Enclosure fires

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is fi lled with fuel molecules, contains too little oxygen for combustion to be able to take place. The fuel-rich atmosphere is therefore not self-combustible. Combustion takes place, instead, at the periphery, where the fuel and oxygen have diffused into each other. The reaction layer is the place where they are mixed in the right proportion with each other. Fuel and air are very well mixed in this thin layer. Diffusion fl ames are usually yellow, which is due to soot forming. Premixed fl ames do not have the same tendency. It should also be added that there are diffusion fl ames which do not produce so much soot. They are therefore similar to premixed fl ames. Diffusion fl ames differ in that combustion takes place at roughly the same rate as the fuel gas and oxygen from the air diffuse into each other. Diffusion fl ames result from a combustion process where fuel molecules are mixed with oxygen through laminar and/or turbulent mixing. This gives rise to laminar and turbulent diffusion fl ames respectively. Turbulence helps to speed up the mixing process. Laminar diffusion fl ames When a candle burns this produces a typical diffusion fl ame, where fuel and oxygen from the air fl ow side by side with each other at low velocity. They mix together laminarly and combustion occurs evenly in the reaction layer. If diffusion takes place slowly the oxygen and fuel need to be mixed for a long time to be able to burn. This is similar to how fi re can spread in buildings. A combustible mixture of fuel and oxygen can occur and ignite far away from the original fi re source. Turbulent diffusion fl ames The following example is based on a gas burner. If the speed of the fuel is increased the fl ame will gradually change from being laminar to turbulent. When the rate at which the fuel is fl owing out is higher than that at which it is mixing with the oxygen from the air, the mixing process occurs in whirls. This is known as turbulent mixing. In this instance too, mixing with oxygen occurs via diffusion, but the combustion process is uneven and irregular. Even though turbulence causes the All natural fl ames are actually diffusion fl ames, as they are dependent on diffusion. The type of fl ames which fi refi ghters mainly have to tackle are diffusion fl ames. 51
Figure 32. Laminar diffusion fl ame on the left. Turbulent diffusion fl ame on the right. Turbulent diffusion fl ames are the type of fl ames seen in most fi res. 52 combustion rate to increase, it is much lower for turbulent diffusion fl ames than for premixed fl ames. Turbulent fl ames have the following main features: • irregular swirling motion • rapid diffusion • thin, irregular reaction layer Unlike with laminar fl ames, turbulent fl ames are often accompanied by sound and rapid changes in appearance. Turbulence can occur in all gaseous media. Diffusion fl ames occur in the boundary layer between the fuel and air. This means that we cannot use fl ammability limits to describe diffusion fl ames or concepts such as “lean” and “rich”, which are associated with premixed fl ames.
Page 1 and 2: Lars-Göran Bengtsson Enclosure fi
Page 3 and 4: The content of this book may not be
Page 5 and 6: 4.4 Risk assessment 99 Smoke gases
Page 8 and 9: Foreword The aim of this book is to
Page 10: Overview This book assumes that its
Page 13 and 14: Figure 1. Fire growth curve featuri
Page 15 and 16: Fire extinguished/burnt out Just sm
Page 18 and 19: chapter 2 How a fi re starts People
Page 20 and 21: smoke gases in the compartment, but
Page 22 and 23: Pyrolysis gases material is subject
Page 24 and 25: Ignition time Ignition time can als
Page 26 and 27: Smouldering fi res can therefore re
Page 28 and 29: 2.4.1 Thermal inertia kρc The fl a
Page 30 and 31: grow to 50 cm, then a 1 m high fl a
Page 32 and 33: Flammability in solid materials is
Page 34: 6. The fl ame spread rate varies wi
Page 37 and 38: Figure 22. Smoke gases start to acc
Page 39 and 40: Figure 25. The different sections i
Page 41: The production of unburnt gases is
Page 44 and 45: Heat release rate from a fuel surfa
Page 46 and 47: Carbon monoxide (CO) is the next mo
Page 48 and 49: ment to accelerate, but this is obv
Page 50 and 51: is far too high for a premixed fl a
Page 54 and 55: Figure 33. Flames on the under side
Page 56 and 57: Flammability limits Vent For a prem
Page 58 and 59: CH4 + 2 + 2 + + 2O2 = CO2 + 2H2O +
Page 60 and 61: Temperature impact on fl ammability
Page 62 and 63: energy is required, which can be ex
Page 64 and 65: ustion takes place in under-ventila
Page 66 and 67: If turbulence affects the system th
Page 68 and 69: Neutral plane The differences in pr
Page 70 and 71: Where the differences in temperatur
Page 72 and 73: Positive pressure Negative pressure
Page 74 and 75: Dp = 353(1/Ta - 1/Tg)gh Let us take
Page 76 and 77: Figure 51 (top). The pressure condi
Page 78 and 79: Vent just during the early stage of
Page 80 and 81: In Figure 57 calculations have not
Page 82 and 83: 3.4 Summary In many situations the
Page 84: oom and in a fairly closed room (on
Page 87 and 88: Figure 59. The fi re has spread to
Page 89 and 90: Figure 60. Flashover occurrence ove
Page 91 and 92: 90 precisely the exact moment when
Page 93 and 94: The amount of energy released when
Page 95 and 96: Figure 63. Thermal balance on a fue
Page 97 and 98: Figure 64. Flame spread under the c
Page 99 and 100: Access to fuel Figure 67. Fire’s
Page 101 and 102: The signs indicating that a fl asho
Page 103 and 104:
Figure 69. Different signs indicati
Page 105 and 106:
Smoke gas temperature 700 600 500 4
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106 clear that we need to reach the
Page 109 and 110:
This example illustrates how crucia
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110
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112
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114
Page 117 and 118:
Figure 78. Fire pulsating. Figure 7
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Figure 81 (above). The fi re is abo
Page 121 and 122:
Figure 85. The fi re resumes its de
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122 is suffi ciently big. A fi re
Page 125 and 126:
Figure 89. A backdraught entails a
Page 127 and 128:
Figure 90. The picture shows a vent
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128 The current which results in sm
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130 fans should not be used. You sh
Page 133 and 134:
Figure 97. A larger part is premixe
Page 135 and 136:
Figure 101. There are still combust
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Figure 103. Sauna. A common scenari
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Fires in enclosed spaced with minim
Page 141 and 142:
Figure 107. The smoke gases are str
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Figure 110. The fi xed lance versio
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Four fi re scenarios: 1. The fi re
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146 to occur. If a backdraught occu
Page 149 and 150:
148 Fire at 62 Watts Street The New
Page 152 and 153:
chapter 7 Smoke gas explosion Throu
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ignition source required to ignite
Page 156 and 157:
ity is also affected by turbulence
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7.3.2 Course of action If smoke gas
Page 160 and 161:
Test your knowledge! 1. Why do smok
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Corridor Space in between Sauna app
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164 gases the fl ames would come ou
Page 167 and 168:
Figure 119. Plan drawing of the bas
Page 169 and 170:
168 Glossary Adiabatic fl ame If al
Page 171 and 172:
Heat of combustion, This measures t
Page 173 and 174:
172 Suggested solutions to test que
Page 175 and 176:
174 13. In a closed room there will
Page 177 and 178:
176 6. By cooling down the smoke ga
Page 179 and 180:
Sample calculations Flammability li
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You should bear in mind that this c
Page 183 and 184:
Backdraught Sample calculation: The
Page 185 and 186:
E = (T f/T i)( N b/N u) Tf Ti Nb Nu
Page 187 and 188:
v* = v/(g × h × b) 0.5 , which gi
Page 189 and 190:
188 gram), Institutionen för brand
Page 191 and 192:
Index 62 Watts street 148 Adiabatic
Page 193 and 194:
192 List of fi gures Drawn illustra
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Enclosure fires

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