Enclosure fires

4.3 Processes taking place in the smoke gas layer
The combustion of the smoke gas layer is a typical stage in the
fl ashover process. The smoke gas layer usually ignites immediately
before a fl ashover takes place.
We discussed fl ames in smoke gases earlier on and noted
that they were diffusion fl ames. This is because the smoke
gases do not have the time to mix together before they ignite,
as the whole time is “consumed” by the ongoing fi re. For this
reason, it is not possible for any major amounts of premixed
gases to accumulate.
If the smoke gas layer contains unburnt smoke gases this
can result in combustion, which can then lead to a signifi cant
rise in the level of external radiation.
The unburnt smoke gases accumulate when the oxygen
supply is not suffi cient. But even when there is suffi cient oxygen,
a certain amount of unburnt smoke gases always accumulate.
The less oxygen there is, the more unburnt gases there
are. In these situations there are also a lot of pyrolysis products,
mainly from the fl ammable ceiling material.
When the fl ames grow in size, a stage is reached where the
top of the fl ames penetrate the smoke gas layer and touch the
ceiling. The situation is critical when the top of the fl ames
reach the ceiling. When the fl ames spread along the ceiling a
large amount of radiation is emitted to the surfaces in the
lower part of the room. When the smoke gas temperature under
the ceiling has reached about 600 °C the fl ashover process
may start. 25
Figure 65. An armchair
which is affected by
radiation releases more
heat.
Figure 66. In this case,
we are assuming that
there is only fuel at fl oor
level. This can make the
description simpler.
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