Fire Fundamentals

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:
  1. Fuel: The materials that will 'burn' or be oxidized.
  2. 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.
  3. Air: In order to burn, fire must have a supply of oxygen. Different concentrations of oxygen will lead to different fire behavior. 
  4. 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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