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Swedish Flashover Terminology
The established scientific terminology associated with various forms of
rapid fire phenomena have existed for decades. Flashover, for example, is a term
well documented throughout fire science and its use in a generic format, to
describe sudden fire escalations, has been commonly used by firefighters.
However, the main parameters which describe the ability of mixtures of
substances in air to propagate a flame and/or explode are flammable limits,
flash points and auto-ignition temperatures. UK fire scientist Richard Chitty explains some
rather inappropriate definitions
of various rapid fire phenomena, as used by the Swedish Fire Service in the
1980s, some of which may have found their way into various fire service training texts - 'Two
key stages in Giselsson and Rosander's description of "Indoor fires"
are lean and rich "flashovers". These seem particularly
confused. Their description of "lean flashover" is as
follows:
When a fire begins in a room, lean flashover often occurs. A fire normally
begins as an initial fire in the lower part of a room. This initial fire,
because of oxygen deficiency, secondary heating etc., generates un-burnt gases
which rise and collect under the ceiling. The gases become increasingly
flammable as the concentration and temperature rise. Soon the gases' lower limit
of flammability is reached, where ignition is immediately possible. The initial
fire ignites this "cushion" of combustion gases which has collected in
the upper part of the room. Unless the room is very large this is normally
short-lived and is over before the fire service arrives.
This lean flashover rises from the initial fire up into the upper part of the
room where it spreads out. This takes 5-15 seconds with a moderate pressure rise
of 1 kPa.
The lean flashover dies out itself very quickly if no ventilation is present.
It is self extinguishing due to its oxygen consumption and simultaneous
over-carbureting. A great deal of combustion gases are extracted from wall and
ceiling materials when heated by the flashover.
Lean flashover happens in a similar way even in large premises, such as
industrial buildings. Every stage in the development of the fire takes
considerably longer than it does in a house fire. The premises are often leaky
and well ventilated which also affects the course of the fire. It can take
almost 10 minutes for lean flashover to occur in a large building.
If the room is directly connected to another room, then the combustion gases
can cause a flashover in the next room and this can be far more fast than
flashover in the original room. After lean flashover is finished, the mixture in
the room becomes quite rich.
Normally there are a number of smoldering fires left in the room. If there
are no easily ignited materials in the room which can smolder, i.e. only
synthetic plastic materials, then the fire dies out by itself after the lean
flashover."
It is a little difficult to see how this event would occur suddenly as this
suggests. During normal fire growth, flames from the fire will eventually reach
the ceiling of the room. Under the ceiling it is not so easy for the un-burnt
fuel gases to mix with air because buoyancy forces tend to keep the hot fuel
gases above the cool air. The flames will therefore lengthen considerably after
they impinge on the ceiling. It is the consequent increase in radiant heat
transfer from these now lengthened flames that leads to flashover as described
in Section 3.2. The lengthening of flames following their impingement on the
ceiling is not a particularly sudden event and is unlikely to generate a sudden
pressure increase of 1 kPa. If it did then windows would be blown out more
frequently than they are. Of course the details of how a local flammable mixture
is formed are strongly influenced by chaotic turbulent mixing processes. It is
these processes that may be responsible for the "sudden" ignition of
gases under a layer during fire growth.
For a more "explosive" event to occur a flammable mixture must
develop remotely from the source of ignition. Such an event could occur if
pyrolysis products, from a source unable to sustain flaming combustion,
accumulate forming a flammable mixture which is subsequently ignited. For
example the ignition by a boiler pilot flame of the products from a smoldering
fire. This, of course, could occur anywhere between the upper and lower
flammability limits. These possibilities are not described as "lean
flashover" by Giselsson and Rosander since they require that ignition
results fom the initial fire.
Rich Flashover
Giselsson and Rosander continue to develop their model of a compartment fire
by considering the events after the point when a "lean flashover"
could occur. They argue that if the flammable mixture is not ignited then the
concentration of flammable components could continue to increase until a rich
mixture is created. If "lean flashover" does occur then the atmosphere
will be oxygen depleted and smoldering fuel could continue to produce un-burnt
pyrolysation products which will accumulate and create a fuel rich mixture. A
rich mixture gives the potential for a "rich flashover" which
Giselsson and Rosander describe as follows:
If a room containing an over-rich mixture receives a supply of air, the
mixture enters the flammable range. If there is an ignition source in the room
then a rich flashover will take place.
These descriptions of "Rich flashover" correspond to the
internationally accepted scientific definition of backdraught'.
Hot Rich Flashover
Often the temperature of the combustion gases is sufficiently high for
spontaneous ignition to occur if air is supplied. The ignition takes place at
the air opening and rapidly spreads into the room. The increase in pressure is
very noticeable, about 2 kPa is normal.
These descriptions of "Hot Rich flashover" correspond to the
internationally accepted scientific definition of 'auto-ignition'.
Delayed Flashover
Sometimes ignition does not take place until an igniting flame flares up from
the initial fire. This is similar to a concealed source of ignition where
delayed ignition means that the mixture can be well inside the flammable range,
the ignition can be violent. This phenomenon is known as a combustion gas
explosion. The increase in pressure can reach 10 kPa.
Overall the effect of delayed ignition of combustion gases is explosive.
Sometimes the ignition can be caused by the fire service personnel themselves if
they expose a concealed ignition source when they enter a room.
These descriptions of "delayed flashover" correspond to the
internationally accepted scientific definitions of 'fire gas ignition' or 'smoke
explosion'.
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