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2.2: Fire Terminology

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    Fire

    NFPA defines fire as a rapid oxidation process, which is an exothermic chemical reaction, resulting in the evolution of light and heat in varying intensities.1

    Oxidation Reaction

    • Exothermic reaction – Chemical reaction that releases energy because it takes less energy to break bonds in the reactants than is released when new bonds form in the product.2 Fire is an exothermic reaction that releases energy in the form of heat and light, as well as biproducts in the form of smoke.
    • Endothermic reaction – A chemical reaction that needs a constant input of energy to continue because it takes more energy to break bonds in the reactants than is released when new bonds form in the products.

    Flame

    A body or stream of gaseous material involved in the combustion process and emitting radiant energy at specific wavelength bands determined by the combustion chemistry of the fuel. In most cases, some portions of the emitted radiant energy is visible to the human eye.3

    Premixed flame – A flame for which the fuel and oxidizer are mixed prior to combustion, as in a laboratory Bunsen burner or a gas cooking range.

    premixed blowtorch flame
    Figure \(\PageIndex{1}\): A Premixed Flame.
    Copyright: "Blowtorch" by Clint__Budd, licensed CC BY 2.0.

    Diffusion flame – A flame in which fuel and air mix or diffuse together at the region of combustion. Diffusion flames can be further categorized by the nature of how the fuel and air enter the combustion zone.

    Diffusion Flame
    Figure \(\PageIndex{2}\): A Diffusion Flame.
    Copyright: "flame" by Liz West, licensed CC BY 2.0.

    Laminar flame – A laminar flame has a well-defined luminous body as a result of consistent introduction of fuel and oxygen into the combustion zone. A great example of this flame is a candle.

    a candle as an example of a laminar flame
    Figure \(\PageIndex{3}\): A Laminar Flame.
    Copyright: "Candle flame" by Till Westermayer, licensed CC BY-SA 2.0.

    Turbulent flame – A turbulent flame is characterized by a luminous body that fluctuates greatly as a result of varying involvement of fuel and air. A campfire is a great example of a turbulent flame.

    A campfire as an example of a turbulent flame.
    Figure \(\PageIndex{4}\): A Turbulent Flame.
    Copyright: "Campfire" by Sam Howzit, licensed CC BY 2.0.

    Fire Plume

    The fire plume is the column of hot gases, flames, and smoke, rising above a fire-also called convection column, thermal updraft or thermal column (NFPA 921). While the flame is the visible luminous part of the combustion byproducts, the fire plume represents all visible and invisible combustion byproducts. The fire plume is what typically causes the spread of fire throughout a structure or to new fuels. Keep in mind that the fire plume will typically rise above the fire following the path of least resistance. In a room, the plume will rise until it is blocked by the ceiling, then will move outward.

    Flame Plume

    The flame plume is the visible or luminous portion of the fire plume.

    Heat of Combustion

    The heat of combustion is the amount of heat released during the combustion of a specified amount of fuel.4 Heat of combustion refers to the total amount of heat that will be released if a fuel is completely combusted or burned. Heat of combustion is just one of many factors that influence overall fire behavior. While heat of combustion represents the total amount of heat in a certain amount and type of fuel, it is important to understand that the actual amount of heat that is produced can vary. Different fuels release heat at different rates when burned, and the fuel can be consumed at varying rates during the combustion process.

    Heat Release Rate

    Heat release rate is the rate at which fire releases energy - this is also known as power. HRR is measured in units of Watts (W), which is an International System unit equal to one Joule per second. Depending on the size of the fire, HRR is also measured in Kilowatts (equal to 1,000 Watts) or Megawatts (equal 1,000,000 Watts).5

    Mass Burning Rate

    Mass Burning Rate or Mass Burning Flux refers to the rate at which a fuel is consumed when burned. The mass burning is a key aspect that differentiates legacy fuels like wood and cotton from modern fuels such as foams and plastics. Modern fuels are able to be consumed at a much faster rate.

    Heat of Combustion, Heat Release Rate, and Mass Burning Rate are all important aspects that influence overall fire behavior. Simply put, heat of combustion is the total amount of energy available in a fuel, heat release rate is the speed at which that heat is released, and mass burning rate refers to the speed at which fuel is consumed. Each of these concepts are separate yet related. For example, a faster mass burning rate will result in a higher heat release rate.

    Smoke

    The airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion, together with the quantity of air that is entrained or otherwise mixed into the mass.6 Only about 10% of smoke in a typical fire is visible. Smoke is a key aspect of fire behavior. Smoke should always be considered to be unburned fuel.

    Smoke is Fuel Demonstration

    Video: "Candle Smoke Flashback Demonstration (candle magic trick)" by Firesciencetools.com. YouTube Standard License.

    Smoke (unburned fuel) can spread fire throughout a structure.

    References

    1. ​National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024

    2. Brainard, J. (2014). 8.4 Chemical Reactions and Energy. In CK-12 Physical Science For Middle School. https://www.ck12.org/book/ck-12-physical-science-for-middle-school/section/8.4/. Licensed CC BY-NC.

    3. ​National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024

    4. "Heat of combustion" (n.d.) In Wikipedia. Retrieved August 18, 2026 from https://en.wikipedia.org/wiki/Heat_of_combustion. Licensed CC BY-NC-SA 4.0.

    5. National Institute of Science and Technology. (2025). Fire dynamics. https://www.nist.gov/el/fire-researc.../fire-dynamics. Public domain.

    6. ​National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024

    This page titled 2.2: Fire Terminology was last modified on Fri, 21 Aug 2026 02:07:55 GMT and is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Robert Smith.