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65 backlayering with a normal hydrogen HRR or keep the back- detailed information concerning safety issues and the behav- layering under control with a high HRR. She concluded ior of these energy carriers where a fire can develop. that with a high HRR the flame inside the tunnel may have encountered oxygen deficiency. This will result in the impinge- Systems, not only components, need to be tested to within ment of hydrogen jet flames on the tunnel ceiling, which would different scenarios and that models be developed for these produce high temperature ceiling flows reaching substantial scenarios. When the scenarios are described in a representa- distances and damage the tunnel infrastructure. The oxygen- tive way, technical safety solutions, mitigations systems, and deficient hydrogen fire also poses a risk of flashover inside rescue service tactics can be developed. It is also important the tunnel and ventilation ducts. to study how the different systems (detection, ventilation, and mitigation) interact, and how the models developed are In early 2004, fire tests of FCVs in the event of low pres- altered depending on the scenario. sures of 20 MPa (2900.8 psi) and high pressures of 35 MPa (5076.3 psi) were conducted in Japan in a simulated full-scale The incidents analyzed show that when there is a fire new tunnel 80 m (262.5 ft) long with a cross-sectional area of 78 m2 energy carriers can explode with catastrophic consequences. (840 ft2). Tests were also performed with the natural gas cars The outcome does, however, vary with different scenarios. (CNG) for comparison. CNG cars and FCVs generated a large It is important to learn from incidents that have occurred, and quantity of heat compared with gasoline cars. The flame of that experiments and relevant research be performed to the CNG cars and FCVs tended to rise faster when compared maximize the understanding of the risks. Such incidents also with gasoline cars. The highest air temperature was reached show that safety systems do malfunction, especially in used at 6 m (19.7 ft) above the roadbed at 319C (606F) for vehicles. Such malfunctions can be the result of accidents, CNG cars, 243C (469F) for FCVs with high pressure, mistakes, conversions, or erroneous repairs, but the conse- 228C (442F) for gasoline cars, and 166C (331F) for quences of such malfunctions are always potentially serious. FCVs with low pressure. The maximum radiation heat for CNG cars was 5125 W/m2 (1625 Btu/hr/ft2); for gasoline cars, The field of new energy carriers is very diverse and con- 4471 W/m2 (1417 Btu/hr/ft2); for FCVs with high pressure, stitutes many different areas of research. This makes a detailed 4141 W/m2 (1313 Btu/hr/ft2); and for FCVs with low pres- review of all aspects of risks associated with new energy sure, 1774 W/m2 (562 Btu/hr/ft2). In all cases, the temperature carriers and safety in tunnels beyond the scope of this study. rose to 1100C (2012F). In the case of FCVs with high pres- On the other hand, this is exactly why this issue is so important. sure the temperature grew rapidly to 1435C (2615F) within When new energy carriers are developed and used in vehicles 290 s. According to an inspection of the concrete above the traveling through tunnels, a variety of different safety aspects fire, damage was limited, with little impact on its compres- converge and need to be dealt with properly and promptly. sion strength. At its conclusion, the CNG and FCV cars Clearly, more research is needed concerning how safety in caught fire rapidly and burned intensely. With air velocities tunnels is affected by the introduction and development of of 2 m/s (394 fpm), stratification was observed; therefore, the tenable environment was maintained at 1.5 m (4.9 ft) new energy carriers. from the roadbed. A concern was raised of possible gas deto- nation if tunnel air velocity reached close to 0. Additional FIRE SMOKE AND SMOKE PRODUCTION-- research and modeling is needed. LITERATURE REVIEW It is difficult to properly evaluate what are the emerging Almost all fires generate smoke. Smoke is a mixture of gases, trends concerning use and what risk scenarios are possible fumes, and particles. The generation of smoke is affected by or most likely with alternative fuel vehicles. This can be, for the following factors: example, a problem for the rescue services, because they will be exposed to incidents involving different types of fuels and Possible reduced supply of oxygen to the fire site, energy carriers. This means that they must have information Heat release, concerning not only the situation itself but also the energy Heat convection, carriers involved. Some tunnels require drivers of vehicles Longitudinal slope, running on CNG or LPG to report this before entering the Type of ventilation, tunnel and to correspondingly label their vehicles. It is impor- Dimensions of the traffic space and possible obstructions, tant that an overall system be developed as the diversity of Thrust caused by any moving vehicles, and vehicles increases. Meteorological influences (wind strength and direction). There are a variety of views on how vehicles running on Smoke mixes with the surrounding air and dilutes in the LPG, CNG, or similar fuels are treated and what safety mea- plume. This process depends on the size of the source of sures are needed. It is important that restrictions are premised fire, fire and air temperature, buoyancy, and height in the on correct information based on additional systematic research plume. With no obstructions and no longitudinal air move- on new energy carriers. It is important to provide correct and ment, the plume of smoke and hot gases rises to the tunnel

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66 ceiling directly above the source of the fire and spreads in assume that there is a correlation between the local tempera- both directions, fire forming a relatively dense smoke layer. ture stratification, the gaseous composition (CO, CO2, O2, etc.), A relatively low-density cold smoke layer sits below the and smoke stratification in tunnels. The temperature stratifi- hot layer. cation is, however, not only related to the air velocity but also to the HRR and the height of the tunnel. These parameters Basically, it can be said that as a result of the heat released can actually be related through the local Froude number (Fr) around the fire site and thermal buoyancy, the smoke is lifted or Richardson number. up to the ceiling near the fire site and spread in the upper area of the tunnel. The smoke continues its flow in one direction Three distinct regions of temperature stratification are when the longitudinal velocity is high (with or without back- defined by the Froude number (Fr) or Richardson number. layering), but in both directions when the longitudinal velocity The first region (region I), when Fr < 0.9, results in severe is low. Thus, there is a limited space above the road surface stratification, in which hot combustion products travel along without any smoke gases, at least for a short period of time. the ceiling. For region I, the gas temperature near the floor Note that this may not be true for small fires with limited heat is essentially ambient. This region consists of buoyancy- dissipation, because the smoke can be relatively cold. dominated temperature stratification. Also, this region is next to the fire location and allows for the evacuation of motorists. A smoke layer may be created in tunnels at the early stages of a fire with essentially no longitudinal ventilation. The second region (region II), when 0.9 < Fr < 10, is However, the smoke layer will gradually descend further dominated by strong interaction between imposed horizontal from the fire. If the tunnel is very long, the smoke layer may flow and buoyancy forces. Although not severely stratified descend to the tunnel surface at a specific distance from the or layered, it involves vertical temperature gradients and is fire depending on the fire size, tunnel type, and the perimeter mixture-controlled. In other words, there is significant inter- and height of the tunnel cross section. When the longitudinal action between the ventilation velocity and the fire-induced ventilation is gradually increased, the stratified layer will buoyancy. gradually dissolve. A backlayering of smoke is created on the upstream side of the fire. Downstream from the fire there is a The third region (region II1), when Fr > 10, has insignificant degree of stratification of the smoke that is governed by the vertical temperature gradients and consequently insignificant heat losses to the surrounding walls and by the turbulent stratification. mixing between the buoyant smoke layers and the normally opposite moving cold layer. The particular dimensionless Because a tunnel can be used by different types of vehicles, group, which determines whether a gas will stratify above such as cars, buses, trucks, and special vehicles, which may another, is the Richardson number (Ri) defined by Eq. 16. have different loads (persons, nonflammable cargo, flammable The Richardson number is similar to the inverse of the Froude cargo, explosives, toxic goods, etc.), it is possible that tunnel number (Fr) defined by Eq. 15; however, the Richardson fires may differ in terms of quantity and quality. In most number is thought of as controlling a mass transfer between cases, car fires are relatively harmless for small tunnel fires layers, whereas the Froude number gives the general shape of with minor temperature and smoke development. However, a layer in an air stream. it is very dangerous when there is a tanker fire with the result- ing high temperatures and enormous smoke production, plus The destratification downstream from the fire is a result of the danger of explosion. Therefore, it is not possible to the mixing process between the cold air stream and the hot describe the temperature and smoke development for every plume flow created by the fire. The phenomenon is 3D in the possible kind of tunnel fire. region close to the fire plume. The gravitational forces tend to suppress the turbulent mixing between the two different The main design parameter is the smoke flow rate produced density flows. by the fire. For the smoke flow rates by fires of passenger cars, buses, and trucks, the PIARC assumptions in the Brussels' It becomes possible for cold unreacted air to bypass or report were confirmed by the EUREKA fire tests. German pass beneath the fire plume without mixing, even though the regulations (RABT from the year 1994) quote smoke produc- flow is turbulent. The longitudinal aspect of the fuel involved tion rates somewhat higher than those of PIARC. in the fire, therefore, may play an important role in the mix- ing process between the longitudinal flow and the fuel vapors CFD calculations made in France by CETU (Centre generated by the fire. d'Etudes des Tunnels) show a decrease in smoke volume flow with increased distance to the fire for HRRs above 60 MW There is a correlation between temperature stratification at (205 MBtu/hr) (49), as shown in Figure 14. a given location and the local mass concentration of chemical compounds. There is also a correlation between local smoke For fires up to 60 MW (205 MBtu/hr), the volume flow OD (or visibility), the local density (or temperature), and the does not depend on this distance. From at least 10 to 120 m oxygen concentration in tunnels. Therefore, it is reasonable to (32.8 to 393.7 ft) from the fire, the smoke cools down; how-

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67 FIGURE 14 Variation of smoke volume flow with (plume flow) distance to fire (CETU)(9). FIGURE 15 Smoke flow rate versus fire heat release rate (9, 50). ever, fresh air is entrained so that the volume flow does about 250 m3/s (8,828.7 ft3/s) at approximately 150 MW not change. For 100150 MW (341512 MBtu/hr) fires, the (512 MBtu/hr), as shown in Figure 15. entrainment of fresh air does not compensate for the very strong reduction of smoke temperature 50 to 100 m (164 to 328 ft) Table 18 presents smoke production rates, CO, and CO2 from the fire. as published in different literature sources (summarized exper- imental results and standards values). To convert the smoke These calculations were performed with no longitudinal masses produced to smoke volumes it is necessary to know ventilation airflow. The smoke flow rate was calculated as the smoke temperatures. The theoretical stoichiometric com- the volume flow of gases that moved away from the fire in the bustion temperatures of regular gasoline are about 2000C upper part of two cross sections located at given distances at (3632F). The real fire temperatures are usually much lower, both sides of the fire. primarily because the combustion is not stoichiometric or because the smoke mixes with air. Also, according to the CFD results, the smoke flow rate varies nearly linearly with the HRR--from about 50 m3/s The dangerous nature of smoke gases in tunnel facilities not (1,765.7 ft3/s) at approximately 10 MW (34 MBtu/hr) to only results from the visibility obscuring effect but also from TABLE 18 SMOKE, CO2 AND CO PRODUCTION Smoke Flow [m3/s (ft/s)] CO2 Production (EUREKA CO Burning PIARC RABT EUREKA tests) Production Vehicle (1987) (1994) Tests CETU (1996) [kg/s (lb/s)] [kg/s (lb/s)] Passenger Car 20 (706) 2040 -- 20 (706) -- -- (706 1,412) Passenger Van -- 30 30 (1,060) 0.40.9 0.0200.046 (plastic) (1,059.4) (0.882) (0.040.1) 23 Passenger -- -- 30 (1,060) -- -- Cars 1 van -- -- 50 (1,765) -- -- Bus/Truck 60 (2,120) 6090 5060 80 (2,825) 1.52.5 0.0770.128 Without (2,120 (1,765 (3.35.5) (0.170.28) Dangerous 3,180) 2,120) Goods Heavy Goods -- -- 5080 6.014.0 0.3060.714 Vehicle (1,765 (13.230.9) (0.671.57) 2,825) Gasoline 100200 150300 -- 300 (10,600) -- -- Tanker (3,531 (5,300 7,063) 10,600) Sources: Fire in Tunnels (9) and PIARC (21).

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68 the possible toxicity of gases including CO, carbon dioxide Approximately 1.3 for the plastic passenger van fire, (CO2), and other gases depending on the burning materials, Approximately 0.5 for the bus fire, and especially toxicity caused by cargo. To address these con- Approximately 0.8 for the HGV fire. cerns, during the EUREKA and Runehamar fire tests, the CO and CO2 levels were monitored at several measuring points Another approach is based on the mass OD. Visibility along the tunnel. depends on: During the EUREKA fire tests, the CO level was monitored Smoke density, at several measuring points along the tunnel. In the region Tunnel lighting, from approximately 20 to 30 m (65.6 to 98.4 ft) downstream Shape and color of objects and signs, of the burning vehicles, the following peak CO concentrations Light absorption of smoke, and were measured at head height: Toxicity of smoke (eyes irritating). Passenger van (plastic): 300 ppm The visibility in smoke can be related to the extinction Public bus: 2,900 ppm coefficient, K, by the following equation: HGV: 6,500 ppm. ln (10 ) OD CO concentrations of more than 500 ppm were exceeded K= (19) X from about 10 to 15 min from the start of the fire and lasted approximately 2 h during the bus fire and approximately 15 min during the HGV fire. During an experiment with a mixed fire where: load, CO concentrations of 500 ppm and more occurred not before about 80 min after the start of the fire and lasted for OD is the optical density, and 90 min. X is the path length of light through smoke. The EUREKA results depend very much on the different The optical density per unit optical path length can also be ventilations of the test tunnel during the fire tests. Furthermore, expressed as: they are related to the type of burning material. Therefore, the EUREKA results may not be transferred directly to other OD Q tunnels. However, the EUREKA results indicated that down- = g Ys g m f VT = Dmass (20) X uAH stream of the fires there is, at least for larger fires, a need for escape and rescue within about 10 to 15 min from the start of where: a fire. Harmful CO concentrations are also expected in the progressive stage of vehicle fires. The mass generation of CO2 is the specific extinction coefficient of smoke or particle can be estimated using a ratio of 0.1 kg/s per MW of HRR. OD (m2/kg); A reasonable linear correlation between the production rates Ys is the yield of smoke (g/g); of CO2 and CO was found when analyzing the EUREKA test mf denotes the mass flow of material vapors of the burning data. These results suggest an average ratio of 0.051, with a material; standard deviation of 0.015. This average is used for the VT is the total local volumetric flow rate of the mixture of calculation of the CO production rates. As an order of mag- fire products at the actual location (measuring point) nitude, the volume concentration of CO is also approximately and air (m3/s); 5% of the concentration of CO2. Ys is defined as mass OD, Dmass (m2/g); Q is the HRR in kW at the actual location and H is the The correlation of the smoke-dependent visibility mea- effective heat of combustion (kJ/kg) obtained from the sured by the OD and the concentration of CO2 produces a lin- tables for different materials (but not of the burning ear relation when a correction for the smoke gas temperature vehicle); and is made. The following formula can be used to estimate the u (m/s) is a unified longitudinal ventilation velocity across OD from the CO2 volume concentration: the tunnel cross section A (m2). OD = g ( T0 T ) g CO 2 (18) For objects such as walls, floors, and doors in an under- ground arcade or long corridor the relation between visi- where: bility and the extinction coefficient was defined earlier by Eq. 13. T is the local temperature in Kelvin, T0 = 273 K; [CO2] is the concentration in percent of volume; and Thus, by combining the equations, a correlation between is a coefficient which is: the visibility V and the HRR in a tunnel at an actual position