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Hostile Fire Events: Flashover

Hostile Fire Events: Flashover

Pyrolysis: When a fire starts within a closed compartment, like a room and contents fire, it passes through the three phases of combustion. The temperature rises, combustion continues to consume off-gassing fuel vapor, forms a plume, draws air in (entrainment), and releases; several gasses (over 2000, I am told), heat, light, flame, and smoke. Early in the process, the dominant heat transfer methods are first conduction (flame-contact) and then convection (heated gasses rising up the plume).

As pyrolysis continues, the products of combustion move toward the ceiling. Fire wants to burn upward, so it does.

When these gasses, flames, and smoke hit a ceiling and cannot continue upward, they start to spread horizontally. They will continue to do this until they hit a wall, then they will begin to bank down, giving us the thermal layering within the compartment. The dominant heat transfer method is now radiant heat. As soon as the fire becomes oxygen starved, it focuses on any source of more oxygen.

Once an open window, door, or other vent hole is found (fire is really not that particular) the heat, smoke, flame, and gasses move in that direction, spreading the fire. As the heat builds up within the compartment, though, the radiant heat comes back on itself. This is called radiant heat feedback.

If enough heat builds up within the compartment due to this radiant heat feedback, the temperature rises to the auto-ignition temperature of all combustible material in the room (about 1100F / 590C). All combustibles self ignite and you will experience the hostile event- “flashover”.

We observed this first-hand several years ago, during a Fire Marshal training exercise for accelerant detection, using a compartment with only three walls, a floor and a ceiling (there was NO accelerant in this compartment). The compartment was built in a field outside, and the weather was cool and windy. As you can see in these photos, we had no problem getting the room to flash. It took less than 4 minutes.

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