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3. This is usually split into normal pressure differences and<br />
pressure differences caused by the fi re. Normal pressure<br />
differences are caused by the wind, air conditioning and<br />
temperature differences between outside and inside air.<br />
Pressure differences created by the fi re are inhibited<br />
thermal expansion and thermal buoyancy.<br />
4. Premixed fl ames occur when fuel and air are premixed<br />
before ignition occurs, whereas diffusion fl ames occur<br />
when fuel and air are not premixed when ignition occurs.<br />
5. A premixed fl ame front can travel at around 3–5 m/s.<br />
6. A defl agration is when fl ames spread in a premixed<br />
gaseous mass.<br />
7. A detonation is when fl ames spread in a premixed<br />
gaseous mass where a shock wave is associated with the<br />
fl ame front, which can produce very high pressure and<br />
speeds.<br />
8. When the fi re is fuel controlled the heat release rate is<br />
controlled by the fuel. In ventilation-controlled fi res the<br />
heat release rate is controlled by the size of the openings.<br />
It is very important to know whether the fi re is fuel or<br />
ventilation controlled, for instance, when you are using a<br />
fan. It is then possible to predict the consequences resulting<br />
from supplying air.<br />
9. A number of products are formed during a fi re, including<br />
carbon dioxide and water, which are the most<br />
common. When the combustion effi ciency drops<br />
unburnt gases such as carbon monoxide and unburnt<br />
hydrocarbons are produced.<br />
10. You can obviously fi nd the products specifi ed in the<br />
answer to question 9, as well as a quantity of air, associated<br />
with the plume.<br />
11. The rate is controlled by the speed at which the molecules<br />
diffuse into each other. This rate increases with<br />
temperature, for instance.<br />
12. In an open room there will be positive pressure at the top<br />
of the room, which results in smoke gases leaking out. In<br />
the bottom part of the room there is negative pressure,<br />
which causes air to get sucked in.<br />
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