Fire is defined as the
rapid oxidation of a material during
combustion, releasing heat, light, and
various reaction products2.
Oxidation is when a substance loses
electrons, generally to oxygen, and the
substance becomes ionized. The oxygen and
the ionized substances go through various
other chemical reactions and combinations.
A slower and less dramatic oxidation
reaction can be observed when metal rusts.
In fires, though, this process occurs
dramatically. The released heat from the
reactions can raise ambient temperatures
by thousands of degrees, and the oxidation
reactions (as well as incomplete
combustion) generates smoke2,8,9.
The increased temperature further
increases the volume of gasses by the
processes discussed previously on this
webpage.
While all of this is interesting, a
more practical approach to fire might
serve more useful. Since fire is a process
and not a material, we'll focus more on
what conditions lead to fire.
The Fire Tetrahedron
In order to have a
fire, there are four things that must
be present:
- Fuel: The materials that will
'burn' or be oxidized.
- Heat: In order to ignite a fire
must have a heat source that begins
the reaction. In chemistry this is
equivalent to the activation energy.
- Air: In order to burn, fire must
have a supply of oxygen. Different
concentrations of oxygen will lead to
different fire behavior.
- Uninhibited Chain Reaction: Once
the initial ignition occurs, the
resulting heat and incomplete
combusted fuel will ignite further
oxidation reactions.
In order to halt a fire, we only
need to remove any of the four parts of
the fire tetrahedron.
When discussing fully developed fires,
we can think of them by what prevents them
from growing larger. There are two major
types, fuel limited and ventilation
limited.
If a fire is fuel limited,
it self extinguishes once it runs out of
availible fuel. When camping, you've
probably observed fires die due to fuel
limitations. However, most modern
house fires are not fuel limited at all,
and the logistics of moving all
combustible materials safely away from
materials that are currently in combustion
becomes logistically difficult.
In ventilation limited fires, there is
adequate material for combustion but the
oxygen concentration in the air has
decreased too much for continued growth.
These fires can be suprising as they tend
to smoulder while ventilation is limited,
but as soon as a door or window is opened
they quickly escalate and become fuel
limited (since now the fire has access to
all of the atmospheric oxygen).
Newer innovations in fire engineering
involve working on breaking the
uninhibited chain reaction.
Flow Paths
Since fires, like us, must
breath to continue to grow, we need to
consider how the oxygen will reach them.
We also need to consider where the heat
and combustion products will go.
A flow path is the movement of the
heat and combustion products from the high
pressure at the fire itself (created by
the rapid expansion of gases when exposed
to heat and the conversion of solids and
liquids to gasses) to lower pressure areas
in the surrounding area. The smoke and
combustion gasses will also have a lower
specific volume and will tend to rise.
In the case where there is only one
opening, the balance between the fire's
need for oxygen and the rejection of
combustion gases can be seen quite
clearly.
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