An experimental study of the impact of tunnel suppression on tunnel ventilation Yoon J. Ko and George Hadjisophocleous Civil and Environmental Engineering, Carleton University 1125 Colonel By Drive, Ottawa, Ontario, Canada, K1S 5B6 ABSTRACT Water-based fixed fire fighting systems (WFFFS) are now considered to be an option for mitigating the effects of tunnel fires as these systems can control the fire and prevent fire spread. The mitigation effects can be enhanced when WFFFS is used with ventilation systems. There is a great need to study the interaction between WFFFS and ventilation systems; and to identify design elements for both systems to make them efficient. This paper presents results from a preliminary test conducted in a full-scale laboratory tunnel furnished with a sprinkler system. The absolute cooling effect and radiation attenuation were examined by activating the sprinkler system over a propane fire which generated a constant HRR. The test examined the effectiveness of the longitudinal ventilation system with the sprinkler system active. The paper also discusses the impact of the water vapour on the measurement of HRR, which uses oxygen consumption calorimetry. KEYWORDS: tunnel fires, longitudinal ventilation, suppression, heat release rate measurement INTRODUCTION Water-based fixed fire fighting systems (WFFFS) are now considered to be an option for mitigating the effects of tunnel fires. The ventilation system alone is not capable of achieving neither structural protection nor life safety for severe heavy good vehicle (HGV) fires although, in particular, the longitudinal ventilation system is quite effective in reducing the temperature downstream of the fire and keeping the upstream areas free of smoke. WFFFS can reduce the severity of the fire and prevent fire spread. The mitigation effect can be enhanced when WFFFS is used with ventilation systems, provided that both systems are suitably designed. There is a great need to study the interaction between WFFFS and ventilation systems; and to identify design elements for both systems to make them efficient. In general, the effectiveness of suppression is greatly influenced by the degree of ventilation. The greater the ventilation rate, the greater the water consumption or the time to extinguish the fire [1]. In tunnel application of WFFFS, oxygen depletion by WFFFS cannot be very effective if the fire is under continuous air flow in the tunnel. Direct cooling of the fuel surface is also hard to achieve since the tunnel fire is often shielded by the vehicle body. Thus, the dominant effect to be expected in tunnel fire suppression is cooling the hot gases and the surroundings of the fire. In recent years, a great deal of research has been done to study the effectiveness of water-based suppression systems on tunnel fires. It was found that the suppression system can effectively control fires and prevent fire spread by reducing the heat release rate (HRR) and cooling the hot gases [2, 3, 4, 5]. Figure 1 shows the published experimental data from previous studies [2, 4, 5], which examined a range of fires with various types of WFFFS, such as water mist systems and sprinkler systems. The ratio of the maximum HRR in a suppression test (Q S ) to the corresponding maximum HRR in a free burn test (Q D ) was plotted with respect to the water flow density (l/min m 2 ) used in each test. The ratio, Q S /Q D represents the rate of suppressed HRR. This shows that WFFFS can reduce the HRR by about 40 ~80% of the design fire (Q D ) depending on the water flow density. A successful suppression system with sufficient water flow rate and fast activation can hold the development of fires and 341
maintain the HRR at the time of activation. The strategy in using both systems can be that WFFFS limits the fire size to be manageable by the ventilation systems, and longitudinal ventilation systems control smoke and reduce the hazard. Figure 1 The ratio of the maximum HRR in a suppression test (Q S ) to the corresponding maximum HRR in a free burn test (Q D ) was plotted with respect to the water flow density (l/min m 2 ) So far, the most reliable method of studying water-based suppression system is real-scale tests since there are difficulties in scaling the water drop sources and their interaction with fire and smoke. To properly investigate the effects of water-based suppression systems on convective flows and thermal radiation, a study in a full-scale laboratory tunnel is being conducted at Carleton University. This paper presents the preliminary study conducted to examine the mitigation effects of both systems and the effect of WFFFS on the efficiency of longitudinal ventilation systems. DESCRIPTION OF THE LABORATORY TUNNEL The large scale experiments are performed in a laboratory tunnel of Carleton University located in Almonte, Ontario, Canada is used in this study for performing the large scale experiments. The tunnel is 37.5 m long, and the cross section is 10 m wide and 5.5 m high. The tunnel has a shutter opening (3.8 m wide and 4.0 m high) and two louver openings (1.2 m wide and 4.5 m high) at the east end. Figure 2 shows a schematic diagram of the tunnel facility. The temperature profiles, combustion gas generation as well as the propagation of heat depend strongly on the cross section of the tunnel. Proper investigations of the convective flows and thermal radiation are possible in this tunnel because the cross section is typical of tunnels with two traffic lanes. In a previous study [6], temperature profile and smoke flows measured in the Carleton's laboratory tunnel were found to be reasonably extrapolated to longer tunnels. In that study, it was concluded that the length of the tunnel does not significantly affect on the temperature development close to the fire location. 342
Figure 2 A schematic diagram of the laboratory tunnel facility of Carleton University Tunnel ventilation system On top of the tunnel, a mechanical exhaust system with three fans capable of exhausting a total of 132 m 3 /s (285,000 cfm) of gas is located at the east end of the facility. Two fans are located at the fan chamber level, and one fan is located at the attic level as shown in Figure 2. The fans draw smoke from the tunnel through two ceiling openings in the tunnel, and the smoke is directed to the upper middle chamber. A longitudinal air velocity ranged from 1-8 m/s can be created by the ventilation system in the tunnel. HRR MEASUREMENT SYSTEM A HRR measurement system has been developed for the laboratory tunnel to be used in measuring the heat release rate of fire in the tunnel. The HRR measurement system for the laboratory tunnel is able to measure various types of tunnel fires and sizes up to low severity HGV fires of about 20 MW. The system uses oxygen calorimetry for accurate and reliable measurements under different ventilation conditions and with the suppression system activated. The HRR measurement system collects data at the end of the upper middle chamber (5.66 m wide by 2.94 m high), directing all combustion products through this design area. Figure 3 (a) shows a schematic of the HRR measurement system for the facility. Signals from thermocouples, pressure transducers, and the gas analyzers are transmitted via the data acquisition system (DAS) to the realtime HRR calculation and display system, which is programmed using Labview (version 8.6). Some delay is inevitable due to the long distance from the fire source to the sampling location and gas analysis system. (a) Schematic diagram of HRR measurement system [7] (b) Instrumentation in the duct chamber Figure 3 HRR measurement system and instrumentation The oxygen calorimetry requires accurate measurements of the mass flow rate, CO 2, CO and O 2 concentration. In particular, accurate measurements of mass flow rate of gases were challenging due to the variation of the flow velocity inside the large chamber of the facility. To obtain good representation of flow velocity and other parameters across the entire cross-section of the chamber, the instrumentation in the fan chamber was designed based upon extensive analysis of various CFD simulations as well as manual velocity measurements [7]. Figure 3 (b) shows the arrangement of instruments including thermocouples, gas sampling grid and bi-directional velocity probes. Four bidirectional probes and six thermocouples were installed, as well as a steel gas sampling grid covering the measurement area of the fan chamber. 343
CALIBRATION TESTS The accuracy of heat release measurements based on oxygen consumption is dependent on accurate measurements of the mass flow rate and oxygen concentration [8]. Generally, the calculation of mass flow rate of the gas in the duct needs to be corrected to take into account the reduction of velocity near the walls and the effect of wall friction. The correction factor depends largely on the velocity profile of the flow in the measurement area. For this reason, a series of calibration tests was conducted to experimentally determine the correction factor for the experimental tunnel. Based on results from 16 tests with different fan capacities using propane burners and heptane pool fires, the factor was found to be approximately 0.85. The correction factor was calculated by comparing a time integrated HRR with the energy content of the fuel used in the calibration tests. Figure 4 shows the HRR measurement results from a calibration test using propane burners. The fans were running at 25% capacity, generating about 2 m/s of air velocity in the tunnel. The calibration propane burners, which can produce a range of HRR up to 10 MW, were placed at 10 m from the east end of the tunnel. The HRR started with 8 MW and decreased step by step at a regular time interval. The system shows a reasonable estimate of the HRR. Figure 4 Results from a calibration test of the HRR measurement system using propane burners (25%, 8-2 MW propane test) Figure 5 Results from a calibration test of the HRR measurement system using propane burners and heptane pool fire (100% fan speed, 23 MW) To test the capacity of the HRR measurement system, a calibration test was conducted using both a 344
heptane pool fire and propane burners (see Figure 5). In the test, a pan fire (1.6 m x 1.6 m) with 123 kg of heptane and propane burners of 10 MW were ignited at the same time. The fans operated at full-capacity in this test, which generated about 3 m/s of longitudinal air flow in the tunnel. The propane burners were turned off at about 6 min after ignition, and the heptane pool burned off at about 7 min after ignition. The heptane pool produced a HRR of about 13 MW, which is much greater than the theoretical HRR of 9 MW, which can be realized by the same size of a pool fire in a windfree environment, as suggested by Babrauskas [9]. The longitudinal air flow caused intense and fast burning of the heptane fire. The HRR measurement system was successful in measuring in real-time a fire of 23-MW. SUPPRESSION TESTS A preliminary test was conducted in the laboratory tunnel to investigate the interaction between WFFFS and the longitudinal ventilation system. This test is part of a study aiming to investigate the impact of WFFFS under longitudinal air flows on smoke spread and temperature propagation; and to examine the influence of WFFFS on the efficiency of longitudinal ventilation systems. A propane burner generating a constant HRR of 5 MW was used. The top of the fire area was shielded by a metal plate (2.5 m x 4.9 m) built over the fire area at a height of 2.8 m. The tunnel ventilation system was controlled to create a longitudinal air flow of 1.8 m/s, which is the critical velocity for a 5-MW fire [10]. During the test, the HRR measurement system was tested to evaluate the system accuracy in the case when the suppression system is active. The large quantities of water vapour generated by the suppression system may affect the calculation of the HRR by oxygen calorimetry since the system does not measure the water vapour content of the exhaust gases [11]. Sprinkler system Figure 6 (a) shows the test arrangement and the sprinkler system in the tunnel. The sprinkler system is installed near the ceiling consists of 5 parallel branches (9 m long) spaced apart at a distance of 3.7 m, which are connected to a main pipe. Each branch has four sprinkler heads spaced at 2.5 m. A total of 20 sprinkler heads are fitted in an upright position. The type of sprinkler nozzles has a thread size of 15 mm and K-factor of 11.2, generating large droplets. The maximum capacity of the system is 12.0 l/min m 2 with operating area of 185 m 2, which is designed based on NFPA 13 [12], design standard for automatic sprinkler systems in industrial buildings. The maximum application rate suggested is 12.2 l/min/m 2 for extra hazard with minimum design area of 230 m 2. The water flow rate and application area can be adjusted to test variable water flow rates and operating areas. The system is open and is manually operated. For this preliminary test, only two branches over the fire area were activated (See the shaded area in Figure 6). The sprinkler system was activated at 11min after ignition of the propane burners and lasted for 9 min. The water application rate for the test was 530 l/min (140 gpm), and the pressure was about 138 kpa (20 psi). Instrumentation Figure 6 (b) shows the instrumentation used in the tunnel. The ceiling temperatures at a distance of 0.2 m from the ceiling were measured along the centre line of the tunnel. 4 thermocouple trees were placed as shown in Figure 6 (b). Heat fluxes at a height of 1.5 m were measured 6 m downstream of the fire and 5 m upstream of the fire. Gas concentrations (O 2, CO 2 and CO) and mass flow rates of the exhaust gases were measured at the instrumentation station (see Figure 3 (b)) in the fan chamber and used in the HRR measurement system. 345
(a) Sprinkler system (Plan view) (b) Instrumentations (Section view) Figure 6 Sprinkler system and instrumentation in the tunnel RESULTS The ventilation system generated a longitudinal air flow of 1-2 m/s in the tunnel and successfully controlled smoke without the sprinkler system active. When the sprinkler system was turned on, some smoke escaped out of the tunnel openings, but overall the ventilation system was able to control the smoke. The sprinkler system cooled smoke, caused steam formation, and lowered the visibility. Figure 7 shows a photo taken right after the closing of the sprinkler system. Figure 7 A photo taken right after the closing of the sprinkler system 346
With the sprinkler system active, ceiling temperatures upstream of the fire and in the spray section dropped drastically. Figure 8 compares the ceiling temperature profiles with and without the sprinkler system active. The ceiling temperatures downstream of the fire dropped about 20 C, yet the profile was similar to that without suppression system. Figure 9 shows the vertical temperature profiles 5 m upstream and 6 m downstream of the fire. The sprinkler system cooled down the temperature, and the cooling was more significant for the gas temperatures measured 5 m upstream since the sprinkler system is located as close as 4.4 m away. Figure 8 A comparison of the ceiling temperature profiles with and without the sprinkler system active Figure 9 A comparison of vertical temperature profiles with and without the sprinkler system active Figure 10 compares heat fluxes measured 5 m downstream and 6 m upstream over time. Before the sprinkler system was activated, heat fluxes measured at both locations were about 4-5 kw/m 2, which may not be lethal but can cause irritation [13]. After the system commenced, heat fluxes at both locations decreased to 1-3 kw/m 2. With the sprinkler system active, the heat flux upstream was lower than the heat flux downstream, because some water droplets wetted the surface of the heat flux meter. 347
Figure 10 A comparison of heat fluxes measured 5 m downstream and 6 m upstream over time Figure 11 The measured HRR and O 2 concentration over time When the suppression system was active, the temperatures in the fan chamber immediately dropped, and temperatures came back up when the suppression system was stopped. With the suppression system active, CO and O 2 concentrations slightly decreased; however, gas concentrations were pretty consistent throughout the test. Figure 11 shows the measured HRR and O 2 concentration over time. O 2 measurements were unstable for a short time after the opening of the sprinkler system as well as after the closing of the system. This affected HRR calculation a little bit, but the measured HRR was quite consistent throughout the test. DISCUSSION This paper presented test results demonstrating how the ventilation and suppression system affect the temperature propagation and the smoke condition in the tunnel. The absolute cooling effect and radiation attenuation were tested by activating the sprinkler system over the propane fire which generated a constant HRR. The condition generated by the test represents the situation when the suppression system successfully controls the development of a fire and limits the HRR to that at the time of activation. It was found that the sprinkler system and ventilation system cooled down smoke and reduced the heat flux effectively. The measured heat fluxes showed that the absorption of thermal radiation and transmission of the radiation can be affected by the sprinkler system and air flow in the tunnel. The longitudinal air flow in the tunnel was affected by the discharge of water sprays because the air flow velocity was as low as 1-2 m/s. However, the ventilation system was able to control smoke in the tunnel. As the sprinkler system reduced the smoke temperature, it could be expected that the 348
driving force to propagate the smoke decreased, thus enabling the longitudinal ventilation system to prevent backlayering of smoke. Tests are on-going to further study the interaction of the suppression system with the longitudinal air flows and re-examine the effectiveness of the longitudinal ventilation system with suppression system active. In addition, the impact of the water vapour on the measurement of HRR, which uses oxygen consumption calorimetry, was tested. Results showed that the sensitivity of HRR calculations to water vapour was very small, and the HRR measurement system yielded HRR with no significant error. However, it is necessary to evaluate the system for different conditions with larger water vapour. REFERENCES 1. Grant, G., Brenton, J., and Drysdale, D., "Fire Suppression by Water Sprays", Progress in Energy and Combustion Science, 26, 79-130, 2000. 2. Ministry of Transport, Project Safety Test Report on Fire Tests, Public Works and Water Management of the Netherlands, August 2002. 3. Hejny, H., UPTUN Report work package 2 Fire development and mitigation measures D231 Evaluation of Current Mitigation Technologies in Existing Tunnels, Task 2.3 Final Report, July 2006. 4. Opstad K., Stensaas, J. P., and Brandt, A.W., UPTUN work package 2 Fire development and mitigation measures D241 Development of new innovative technologies October 2006. 5. Ingason, H., Model Scale Tunnel Tests with Water Spray, Fire Safety Journal, 43(7), 512-528, 2008. 6. Ko, Y., Kashef, A., and Hadjisophocleous, G., Modeling of smoke movement during the early stage of tunnel fires under different ventilation conditions. Proceedings of the International Congress of Smoke Control in Buildings and Tunnels, 305-321, Santander, Spain, 16, October, 2008. 7. Ko, Y., Michels, R., and Hadjisophocleous, G., Instrumentation Design for HRR Measurements in a Large-scale Fire Facility. Fire Technology, [Online]. Available: http://dx.doi.org/10.1007/s10694-009-0115 8. Huggett, C., Estimation of the Rate of Heat Release by means of Oxygen Consumption, Fire and Materials, 4, 61-65, 1980. 9. Babrauskas, V., Estimating Large Pool Fire Burning Rate, Fire Technology, 19, 251-261, 1983. 10. Wu, Y., and Bakar, M. Z. A., Control of Smoke Flow in Tunnel Fires using Longitudinal Ventilation systems a Study of the Critical Velocity, Fire Safety Journal, 35, 363 90, 2000. 11. Dlugogorski, B. Z., Mawhinney, J. R. and Duc, V. H., "The Measurement of Heat Release Rates by Oxygen Comsumption Calorimetry in Fires under Suppression," Proceedings of the Fourth International Symposium, International Association for Fire Safety Science, 877-888, June 1994. 12. National Fire Protection Association (NFPA), NFPA 13 Standard for Installation of Sprinkler Systems, NFPA, Quincy, Massachusetts, 2007. 13. Milke, J. A., Hugue, D. E., Hoskins, B. L., and Carroll, J. P., Tenability Analyses in Performance-Based Design Methods for Appraising the Effects of Exposure to Smoke or Heat from a Fire, Fire Protection Engineering, 28, 50-57, 2005. 349