Flashover in a Bottle

One of the most feared words in the vocabulary of modern firefighters is "backdraft." We fear the very thought of one, and with good reason. Despite what it is called in the fire service, the scientists that study fire and it's development simply see this as another form of a flashover, which by it's definition is a rapid change in a developing room fire to a full room fire. So, a backdraft would fit the classification of a flashover.

The backdraft phenomena is often classified into a group of dramatic fire behavior situations called smoke explosions. I would propose to you that a smoke explosion is simply a "bottled flashover."

Explanation

During a house fire, generally, the percentage of available oxygen to support combustion will drop until the fire self-ventilates. What you get as the level of oxygen drops the self sustaining chemical chain reaction will become less efficient and less complete, however the fire will continue to burn openly. When the fire self ventilates, either through burning through the area above the fire or by a window failure it will become less and less ventilation limited and more fuel limited.

If the fire is completely contained (such as in concrete buildings, building with energy efficient windows, or without windows at all) the heat will remain but will not trigger a reaction with the fuel in the air simply because the oxygen concentration is too low. The fire has become ventilation limited, and the fire enters a "hot smoldering" phase.

At this point, the heat remains very high at the ceiling level but may drop off as it gets closer to the floor. The smoke layer will be very low to the floor but may be cooler than expected. The rapid progression of the fire to flashover has been slowed down by the exclusion of oxygen by combustion products.This where the flashover becomes bottled, unable to find another oxygen source, it sits and it waits like a shaken soda bottle until someone comes along to open it.

With the opening of a secondary oxygen source (say during a poorly coordinated horizontal ventilation effort, or plain ignorance through improper size up) and the percentage of oxygen suddenly rising, the fire now has free reign to continue open burning as it is now no longer ventilation limited. The result as the fuel-oxygen ratio drops rapidly back into the explosive range is an equally rapid oxidation reaction.

Mitigation

The best way to handle the danger is to vertically ventilate as high as possible. It makes sense to ventilate superheated smoke and gas up and out of a structure. This removes the threat of explosion as you make entry into the structure by not only removing heat but also allowing the gases to exit. Horizontal ventilation is not to be used where you can help it.

Sometimes, however, you may be forced to use horizontal techniques to access a structure where these events are seeming to be occurring. John Norman (FDNY, ret) in his book Fire Officer’s Handbook of Tactics (2005) had an excellent example, and suggests creating a small penetration of the space and large amounts of water immediately directed in, utilizing an indirect attack method FROM THE EXTERIOR. The superheated gas will vaporize the water into steam, creating a barrier and absorbing the heat necessary to trigger an explosion as well as cool the seat of the original fire if it can be reached.

Since we are discussing the use of an indirect attack, it should be mentioned that at this point you are no longer concerned about rescuing occupants as the heat and fire gases will probably have already killed them. An indirect attack should only be used in situations where the rescue of occupants cannot be undertaken because of fire progression or structural instability.

Sources

Norman, J. (2005). Fire Officer's Handbook of Tactics (3rd ed). Tulsa, OK: Penwell Publishing.

Quintiere, J. (1998). Principles of Fire Behavior. Albany, NY: Delmar Publishers.

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Starting From The Beginning

The candle post was a bit cryptic, but it is probably the simplest form of a flame I can muster and easily explain afterwards.

It is a great way to start to explain concepts that affect fire spread in a confined space, and why fire behaves the way it does. The best way to start is from the beginning on what combustion is, what it does, and later on I will launch into an explanation of tactics and strategies with these things in mind.

Simply, combustion is a sequence of chemical reactions that create heat, light, and the conversion of chemicals. The candle does a very good job of explaining this process because you can observe it safety.

The candle flame is created by the heating of the wick by an outside source. The heat begins the process of pyrolysis, which is the conversion of a solid into a gas by heating.

What happens as the heat rises into a plume, is that it creates air entrainment. Fresh air comes in at the bottom of the plume, enters the intermix zone where heated fire gases are chemically combining with oxygen to oxidize and burn, and the hot gases rises away from the plume. Generally, a single flame separate from other fuel sources isn't dangerous. It is when the heat plume is allowed to contact other fuels that it becomes dangerous.

See, when a fuel source is ignited and gives off heat, it will mushroom at the level where the gases are too cool to continue to rise. Put that in a box, and now you have a problem. The heat plume will mushroom way too early, causing the heat to bank back down to floor level. Through radiation alone the heat flux to surrounding fuels will not be enough to heat them to their ignition temperatures, but with the plume interrupted the heat will bank back down and multiply the heat flux, and things in the room will heat through convention.

As the inside temperature approaches 1000 degrees F, auto-ignition occurs and results in a flashover. The fire transition from ventilation limited (limited only by air flow) to fuel limited (limited only to available fuel sources).