Heat / Smoke / Pressure in a confined box?

[sbfd0551]

Heat / Smoke / Pressure in a confined box??

I am a fireman with 15 years on the job. I am also an instructor at 2 different fire academies and within both my career agency as well as in my volunteer department.

I often watch numerous videos and try to extract "teachable" moments. Recently I have noticed that during some uncontrolled fires within buildings a "cycling" of smoke conditions exist.

We in the fire service often rely on "reading smoke" (that is evaluating the volume, velocity, density and color of the smoke) to provide us with information on where the fire is, its size, where its going, and how fast it is going to get there.

Not long ago I was at a fire where we had some firefighters get injured and hospitalized. Watching the available videos I noticed that the smoke conditions would "cycle" between seemingly ominous to relativity benign...this occurred for at least 3 complete cycles before the exterior wall collapsed injuring numerous firefighter.

Why would this occur? what would be the principal behind the "cycling" of smoke...I assume it has something to do with the combined pressure created by not only the heat but also the volume of smoke contained within the "box" (structure)...

My theory is this : As the pressure builds within the box, oxygen is being denied access to the fire, as the heat production decreases and the products of combustion escape through "cracks" in the box (existing openings, roof lines, vents, etc..) the pressure begins to equalize, thus allowing more oxygen to access the seat of the fire... starting the process all over again.

Not being a science guy would this be an acceptable theory about smoke "Cycling"?... one that I could include in my "fire scene size-up" classes?? If so are there any scientific principals that I could study to support this theory?

Thanks in advance!

 

[256bits]

I would assume the cycling is related to that observed when a candle or oil lantern flickers giving off more smoke and soot. Incomplete combustion due to a lack of oxygen will produces more smoke. Your case would be scaled up version with other variables in play such as that you have already mentioned. Since you are a firefighter, you do know that smoke itself is flammable and is what burns when you have a flashover with tremendous heat and velocity of the flame front.
Not much but it is a start for more discussion from more knowledgeable individuals on this subject.