15 October 2011

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).

No comments:

Post a Comment