Enclosure fires

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