Hostile Fire Events: Flashover
So what, exactly, is a flashover?
A flashover is defined as a hostile event that may occur at the moment when the heat in a compartment is raised to a temperature that all combustibles within that compartment self ignite (the auto-ignition temperature) and burst into flames. When a combustible reaches its auto-ignition temperature, flame impingement (direct flame contact) is not necessary for combustion to occur.
In a nutshell, the life cycle of a fire goes through three phases:
1) Ignition and growth phase
2) Fully developed phase
3) Decay phase.
The transition between phases 1 and 2 is where a flashover could occur.
As I said, flashover is the name given to the phenomenon where heat builds up in a compartment to the point where the ignition temperature of all combustibles in the room is reached almost simultaneously, and the room literally bursts into flames.
The temperature range for this to occur is typically somewhere between 880F and 1100F. Just keep in mind, you can have a fire progress to the fully developed phase without the flashover hostile event taking place.
In high school chemistry class, we performed the following experiment: Place a wood chip in a test tube, heat the test tube with a Bunsen burner, watch the wood chip burn inside the glass (it actually off-gasses and chars as it is heated), and the gasses emitting from the open-ended test tube burst into a small fire-ball when an open flame is drawn near the opening. It was kind-of cool to watch, but the lesson was totally lost on me as a high school student.
Now, looking back, I can appreciate what my poor chemistry teacher, Mr. Faucher, was trying to teach us, but I for one, failed miserably to understand.
He was showing us that heat, not flame, caused the chemical reaction of pyrolysis, or decomposing one form of matter into several other forms of energy and matter. As the wood chip was exposed to the heat from the burner, it off-gassed, produced a flammable vapor, produced smoke, and charred the surface of the wood chip.