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76 After the hot phase, it takes time for the fire to decay if it Definition of the tunnel system; is not extinguished. The German ZTV Tunnel assumes that Identification of fire hazards; it can take about 110 min of linear temperature decaying. The Selection and definition of fire scenarios; EUREKA tests confirmed the duration of fires, but show a Identification of methods of analysis; and steeper decline of temperatures just after the hot phase. On Identification of design options. the other hand, the Nihonzaka fire lasted for four days. Quantitative analysis of design using the appropriate subsystems: Fire ignition, development of heat and smoke; SUMMARY Spread of fire, heat, and smoke; The assessment of fire safety in tunnels is a complex issue, Structural response to the fire; where broad multi-disciplinary knowledge and application of Detection, activation, and suppression; and different physical models are necessary to explore the causes Behavior of tunnel users and the influence of fire on and development of fires to evaluate measures to prevent and life safety. reduce its consequences. The systems to take into account Assessment of the outcome of the analysis and evaluation comprise: against criteria. The occurrence and physics of fire development. The acceptance criteria, which establish the adequacy The tunnel systems; that is, of the design, can be developed according to the following Infrastructure and approaches: Operation. Human behavior of users, operators, emergency services. Deterministic (including, when appropriate, safety Other factors influencing safety. factors). Probabilistic (risk-based used in European countries). The first priority for fire design of all tunnels is to ensure: Comparative (comparison of performance with accepted codes of practice). Prevention of critical events that may endanger human life, the environment, and the tunnel structure and A design fire is an idealization of a real fire that might installations. occur. A design fire scenario is the interaction of the design Self-rescue of people present in the tunnel at the time of fire with its environment, which includes the impact of the the fire. fire on the geometrical features of the tunnel, the ventilation Effective action by the rescue forces. and other fire safety systems in the tunnel, occupants, and Protection of the environment. other factors. Limitation of the material and structural damage. Nobody can precisely predict every fire scenario given the Fire prevention measures reduce either the probability or range of variables and people behavior. Therefore, the designer the consequences of an incident in a tunnel. They are related to: makes a number of assumptions to make sure that the design will save lives and retain structural integrity under most of Tunnel design and maintenance; the foreseeable fire scenarios. Traffic and vehicles; and Notification, communication, operator, and rescue For design purposes, it is necessary to choose fire charac- services. teristics corresponding to the traffic that uses a particular Mitigation measures are conceived to limit the conse- tunnel. Conditions, such as the allowance of transporting quences once the ignition has taken place and developed into hazardous vehicles and materials, have to be taken into account. a fire. The mitigation measures may be related to: Design fires in tunnels are usually given as the peak fire Reduction of the fire development, HRR. There is no common ground on how to calculate the Reduction of the consequences to humans, and HRR. One possibility is weighting of the burning components Reduction of the consequences to structure and of a vehicle, the other is the analytical method. Some calcu- equipment. lations incorporate burning efficiency, which means that the fire may not consume the entire heat load available. The left- The fire safety engineering will normally involve the following over content is typically in the form of either a char residue steps: or as soot and smoke particles displaced by the combustion gas stream. Qualitative design review: Definition of objectives and safety criteria, with The magnitude and development of fire depends on: reference to performance-based standard require- ments and coordination with the authorities having Vehicle combustion load (often called the fuel load, jurisdictions; which is usually greater than the potential fire size),

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77 Source of ignition, Quadratic growth curves are defined in the NFPA standards. Intensity of ignition source, They can be categorized as: Distribution of fuel load in the vehicle, Fire propagation rate, Ultrafast growth rate Tunnel and its environment (including available oxygen), Fast growth rate and Medium growth rate Other factors discussed in the next chapters. Slow growth rate. Studies showed that the fire growth rate is more important The ultrafast fire growth curve with the fire growth coeffi- than the peak HRR when investigating the safety of people in cient of 0.178 kW/s2 meets most of the Runehamar Tunnel the tunnel. fire tests. Fire duration can be determined by the amount of avail- If the fire remains isolated to the first item ignited, the fire able combustible material. The amount of fuel is different for will likely become fuel-controlled and decay. However, if each study based on the type of vehicles, loads, and traffic the fire spreads to other combustibles, this can lead to the patterns. The duration of the hot phase of a fire normally onset of rapid transition from a localized fire to the combustion covers a time interval of about 30 to 60 min after ignition of all exposed surfaces within the vehicle. This phenomenon stage, unless there are unusual circumstances such as a big is referred to as flashover, which is a sudden transition from pool fire caused by a gasoline where a hot phase of about 2 h localized to generalized burning, where all of the items inside is considered. a vehicle or compartment ignite. Usually this phenomenon occurs during a short period and results in rapid increase The specification of a design fire may include the following of HRR, gas temperatures, and production of combustion phases: products. The largest HRRs are expected just after flashover occurs (post-flashover) and are often the basis for tunnel smoke 1. Incipient Phase--characterized by the initiating source, control system designs. During this period the HRR is driven such as a smoldering or flaming fire. by the oxygen flow and the fire is therefore often considered 2. Growth Phase--the period of propagation spread, to be "ventilation controlled." potentially leading to flashover or full fuel involvement. 3. Fully Developed Phase--the nominally steady venti- The key characteristic of fully developed fires is a signifi- lation or fuel-controlled burning. cant steady-burning phase. Fully developed fires may refer to 4. Decay Phase--the period of declining fire severity. either fuel- or ventilation-limited fires. 5. Extinction Phase--the point at which no more heat energy is being released. Usual tunnel fires are fuel-controlled fires; however, in a severe fire with multiple vehicles involved, the fire can be a A smoldering fire is caused by a combination of the fol- ventilation-controlled (oxygen-limited) fire. There are a lowing (input) parameters: limited number of studies found in the literature on fire spread between vehicles in tunnels. 1. Nature of fuel 2. Limitation of ventilation Almost all fires generate smoke. Smoke is a mixture of 3. Strength of the ignition source. gases, fumes, and particles. Its generation is affected by the following factors: The principal hazards posed by a smoldering fire are high concentrations of CO and low visibility conditions. Possible reduced supply of oxygen to the fire site Heat release Pre-flashover fires include the incipient and growth phases, Heat convection and are of primary interest in life safety analyses. The growth Longitudinal slope of a fire is dependent on fuel and the availability of oxygen Type of ventilation for combustion. Typically, as the fire grows in size, the rate Dimensions of the traffic space and possible obstructions of growth accelerates. The rate of fire growth may be modified Thrust caused by any moving vehicles owing to compartment effects, radiative feedback, activation Meteorological influences (wind strength and direction). of sprinklers or the application of other suppressants, avail- ability of fuel, and the availability of oxygen, among other The main design parameter is the smoke flow rate produced factors. It is important to recognize that the total fuel load has by the fire. The smoke flow rate varies nearly linearily with little bearing on the rate of fire growth; however, the rate of the HRR--from about 50 m3/s (1,765.7 ft3/s) at approximately fire growth is governed by the HRR of the individual fuel 10 MW (34 MBtu/hr), to about 250 m3/s (8,828.7 ft3/s) at items burning. approximately 150 MW (512 MBtu/hr).

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78 Smoke reduces visibility in tunnels. Visibility depends on: do represent a new situation and imply new risks. These risks need to be evaluated and considered. The incidents analyzed Smoke density, show that new energy carriers can lead to explosions with Tunnel lighting, catastrophic consequences when there is a fire, although it Shape and color of objects and signs, does not mean that all vehicles running on new energy carriers Light absorption of smoke, and will explode when used or when exposed to fire. However, Toxicity of smoke (eyes irritating). seeking the worst case scenarios is important when new energy carriers are developed. It is also important to realize that all The dangerous nature of smoke gases in tunnel facilities risks are not eliminated by introducing PRDs. Safety systems not only results from the visibility obscuring effect but also do malfunction, especially in used vehicles. The outcome still from possible toxicity of gases including CO, CO2, and other depends on the design of these devices and on the fire scenario. gases depending on the burning materials, especially toxicity caused by cargo. The mass generation of CO2 can be estimated Hydrogen can be used either for ICE vehicles or fuel cell using a ratio of 0.1 kg/s per MW of HRR. A reasonable linear vehicles. Hydrogen cars generate fast, high rising flames that correlation between the production rates of CO2 and CO was reach high temperatures and can lead to higher temperature found at an average ratio of 0.051 with a standard deviation ceiling flows and damage to tunnel structures. The oxygen- of 0.015. The correlation of the smoke-dependent visibility deficient hydrogen fire also poses the risk of flashover inside measured by the OD and the concentration of CO2 produces the tunnel and ventilation ducts. As a result of the nature of a linear relation when a correction for the smoke gas temper- flame/fire development, tunnels with greater slopes and with ature is made. horseshoe cross sections (compared with equivalent rectan- gular cross sections) present lower hazards. The aim of the A lack of sufficient experimental data and test results HyTunnel European on-going project is to develop codes, requires designers to make an assumption by replacing an standards, and regulations so that additional risks owing to actual vehicle fire with a pseudo-fuel. Different designers use the introduction of hydrogen vehicles into tunnels can be different fuels and different values to approximate the actual handled safely. fuel, which causes inconsistency in modeling and design results. There is a need to use actual (or mutually agreed upon) Electric cars that use batteries as an energy source are seen design values for fires involving HGVs, buses, cars, and as the single most promising future energy carrier, in partic- tankers. Additional testing results are needed. ular, for city traffic. Some countries have restrictions on the use of some energy carriers in confined spaces. Many of the Tunnel fires significantly increase the air temperature in restrictions can be related to LPG, which is also considered the tunnel roadway and in the exhaust air duct. Therefore, to be as an alternative fuel, together with LNG, CNG, hydro- both the tunnel structure and ventilation equipment are exposed gen, propane, methanol, ethanol, and biodiesel in accordance to the high smoke and gas temperatures. Different fire char- with the U.S. Energy Policy Act of 2005. LPG and CNG acteristics are needed depending on whether the purpose is to vehicles are restricted in tunnels in New York, New Jersey, design the tunnel structure or the ventilation facilities. Massachusetts, Maryland, and Virginia, as well as in Italy, France, and Austria. However, there are no restrictions on The design of structures for fire resistance is based on LPG vehicles in tunnels in Japan and many other countries. the temperature of the hot air (centigrade or Fahrenheit) and radiation heat versus time. The issue of new energy carriers is very diverse and con- The design of ventilation is based on the HRR (thermal stitutes many different fields of research. There are a variety power in megawatts or million British thermal units per of views on how vehicles running on LPG, CNG, or similar hour) or the smoke release rate (flow at the temperature fuels are treated and what safety measures are needed. It is of the hot smoke in cubic meters per second) versus time. important that restrictions are based on correct information that is based on additional systematic research on new energy The dependence on time is very important for evaluating carriers. It is also important to provide correct and detailed the conditions at the beginning of the fire, taking into account information concerning safety issues and the behavior of these the self-rescue phase (time for the fire department to arrive energy carriers where a fire can develop. Unless this occurs and get organized). Memorial Tunnel Fire Tests, EUREKA in a timely manner, there is a risk that decisions will be based tests, and Runehamar Tunnel fire tests provided ample data on too little or erroneous information. The concern was raised that allow for the estimating of a maximum temperature of possible gas detonation if tunnel air velocity is close to 0. experienced by the tunnel ventilation equipment and by the Additional research and numerical modeling is needed to tunnel structure. address the risk posed by alternative fuel carriers and structural protection against their fires or explosions. The risk to humans New energy carriers or vehicles transporting fuel for new from explosions and from oxygen displacement may also be energy carriers do not necessarily mean higher risks, but they critical and needs to be studied.