Enclosure fires

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Diffusion fl ame A diffusion fl ame occurs when the fuel and air are not mixed with each other at the moment of ignition. Fuel and air diffuse into each other, creating a combustible area in the boundary layer between them. Candles are an everyday example of this. Early stage of fi re This is the period from when the fi re starts until a fl ashover development occurs. During this period the fi re can spread from the initial object and people who are in the building are at major risk of getting injured. Expansion factor When a gas mixture ignites, the temperature can rise by a factor of 8. This causes the volume to expand by the same magnitude. Explosion An exothermic chemical process which, when it happens at a constant volume, generates a sudden, signifi cant increase in pressure. Flammability range This is the range within which gas/air mixtures can ignite. Flashover A fl ashover is a transition period from when the fi re is burning locally until the whole room is involved in the fi re. A fl ashover occurs when the fi re’s heat release rate exceeds a particular critical level. Factors contributing to the increase in the fi re’s heat release rate include fl ame spread over combustible surfaces and reradiation from the hot smoke gas layer. A fl ashover marks the transition from the early fi re development stage to a fully developed compartment fi re. Fuel control After ignition and at the start of a fi re’s development, the fi re is described as fuel controlled as there is suffi cient air for combustion and the fi re’s development is controlled entirely by the fuel’s properties and arrangement. A fi re can also be fuel controlled at a later stage in its development. Fully developed This stage is reached when a fl ashover occurs. In this compartment fi re instance, the fi re is ventilation controlled and it is usual for smoke gas temperatures to reach the order of 800–900 °C. Flames spread via the building’s openings. This means that part of the combustion process takes place outside the compartment. Heat release rate When a material combusts heat will be released. The heat released is measured in watts (J/s). 169
Heat of combustion, This measures the amount of energy the material releases DH c when it burns. The unit used is MJ/kg or kJ/g. Heat of vaporisation, The heat of vaporisation is the amount of heat required to DH v (sometimes L v) produce 1 g gas from the fuel surface. 170 Laminar fl ow There are two different types of fl ow. This is evident when, for instance, smoke fl ows up and out of a narrow chimney. First of all, the smoke moves in a parallel layer without mixing. But at a certain distance from the outlet the fl ow type changes and smoke particles move in irregular paths, in a swirling motion. The layered type of fl ow is known as laminar and the irregular as turbulent. The word “laminar” is taken from the Latin word “lamina”, meaning plate, board, and “turbulent” is taken from the Latin word “turbulentur”, meaning disturbed or tempestuous. Mass loss rate The mass loss rate is the speed at which pyrolysis occurs from a material, sometimes also known as the rate of pyrolysis. This is often measured in the unit g/m 2 s. Premixed fl ames A premixed fl ame occurs when the fuel and air are well mixed with each other before ignition occurs. Pulsation Pulsations occur occasionally when the fi re is entering a ventilation-controlled stage. If there is an opening in the room the fi re can have access to air to allow combustion to take place. As a result of this, positive pressure builds up in the room and the temperature rises. The fi re subsides due to oxygen defi ciency and the temperature falls as new air can be drawn into the room. Pyrolysis Pyrolysis is a chemical decomposition process or other chemical conversion from complex to simpler constituents, caused by the effect of heat. Smoke gas explosion When unburnt smoke gases leak into an area adjacent to the fi re room they can mix very well with air to produce a combustible mixture. If there is an ignition source available or one becomes available some other way, the smoke gases can ignite, with an extremely devastating effect. As a rule, this phenomenon occurs seldom.
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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
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3.4 Summary In many situations the
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oom and in a fairly closed room (on
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Figure 59. The fi re has spread to
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Figure 60. Flashover occurrence ove
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90 precisely the exact moment when
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The amount of energy released when
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Figure 63. Thermal balance on a fue
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Figure 64. Flame spread under the c
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Access to fuel Figure 67. Fire’s
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The signs indicating that a fl asho
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Figure 69. Different signs indicati
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Smoke gas temperature 700 600 500 4
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106 clear that we need to reach the
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This example illustrates how crucia
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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 and 132: 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
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: 168 Glossary Adiabatic fl ame If al
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
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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