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71 tunnel structure and the cargo-traffic regulations for specific tunnels: Passenger car--400C (752F)* Bus/small truck--700C (1292F)* HGV with burning goods (not gasoline or other danger- ous goods)--1000C (1832F) Gasoline tanker (general case)--1200C (2192F) Gasoline tanker (extreme cases: e.g., no benefits owing to tunnel drainage and limited leakage rate; large tanker; avoidance of the flooding of an immersed tunnel)-- 1400C (2552F). FIGURE 16 Maximum gas temperatures These temperatures were estimated for a location 10 m in the ceiling area of the tunnel during tests (32.8 ft) downwind of the fire near the tunnel walls at the with road vehicles (21). minimum air velocity to prevent backlayering. The EUREKA tests confirmed these maximum temperatures. The tests them- selves gave slightly higher results for the passenger cars EUREKA and Runehamar fires covered normally a time [up to 500C (932F), depending on type] and the coach interval of about 30 min after the ignition stage. On the other [800C (1472F)] because of the small cross-sectional area hand, the Mont Blanc and Nihonzaka fires lasted significantly and the low air velocity used [0.3 m/s and 0.5 m/s (59.1 and longer. The EUREKA and Runehamar tests showed a steep 98.4 fpm)] in the test tunnel. The fire tests of EUREKA and decline of temperatures just after the hot phase. Runehamar also showed that fires resulting from HGVs can produce maximum temperatures between 1000C and 1350C (1832F and 2462F) at the tunnel ceiling. For fully devel- FIRE DEVELOPMENT BASED oped fires of gasoline tankers, temperatures between 1200C ON LITERATURE REVIEW and 1400C (2192F and 2552F) are studied. Combustible materials in a vehicle or tunnel are set on fire by an external ignition source. Energy is released and part of the As can be seen by Figure 16 in the EUREKA tests, temper- solid matter of the fire material is converted into gases being atures of more than 300C (572F), which can be dangerous to the steel reinforcement of the concrete tunnel lining, were part of the smoke. These gases mix with ambient tunnel air. found as far as approximately 100 m (328 ft) downstream of The constant release of energy greatly heats up the mixture of the fire. In addition, because of backlayering, this tempera- combustion gases and air, forcing it upwards, the phenomena ture can be reached about 30 m (98.4 ft) upstream of the fire. of buoyancy effect. There is also direct radiation from the According to actual fires and to the Memorial Tunnel tests, flames. The heated gasair mixture comes into contact with the extension of this region can be quite different from these the ceiling and walls. The mixture conveys part of its heat to values owning to many factors, such as the ventilation, tunnel surrounding surfaces through radiation and thermal conduc- grade, surface roughness, and fire-resistant coatings. tion and continues spreading it through the tunnel as smoke, with the temperature progressively declining as it moves Many known real tunnel fires and also the EUREKA and away from the fire source. The thickness of the smoke and Runehamar fires showed a very fast development during the its concentration are reduced as it mixes with the tunnel air. first 5 to 10 (sometimes 15) min. The gradient of temperature The ability to escape the smoky environment depends on the is especially steep at the beginning of a full car fire, with a smoke's concentration and the height of the smoke layer corresponding high emission of heat and smoke. Between 7 above the roadbed. and 10 min after ignition a flashover needs to be taken into account (even sooner in the case of a passenger car). The combustion process efficiency depends on sufficient oxygen availability. The air stream caused by the fire often The temperature during the Runehamar fires followed the creates a suction effect that assures oxygen supply from Rijkswaterstaat (RWS) curve. That test comprises the largest the portals or shafts. This results in continuous feeding of the amount of combustible material of the four tests conducted. fire with oxygen, which allows for continuous heating of the fire materials, and possible re-ignition. The process continues With the lowest calorific energy output the temperatures until either the combustible material is completely burned or were recorded to be in the same magnitude, although for a the fire extinguishing measures interrupt the burning process. shorter period of time. The duration of the hot phases of the The growth and development of a fire will be influenced in its early stages by the ignition scenario and the fire performance *Higher temperature if flames touch the walls. of the materials. Fires can start developing inside vehicles or

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72 2. Growth Phase, the period of propagation spread poten- tially leading to flashover or full fuel involvement. 3. Fully Developed Phase, the nominally steady ventilation or fuel-controlled burning. 4. Decay Phase, the period of declining fire severity. 5. Extinction Phase, the point at which no more heat energy is being released. Figure 18 represents all phases of fire development. A smoldering fire is caused by a combination of the fol- FIGURE 17 Maximum gas temperatures in the cross section lowing (input) parameters: of the tunnel during tests with road vehicles (21). 1. Nature of the fuel, 2. Limitation of ventilation, and outside in the cargo container. As fires develop, heat builds 3. Strength of the ignition source. up leading to elevated gas temperatures within the enclosure. The elevated temperatures will in turn have a significant impact The smoldering fire generally burns over a long period in on the growth rate of the fire. Elevated gas temperatures will limited ventilation conditions with insufficient oxygen to pre-heat materials that have not been ignited and potentially fully burn the fuel. It produces relatively low levels of heat, accelerate flame spread. Gas temperatures in an enclosure can but considerable unburned combustibles and a higher con- be affected by the size of the enclosure, the ventilation into centration of smoke. This creates a relatively low visibility the enclosure, and the FHRR (see Figure 17). with large toxic products of combustion such as CO and soot (e.g., the burning of rubber tires of those vehicles involved in Development of fires inside vehicles is dependent on a the fire). The relatively low temperatures generated create less number of factors including: (1) the fire performance of buoyancy in the combustion products, and thus decreases the interior materials and features, (2) the fire performance of likelihood of smoke stratification under the tunnel roof as vehicle cargo, (3) the size and location of the initiating fire with hotter fires. Therefore, the principal hazards posed by a event or ignition scenario, (4) the size of the enclosure where smoldering fire are high concentrations of CO and low visi- the fire is located, and (5) the ventilation into the enclosure. bility conditions. The construction and combustible contents of a vehicle, such as electrical fault or overheating parts in the The specification of a design fire may include the following engine compartment, could be a potential source of a smol- phases: dering fire in tunnels. 1. Incipient Phase, characterized by the initiating source Pre-flashover fires include the incipient and growth phases such as smoldering or flaming fire. and are of primary interest in life safety analyses. The growth FIGURE 18 Simplified phases of fire development. Note: Sprinkler activation is shown as a representative example and its impact on fire development depends on its activation time and sprinkler system characteristics discussed later (9).

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73 FIGURE 19 An example of fire development curves linear, quadratic, and exponential proposed as the result of UPTUN tests (28). of a fire is dependent on fuel and the availability of oxygen Max HRR: Q(t) = Qmax at t = t1, where t1 equals time when fire for combustion. Typically, as the fire grows in size, the rate reaches its maximum HRR of growth accelerates. The rate of fire growth may be modified owing to compartment effects, radiative feedback, activation Decay phase: Q ( t ) = Qmax e - b( t - t1 ) - t > t1 of sprinklers or the application of other suppressants, avail- ability of fuel, and the availability of oxygen, among other The quadratic growth curve is defined in the NFPA stan- factors. It is important to recognize that the total fuel load has dards such as NFPA 204; they differ with: little bearing on the rate of fire growth; however, the rate of fire growth is governed by the HRR of the individual fuel Ultrafast growth rate items burning. Fast growth rate Medium growth rate There are numerous methods available to mathematically Slow growth rate. represent a design fire curve in tunnels. These include different types of fire growth rates; for example, linear growth (a t), Figure 20 represents different fire growth quadratic growth quadratic growth (a t2), or exponential growth (see Figure 19). curves. An exponential or power-law is often used for characterizing the transient growth of the HRR. The most common is the The ultrafast fire growth curve with the fire growth coef- "t 2 fire," where the HRR increases with the square of the time. ficient of 0.178 kW/s2 meets most of the Runehamar Tunnel fire These growth and decay functions can be combined with tests. An example of a design fire curve is shown in Figure 21. the maximum fire HRR to obtain the fire curve. No allowance in Figure 21 was made for the possible spread of fire between vehicles, nor for the possible effects of under- Fire growth phase: Q ( t ) = at 2 for 0 < t < t1; ventilation on HRR development. If necessary, these effects FIGURE 20 Quadratic fire growth curves based on NFPA 204 (2007).

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74 which is directly proportional to the fuel mass loss rate, mf (kg/s), can then be calculated using the following equation: Q = m f XHT (24) where: HT is the net heat of complete combustion (kJ/kg), and X is the ratio of the effective heat of combustion to net heat of complete combustion. FIGURE 21 Example of design fire with curve decaying phase. If the air-to-fuel mass ratio is less than the stoichiometric value, then the fire is defined as ventilation-controlled and must be investigated separately. There are a limited number the HRR, Q, is directly proportional to the mass flow rate of of studies found in the literature on fire spread between vehicles air (i.e., proportional to the oxygen supply) available for com- in tunnels. bustion. The following equation assumes complete combus- tion and that all the air, ma, is consumed: If the fire remains isolated to the first item ignited, it will likely become fuel-controlled and decay. However, if the fire Q = ma H T r (25) spreads to other combustibles, this can lead to the onset of rapid transition from a localized fire to the combustion of Where r is the stoichiometric coefficient for complete all exposed surfaces within the vehicle. This phenomenon is combustion. referred to as flashover, which is a sudden transition from localized to generalized burning. A good indication of when a fire has become ventilation- controlled is when the ratio mco/mco2 begins to increase con- The key characteristic of a fully developed fire is a signifi- siderably where mco is the mass flow rate of CO and mco2 is cant steady-burning phase. Fully developed fires may refer to the mass flow rate of carbon dioxide (CO2). Tests show that either fuel- or ventilation-limited fires. The transition from the ratio mco/mco2 increases exponentially as the fire becomes fuel- to ventilation-controlled burning occurs when ventilation-controlled for diffusion flames of propane, propy- lene, and wood crib fires. m f = mox s (23) Fires that grow sufficiently large can reach flashover, where all of the items inside a vehicle or compartment ignite. Usually where: this phenomenon occurs during a short period and results in a rapid increase of HRR, gas temperatures, and production of mf and mox refer to the mass fraction of fuel and oxidant, combustion products. The largest HRRs are expected just respectively, and after flashover occurs (post-flashover) and are often the basis s refers to the stoichiometric oxidant to fuel ratio (8). for tunnel smoke control system designs. During this period, the HRR is driven by the oxygen flow and the fire is therefore The air-to-fuel equivalence ratio can be used to determine often considered to be "ventilation controlled." However, the whether a fire is ventilation-controlled or fuel-controlled. HRR history of a vehicle fire ought to include HRR informa- tion during all stages of the fire: the ignition or incipient phase, Usual tunnel fires are fuel-controlled fires; however, in a the growth phase, potentially the post-flashover phase, and severe fire such as the Mont Blanc fire, with multiple vehicles the decay phase. involved, the fire was a ventilation-controlled (oxygen-limited) fire. If the base of the fire source is completely surrounded by Before undertaking any fire scenario analysis, it is essen- vitiated air it may self-extinguish. The vitiated air, which is a tial that the fundamental aspects of fire science and fire safety mixture of air and combustion products, is usually composed engineering, and limitations of the mathematical models used of about 13% oxygen when the fire self-extinguishes such for hazard analysis are clearly understood. that the flammability limits were exceeded. However, this value can be to some extent temperature-dependent. Increasing Design fires, which are the basis of the design fire scenario temperature tends to lower the flammability limits. analysis, are described in terms of variables used for the quan- titative analysis. These variables typically include the HRR If the air-to-fuel mass ratio is greater than or equal to of the fire, yield of toxic species, and soot as functions of the stoichiometric value, then the fire is assumed to be fuel- time. When the mathematical models are not able to predict controlled and the HRR is directly proportional to the fuel mass the growth of the fire and it spreads to other objects within loss rate. This can be exemplified by stating that the oxygen the tunnel traffic or any other part of the tunnel, such growth concentration in the gases flowing out of the compartment or and spread needs to be specified by the analysis as part of the the tunnel exit is greater than zero. The chemical HRR, Q (kW), design fire or determined on experimental basis. A design fire

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75 scenario would typically define the ignition source and process, the growth of fire, the subsequent possible spread of fire, the interaction of the fire with its enclosure and environment, and its eventual decay and extinction. 1. Input characteristics Each design fire scenario is represented by a unique occurrence of events and is the result of a particular set of circumstances associated with active and passive fire protection measures. Accordingly, a design fire scenario represents a particular combination of events associated with factors such as: a. Type, size, and location of ignition source; b. Type of fuel; c. Fuel load density, fuel arrangement; d. Type of fire; e. Fire growth rate; FIGURE 22 Fire scenario recommendation, UPTUN WP2 f. Fire's peak HRR; proposal by Ingason (28). g. Tunnel ventilation system; h. External environment conditions; i. Fire suppression; Many known actual tunnel fires and fire curves show a very j. Human intervention(s); and fast development during the first 5 to 10 (sometimes 15) min. k. Tunnel geometry. The gradient of temperature is rather steep and the emission of heat and smoke are important. Therefore, several tem- Design fires are further characterized in terms of the fol- perature curves were presented that more closely correlate lowing variables as functions of time: to important phases of a tunnel fire. NFPA 502 recognized the RWS curve. The standard reference curve for tunnel a. Fire characteristics (flame length, air velocity, radiation, fires (the Rijkswaterstaat temperaturetime curve) indicates convection, temperatures). temperatures exceeding 1,200C (2,192F) for a period of b. Critical velocity to prevent backlayering (only relevant about 100 min and a maximum temperature of 1,350 C in longitudinal ventilated tunnels). (2,462F). c. Toxic species (smoke) production rate. d. Time to key events such as fire spread from one vehicle The duration of the hot phase of a fire normally covers a to the next. time interval of about 30 to 60 min after ignition stage, unless there are unusual circumstances such as a big pool fire caused Alternatively, design fires can be characterized by thermal by a gasoline tanker or a situation similar to the Mont Blanc fire. actions on the tunnel structure and equipment, as well as in For a big gasoline tanker, the Dutch regulations indicate a hot terms of timetemperature curves that depend on the emissivity phase of about 2 h. If fire trucks arrive on the scene quickly of the fire, surface temperature, and emissivity of the walls. (within minutes) and deal with the fire effectively, the duration Table 24 and Figure 22 show the application of different design of the hot phase will be shorter. However, it is realized that fire curves developed as the result of the UPTUN project. access to such a fire will be difficult. TABLE 24 FIRE SCENARIO RECOMMENDATION, UPTUN WP2 PROPOSAL BY INGASON HRR MW Road, Examples Vehicles At the Fire Boundary 5 12 cars ISO 834 10 Small van, 23 cars ISO 834 20 Big van, public bus, multiple vehicles ISO 834 30 Bus, empty HGV ISO 834 Risk to Life 50 Combustibles load on truck ISO 834 Risk to Construction 70 HGV load with combustibles (approx. 4 tons) HC 100 HGV (average) HC 150 Loaded with easy comb. HGV (approx. 10 tons) RWS 200 or Limited by oxygen, petrol tanker, multiple HGVs RWS higher Source: Ingason (28).