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Hostile Fire Events: Flashover

Hostile Fire Events: Flashover

2nd Law: This law states more information we already know in the Fire Service: Heat cannot transfer from a colder object to a warmer object, but it DOES transfer from a higher temperature mass to a lower temperature mass (warmer object to a cooler object).

Knowing this, we can confidently affirm that the characteristics of fire as explained by Fire Marshal Rimgale in the movie, “Backdraft” (1991) may be artistically interesting, but are not entirely true:

Donald ‘Shadow’ Rimgale: “It’s a living thing, Brian. It breathes, it eats, and it hates. The only way to beat it is to think like it. To know that this flame will spread this way across the door and up across the ceiling, not because of the physics of flammable liquids, but because it wants to…”

We can see that flame spread isn’t just by volition; it also has a little to do with the second law of thermodynamics. Heat transfers from hotter objects to colder ones, and we also know it does this by conduction, convection, and radiation.

3rd Law: The third law of thermodynamics explains the concept of absolute zero, or the condition where an object experiences NO molecular movement at all. This is, of course, impossible, since all matter experiences some form of molecular movement, so it is more theoretical than the first two laws.

There is no Fire Service value in the third law, from my perspective. (By the way, I lied. There is also a 0th Law of thermodynamics, but that just seems too crazy to talk about)

Here are a few other useful definitions:

Heat Release Rate (HRR): The rate that heat energy is generated/released as a result of a substance burning (or going through pyrolysis).

Pyrolysis: The chemical decomposition of one compound into one or more substances by the exposure to heat.

Thermal inertia: The properties of a substance that describe the rate of increasing surface temperature (how hot something gets) as heat is applied to it. This is determined by the substance’s thermal conductivity (the rate of heat transfer through a material), density (high density materials conduct heat faster than less dense materials), and its heat capacity (the amount of heat needed to raise a unit mass 1 degree).

Like Marshal Rimgale, I believe fire is a living entity. If you, too, buy into the belief that fire is alive, then you should also believe that fire is an insatiable pig. It wants fuel and it wants air. It will consume anything in its path. The more it consumes, the more insatiable it becomes. As long as there is something to consume in its path, and as long as it has air, it will continue to consume fuels.

The more it consumes, the hotter it burns, the faster it moves and the faster it draws air in. Fire has a propensity to burn upward, but will burn horizontally if there is a fuel source available. Especially if its path upward is blocked, horizontal looks really good.

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