Enclosure fires

is far too high for a premixed fl ame to be able to occur.
However, there will be a premixed area at the outer edge
of the cloud, which is where premixed combustion may
occur.
4. If gas leaks from a pipe at high pressure mixing will occur
very quickly, which can result in part of the mixture
falling within the fl ammability range. If there is an ignition
source a premixed fl ame occurs.
5. In a smoke gas layer there is often a large amount of fuel
and air with a low oxygen content. This means that it is
very unlikely that a premixed fl ame will develop in this
layer. A diffusion fl ame will spread along the under side of
the smoke gas layer or higher up in the layer.
6. In some cases, the smoke gases and air mix when air
streams into a fi re room where combustion has occurred
with a lack of oxygen. This can result in a premixed
mixture and ignition can then occur quickly.
7. It is possible in a room adjacent to the fi re for the smoke
gases and air to mix together and fall within the fl ammability
range. If the mixture ignites combustion can occur
quickly. A premixed fl ame occurs and spreads very
quickly. This is usually known as a smoke gas explosion.
Depending on how well mixed the smoke gases are, two different
types of fl ames can occur. It is extremely important to be
able to differentiate between both types.
We will fi rst of all describe diffusion fl ames, as they are the
most common in a normal compartment fi re. We will then
look at premixed fl ames.
3.2.1 Diffusion fl ames*
A burning candle can be used to explain the basic principles of
combustion. The fuel is made up of the melted candle grease
in the small liquid pool on the candle. The fuel is sucked up
into the wick where it converts to gas form. Candles are a common
example of where diffusion fl ames occur.
*This section is based on Julia Ondrus’ book Brandteori (Fire Theory).
There are two
different types
of fl ame which
can occur when
smoke gases ignite:
premixed fl ames
and diffusion
fl ames.
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