Enclosure fires

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fans should not be used. You should just take special care and
be aware of the risks when using a fan.
We will now continue with a discussion about the ignition
of the gaseous mass. To do this, it is important to be familiar
with how smoke gases mix.
6.5.4 Ignition of premixed area
The following description assumes that a room is opened after
a little while. When the room is opened air enters and mixes
with the fuel-rich smoke gases in the room. There needs to
be an ignition source in the premixed area for ignition to be
able to occur. Ignition can occur at various times during the
process. There will be mixed areas in different places and at
different times.
1. When the air current is on its way into the room.
2. When the air current is going to leave the room.
3. With a very long delay after the air current has left the
room.
The premixed areas in these different scenarios will be of different
sizes, which means that when ignition occurs the backdraught
will have different levels of power. The ignition source
in the fi gures below is located right inside the room along the
back wall. In reality, the ignition source can obviously be located
anywhere in the room.
1. When the air current is on its way into the room
If ignition occurs when the air current is on its way into the
room a premixed fl ame will occur in the boundary layer, see
Figure 95. You should note that the pictures only give a rough
representation of what happens. As was the case earlier, the
yellow and green represent some type of premixed area.
When the premixed gases ignite this produces a turbulent
combustion area behind the premixed fl ames (see Figure 96).
This region is created as the hot products rise and force down
the unburnt smoke gases. The gases come into contact with
the air-rich layer and combust. Expansion pushes the rest of
the smoke gases out through the opening. This is what produces
the typical fi re ball. 32