Fundamentals of Fire Science
What is Fire?
Fire is the oxidization of any material that results in the release of heat, light, and other reaction products. For the oxidative process to become fire, the chemical reaction must be rapid. This is the process of combustion.
The visible part of the fire is called the flame. The color of the flame will change according to the materials burning and the environment in which the fire is happening. You can also deliberately change the color of the flame by adding different materials to the fire.
MEL Science | Experiment: 5 colored flames (How to color fire with salts)
Likewise, the intensity of any fire will change according to the materials and environment. For example, dry wood will burn hot and fast, but a fire in fresh wood could simply extinguish itself.
FullServiceChimney | Green Wood vs Seasoned Wood Firewood Smoke Test | Full Service Chimney
Elements of Fire
Fire is commonly represented in what is known as the fire triangle:
All three elements - heat, fuel, and oxygen - are necessary for a fire to happen. If any fire element is removed, the fire will go out.
Here is a demonstration of how the fire triangle works by Mr. Wizard: The Fire Triangle (Mr. Wizard).
Heat
It is a type of energy. In most cases, friction is used to produce heat to start a fire. For example, you strike the head of a matchstick against a prepared surface on the side of the matchbox. This particular side is usually made of red phosphorous, powdered glass, and adhesive.
Fuel
It is any material that burns. Similar to all other types of matter, fuels are classified into solid, liquid, and gas.
Oxygen
It is a chemical element symbolized by the letter O. It is commonly found in the air in its most stable form O2. Plants make it, we breathe it in.
Fire Tetrahedron
Chain reactions become present after the fire starts, thereby sustaining it. As long as the other three elements remain and are continuously available in the right proportions, the fire will continue to burn.
Flames
As previously mentioned, the flame is the visible part of the fire.
Types of Flames
According to Luminosity
- Luminous Flames These flames burn yellow and are sooty. Combustion is incomplete and the flames have limited access to oxygen. They are visibly wavy.
- Non-luminous Flames These burn blue and are not sooty. Combustion is complete and the flames have unlimited access to oxygen. They appear steady.
According to Mixture
- Premixed Flames These occur when the fuel and the oxidizer are thoroughly mixed before reaching the flame zone.
- Diffusion Flames This type of flame occurs when only fuel is supplied. As in a candle, the fire takes its oxygen straight from the air.
- Laminar Flames These follow a smooth path.
- Turbulent Flames These have unsteady irregular flows.
Terms to Remember
Physical Properties
Any measurable aspect of an object
- Specific Gravity The ratio between the density of an object and a specified liquid. Generally, the liquid used for measurement is simply water.
- Vapor Density The weight of a gas relative to air. Air itself is assigned the value of 1.
- Vapor Pressure This refers to the force exerted by a vapor in a given closed container.
- Temperature A physical measure that expresses hotness and coldness.
- Boiling Point The temperature at which a liquid's vapor pressure matches the external pressure around the liquid. Once this is reached, the liquid changes into a vapor.
- Ignition / Kindling Temperature The minimum temperature at which a substance can burn.
- Flash Point The minimum temperature at which vapors ignite.
- Fire Point The minimum temperature at which a substance can sustain a fire.
Pyrolysis
The thermal decomposition of organic (carbon-based) materials in the absence of oxygen is called pyrolysis. It is the first step in combustion but is distinct since oxygen is required for combustion.
Types of Chemical Reactions
Endothermic Reactions
rxn* + heat → products
Chemical reactions are classified as endothermic when heat is absorbed or added into the reaction before it occurs. For example, cooking food.
Exothermic Reactions
rxn* → products + heat
As you can tell from the name, exothermic chemical reactions are ones in which heat is released, i.e. heat exits. For example, combustion. This is why fire feels hot.
* rxn = reaction
Oxidation
Simply put, oxidation is the combination of any substance with oxygen.