Enclosure fires

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chapter 8 Grey areas between the various phenomena We have described up until now some of the phenomena which can occur in a compartment fi re. We pointed out earl - ier on that a fi re can develop in different ways. In reality, we sometimes encounter situations which lie between different phenomena. We are going to describe below fi ve scenarios where it can be very diffi cult to differentiate between the various phenomena. 8.1 Differentiating between fl ashover and backdraught A fl ashover occurs when there is good access to an air supply and there is a certain amount of fuel. A backdraught originates in a totally different way and occurs when there is a limited air fl ow in a building/room, which means that combustion continues with a limited oxygen supply. This then leads to the accumulation of a quantity of unburnt gases, which can ignite at a later stage when air is supplied. So, why should it be so diffi cult to differentiate between these scenarios? We are assuming that we are dealing with a ventilation-controlled fi re where a door or window is opened and air fl ows in. We note immediately that there will be no powerful backdraught unless the fl ames are coming out through the opening slowly. This scenario was earlier described as when “the fi re resumes its development”, which is exactly what is happening. If we had a slightly bigger area with premixed smoke Temp What is important is to understand the various processes involved, but obviously, it is just as important to use standard terminology. Oxygen deficiency Figure 118. Grey area.V 163 Fully developed compartment fire
164 gases the fl ames would come out through the opening more quickly. It goes without saying then that we are now faced with a type of grey area, where the amount of premixed smoke gas controls how quickly the fi re will develop. Are we looking at a backdraught or a continuation of a fi re’s interrupted development? Of course, we have to assess the situation on a case-bycase basis. This is shown in Figure 118. 8.2 Differentiating between smoke gases auto-igniting in the opening and a backdraught The scenario where the smoke gases auto-ignite occurs when the smoke gases have a temperature which is higher than the thermal ignition point. In some cases when this happens, it can cause turbulence, which spreads throughout the room, with considerable force too, which may result in a small-scale backdraught. This is rare, but has occurred in some situations. This scenario obviously lies in the same grey area, but is based on the fi re growth curve which is at a higher initial temperature than earlier (see Figure 118). 8.3 Differentiating between smoke gases auto-igniting and a fi re resuming its development It can be diffi cult to identify the situation where smoke gases are auto-igniting in the opening. Sometimes it seems as if fl ames are appearing there, but they may have actually originated from far inside the room. This then makes it diffi cult to decide whether the smoke gases are auto-igniting or the fi re is resuming its development. 8.4 Differentiating between a backdraught and a smoke gas explosion A backdraught and a smoke gas explosion originate in totally different ways. A backdraught occurs in a room where the ventilation conditions have changed. This may be, for instance, a room where the panes are cracking, allowing air in, which can
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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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110
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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 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 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
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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