Enclosure fires

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The larger the premixed proportion in the volume of gases when ignition occurs, the faster the process will be. Premixed fl ames and diffusion fl ames have major differences in terms of rates of combustion. In this example, the ignition source is located in the premixed area, which is a prerequisite for ignition. In many real-life cases, the ignition source is very low down in the fi re room, e.g. with smouldering fi res. The ignition source is often the initial fi re, which fl ares up. 2. When the air current is going to leave the room If ignition does not occur until the air current has refl ected off the back wall the premixed area will be very large (see Figure 97). If the gas mixture ignites the fl ames will spread in a more spherical formation and the there will be a larger increase in pressure due to a larger proportion of the gaseous mass being Figure 95. The air current has just reached the back wall. The premixed area is large. Figure 96. This shows the different types of fl ames developing. In the under side a premixed fl ame can be seen and a diffusion fl ame against the back wall. 131
Figure 97. A larger part is premixed when the air current reaches the back wall, almost half the volume of the room. 132 premixed. The smaller the opening in the room, the larger the increase in pressure in the room. Figure 98 shows how the fl ames spread. 32 This expansion will result in a large fi re ball outside the room. The size of the fi re ball depends on the amount of unburnt gases which have accumulated in the room. 3. With a very long delay after the air current has left the room When the incoming air current has mixed with the fuel-rich gases, bounced off the back wall and then left the room, the lower part of the room will be fi lled almost solely with pure air. This means that there may still be combustible gases above the doorframe in the room (see Figure 101). When ignition occurs the fl ames will spread up to the height of the doorframe. When the volume of combustible gases is smaller the heat release rate will not be as large. But it is important to note that in larger, and especially high premises, there may be larger quantities of combustible gases. There are major risks associated with this. The delay may only last a few minutes. Figure 102 shows how the fl ames spread up to the height of the doorframe and how the remaining gases are ejected from the room and burn as a small fi re ball. In this experiment, therefore, the ignition source has been placed high up in the room. If there is a large distance between the door’s upper edge and the ceiling a large quantity of gases can mix there and with serious consequences, even if the environment lower down in the room is fi ne. This has happened a few times in Sweden.
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Lars-Göran Bengtsson Enclosure fi
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The content of this book may not be
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4.4 Risk assessment 99 Smoke gases
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Foreword The aim of this book is to
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Overview This book assumes that its
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Figure 1. Fire growth curve featuri
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Fire extinguished/burnt out Just sm
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chapter 2 How a fi re starts People
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smoke gases in the compartment, but
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Pyrolysis gases material is subject
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Ignition time Ignition time can als
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Smouldering fi res can therefore re
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2.4.1 Thermal inertia kρc The fl a
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grow to 50 cm, then a 1 m high fl a
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Flammability in solid materials is
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6. The fl ame spread rate varies wi
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Figure 22. Smoke gases start to acc
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Figure 25. The different sections i
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The production of unburnt gases is
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Heat release rate from a fuel surfa
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Carbon monoxide (CO) is the next mo
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ment to accelerate, but this is obv
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is far too high for a premixed fl a
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is fi lled with fuel molecules, con
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Figure 33. Flames on the under side
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Flammability limits Vent For a prem
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CH4 + 2 + 2 + + 2O2 = CO2 + 2H2O +
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Temperature impact on fl ammability
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energy is required, which can be ex
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ustion takes place in under-ventila
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If turbulence affects the system th
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Neutral plane The differences in pr
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Where the differences in temperatur
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Positive pressure Negative pressure
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Dp = 353(1/Ta - 1/Tg)gh Let us take
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Figure 51 (top). The pressure condi
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Vent just during the early stage of
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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
Page 107 and 108: 106 clear that we need to reach the
Page 109 and 110: This example illustrates how crucia
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Page 113 and 114: 112
Page 115 and 116: 114
Page 117 and 118: Figure 78. Fire pulsating. Figure 7
Page 119 and 120: Figure 81 (above). The fi re is abo
Page 121 and 122: Figure 85. The fi re resumes its de
Page 123 and 124: 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
Page 129 and 130: 128 The current which results in sm
Page 131: 130 fans should not be used. You sh
Page 135 and 136: Figure 101. There are still combust
Page 137 and 138: Figure 103. Sauna. A common scenari
Page 139 and 140: Fires in enclosed spaced with minim
Page 141 and 142: Figure 107. The smoke gases are str
Page 143 and 144: Figure 110. The fi xed lance versio
Page 145 and 146: Four fi re scenarios: 1. The fi re
Page 147 and 148: 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
Page 154 and 155: ignition source required to ignite
Page 156 and 157: ity is also affected by turbulence
Page 158 and 159: 7.3.2 Course of action If smoke gas
Page 160 and 161: Test your knowledge! 1. Why do smok
Page 162: Corridor Space in between Sauna app
Page 165 and 166: 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
Page 181 and 182: You should bear in mind that this c
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Backdraught Sample calculation: The
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E = (T f/T i)( N b/N u) Tf Ti Nb Nu
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v* = v/(g × h × b) 0.5 , which gi
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188 gram), Institutionen för brand
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Index 62 Watts street 148 Adiabatic
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192 List of fi gures Drawn illustra
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Enclosure fires

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