transfer to the ceiling, and this region has similar gas temperatures since it is surrounded by the flame in the peak period. Further, the net heat fluxes become negative after about 33 min, that is, the hot walls heat up the gas flow.

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Figure 5: Net heat flux absorbed by the tunnel ceilings at different location in the simulation.

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4.3. Gas temperature and velocity at the measurement station

The measurement station was located at 458 m downstream of the fire. The gas temperatures at five different heights are shown in Figure 6. The gas temperatures are significantly overestimated at all heights. The reason could be the accumulation of underestimation of the heat transfer along the tunnel. This suggests that the heat transfer to the tunnel walls could not be simulated well in the far-field region away from the fire using FDS5.5.

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Figure 6: Gas temperatures at 458 m downstream of the fire source.

Figure 7 shows the gas velocities at five different heights. The gas velocities above 0.7 m are significantly overestiamted and gas velocities at 0.7 m are slightly underestimated. This suggests that the thermal stratification is not simulated well, and further, the wall function in FDS 5 may need to be improved.

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5. CONCLUSION

The simulated ceiling gas temperatures correlate well with the measured values from 10 m to 150 m downstream of the fire, however, the gas temperature at 250 m and 350 m downstream are underestimated by approximately 30 % in the peak period. The gas temperatures at 458 m away from the fire are significantly overestimated at all heights. The gas velocities at 458 m are significantly overestimated at heights above 1.8 m and underestimated at lower height.

The incident heat fluxes at the ceiling are overestimated at 20 m and 40 m downstream of the fire. The maximum net heat flux of about 40 kW/m2 was found at around 10 m from the fire. In the growth period of the fire, the ceilings close to the fire have higher net heat fluxes. However, after around 10 min when the fire grows up to 100 MW, the heat fluxes found at these four locations are approximately the same and decreases sharply with time.

Much attention should be paid on the combustion region and soot production of the fuel.

6. ACKNOWLEDGEMENT

The study was sponsored by SP Tunnel and Underground Safety Centre which is gratefully acknowledged.

7. REFERENCES

[1] Ingason H., Lönnermark A. (2005) Heat release rates from heavy goods vehicle trailers in tunnels, Fire Safety Journal 40, 646-668.

[2] Lönnermark A., Ingason H. (2006) Fire spread and flame length in large-scale tunnel fires. Fire Technology 42, 283-302.

[3] Lönnermark A., Persson B., Ingason H. (2006) Pulsations during large-scale fire tests in the Runehamar tunnel, Fire Technology 42, 283-302.

[4] Ingason H., Lönnermark A., Li Y.Z. (2011) Runehamar tunnel fire tests, SP Report 2011:55, SP Technical Research Institute of Sweden, Borås, Sweden.

[5] McGrattan K., Hostikka S., Floyd J., et al. (2010) Fire Dynamics Simulator (Version 5) Technical Reference Guide, Volume 1: Mathematical Model, NIST Special Publication 1018-5, National Institute of Standards and Technology, USA.

[6] McGrattan K., McDermott R., Hostikka S., et al. (2010) Fire Dynamics Simulator (Version 5) User’s Guide, NIST Special Publication 1019-5, National Institute of Standards and Technology, USA.

[7] Li Y.Z., Lei B., Ingason H. (2010) Study of critical velocity and backlayering length in longitudinally ventilated tunnel fires, Fire Safety Journal 45, 361-370.

[8] Li Y.Z., Lei B., Ingason H. (2011) The maximum temperature of buoyancy-driven smoke flow beneath the ceiling in tunnel fires, Fire Safety Journal 46, 204-210.

numerical simulation of runehamar tunnel fire teststunnel2016/history/tunnel_2012... · 2012. 4....

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