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TABLE 19 stream of the fire can be obtained from the following general
MASS OPTICAL DENSITY FROM BURNING VEHICLES equation:
Average
Type of Vehicle Mass Optical Density X i ,avg = Yi × M a Mi × Q ( T ) ma × HT (22)
Dmass (m2/kg or ft2/lb)
Car (steel) 381 (1,860)
Car (plastic) 330 (1,610) Assuming ma mg, where mg is the mass flow rate of combus-
Bus 203 (991)
tion gases. Here Ma is the molecular weight of air, Mi is the
Truck 76102 (371498)
Source: Fire in Tunnels (9).
molecular weight of chemical species i, and Yi is the mass
yield of species i for well-ventilated fires. The value of Xi,avg
can be converted into a percentage by multiplying it by 100.
The yields of YCO2, YCO, and YHCN for well-ventilated conditions
downstream of the fire with a ventilation air velocity of u
can be obtained for different fuels.
(m/s) is:
Table 20 presents some values for different fuels for well-
uAH ventilated conditions. A lack of sufficient experimental data
V = 0.87 (21) and test results requires designers to use values from this table.
QDmass
By using this table, the designer is making an assumption by
replacing an actual vehicle fire with pseudo-fuel. Different
In Table 19 values of Dmass for different types of vehicles designers use different fuels and different values to approxi-
are given based on large-scale tests. These values may be used mate the actual fuel, which causes inconsistency in modeling
as an engineering tool for determining the visibility in fires and design results.
depending on the fuel load.
The yield values are the mean values for different material
For CFD modeling, engineers use equations and tables of types (polyurethane foam, polystyrene, mineral oil). However,
yields of CO, CO2, HCN, heat of combustion, production of there is a need to replace the simulated materials with design
soot, and mass OD for different types of materials, such as values for fires involving HGVs, buses, cars, and tankers.
wood, polyurethane foam, polystyrene, and mineral oil. Such Additional testing results are needed.
tables can be found in the SFPE Handbook for Fire Protection
Engineering (51) and other literature. Surprisingly, the vehicles TEMPERATURE OF FIRE GASES
are assumed to be one material, which leads to inconsistency AND TUNNEL WALLS
in the results, as there is no uniform agreement on the numbers
to use and to the inaccuracy of the CFD results. Tunnel fires significantly increase the air temperature in
the tunnel roadway and in the exhaust air duct. Therefore,
The average mole fraction Xi,avg of CO2, CO, or HCN over both the tunnel structure and ventilation equipment are exposed
the cross section of the tunnel and at a certain position down- to high smoke and gas temperature. The air temperatures,
TABLE 20
YIELDS OF CO2, CO, HCN, AND SMOKE AND EFFECTIVE HEAT OF COMBUSTION,
FOR WELL-VENTILATED FIRES
Dmass Hec
YC02 YCO YHCN Ys m2/kg MJ/kg
Type of Material kg/kg kg/kg kg/kg kg/kg (ft2/lb) (Btu/lb)
Wood 1.27 0.004 0.015 37 12.4
(181) (5,331)
Rigid Polyurethane Foam 1.50 0.027 0.01 0.131 304 16.4
(1,480) (7,050)
Polystyrene 2.33 0.06 0.164 335 27
(1,640) (11,610)
Mineral Oil 2.37 0.041 0.097 31.7
(13,630)
Swiss Fire Modeling Assumption on 2.07 0.043 0.01 0.13
Average of Three Materials Above
Source: SFPE Handbook of Fire Protection Engineering (51).
Ys = yield of smoke.
Dmass = mass optical density and is proportional to yield of smoke.
Hec = XHT effective heat of combustion.
Mass loss rate of the fuel, kg/s:
mf = Q(T)/ Hec.
Q(T) = fire heat release rate, HRR (kW).
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TABLE 21
MAXIMUM AIR TEMPERATURES EXPERIENCED AT VENTILATION FANS
DURING MEMORIAL TUNNEL FIRE VENTILATION TEST PROGRAM
Nominal FHRR, Temperature at Central Fans, Temperature at Jet Fans,
a b
MW (MBtu/h ) °C (°F) °C (°F)
20 (68) 107 (225) 232 (450 )
50 (170) 124 (255) 371 (700)
100 (340) 163 (325) 677 (1250)
Source: ASHRAE Handbook (22).
FHRR = Fire heat release rate.
a
Central fans located 700 ft (213 m) from fire site.
b
Jet fans located 170 ft (52 m) downstream of fire site.
shown in Table 21, provide guidance in selecting design expo- in Table 23. This also refers to the need to ensure that equip-
sure temperatures for ventilation equipment. ment does not fall when exposed to a temperature of 450°C
(842°F) for at least 120 min.
British standards provided data on distances over which
jet fans were assumed to be destroyed by the fire; this is Different fire characteristics are needed depending on
reproduced in Table 22. BD 78/99 also specifies that heavy whether the purpose is to design the tunnel structure or the
items, such as fans, subjected to temperatures of 450°C ventilation facilities.
(842°F), are to be designed to not fall down during the fire-
fighting phase (52). · The design of structures for fire resistance is based on
the temperature of the hot air (degrees centigrade or
The French Inter-Ministry Circular (2000) specifies that jet degrees Fahrenheit) and radiation heat versus time.
fans must be capable of operating continuously in smoke- · The design of a ventilation system is based on the HRR
laden air at a temperature of 200°C (392°F) for at least 2 h. (thermal power in megawatts or million British thermal
For transverse ventilation systems, a distinction must be units per hour) or the smoke release rate (flow at the
made on the basis of whether the fans are or are not likely to temperature of the hot smoke in cubic meters per second)
be subjected to very high temperatures. In the general case, versus time.
extraction fans, located at the end of a duct, must be capable
of operating at a temperature of 200°C (392°F) for at least The dependence on time is important for evaluating the
120 min. However, under certain circumstances, it may be conditions at the beginning of the fire, taking into account the
necessary for the fans to be capable of withstanding 400°C self-rescue phase (time for the fire department to arrive and
(752°F) for at least 120 min. Rather than providing informa- get organized).
tion on the distances over which jet fans may be considered
as destroyed, the French guidance provides smoke tempera- PIARC recommends the following maximum temper-
tures at various distances (CETU 2003). This is reproduced atures at the tunnel wall or ceiling to be considered for
TABLE 22
DISTANCES OVER WHICH JET FANS ARE ASSUMED TO BE DESTROYED
BY FIRE (BD 78/99)
Fire Size, Distance Upstream Distance Downstream
MW (MBtu/h) of Fire, m (ft) of Fire, m (ft)
5 (1 7 ) -- --
20 (68) 10 (32.8) 40 (131.2)
50 (171) 20 (65.6) 80 (262.5)
100 (341) 30 (98.4) 120 (393.7)
Source: Hall (52).
TABLE 23
SMOKE TEMPERATURES NEAR THE CEILING, WITH AIRFLOW CLOSE TO CRITICAL VELOCITY
Downstream
Distance 10 m 100 m 200 m 400 m
Light Vehicle 250°C 80°C 40°C 30°C
Heavy Vehicle 700°C 250°C 120°C 60°C
Tanker >1000°C 400°C 200°C 100°C
Source: Hall (52).
