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tunnel structure and the cargo-traffic regulations for specific
tunnels:
· Passenger car--400°C (752°F)*
· Bus/small truck--700°C (1292°F)*
· HGV with burning goods (not gasoline or other danger-
ous goods)--1000°C (1832°F)
· Gasoline tanker (general case)--1200°C (2192°F)
· 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)--
1400°C (2552°F).
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 500°C (932°F), depending on type] and the coach interval of about 30 min after the ignition stage. On the other
[800°C (1472°F)] 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 1000°C and 1350°C
(1832°F and 2462°F) at the tunnel ceiling. For fully devel- FIRE DEVELOPMENT BASED
oped fires of gasoline tankers, temperatures between 1200°C ON LITERATURE REVIEW
and 1400°C (2192°F and 2552°F) 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 300°C (572°F), 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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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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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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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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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 1,200°C (2,192°F) 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). (2,462°F).
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).
