Posted on 04/09/2012 2:13:42 PM PDT by Lady Lucky
I've been asked one of those questions so basic that you never know 'til you're asked, that you really don't know. So I tried answers.com and all those other reasonable places and still can't answer. I figure there are enough smart people on FR to improve my chances. Question: When a fire starts, why does it flare up with a sudden burst of energy that is more intense than the subsequent flames? What's going on there? Why does it not kindle gradually, appear first feeble and then grow to its regular intensity?
/8^)
The same process is responsible for the dangerous "flash over" that happens in house fires. Once enough hot gas, smoke, and air has accumulated, it suddenly ignites and the fire spreads rapidly.
A number of variables come into play I suppose. Dryness, water content, wind; other combustible substances, like say, pine sap that might contribute. Ever burn a dried out Christmas tree? Those suckers really burn.
Any ‘flaring’ would be the result of an abundance of oxygen which would wane to a constant level after the fire is established. I’m guessing. I don’t know squat about physics.
Ok but given a presumably abundant quantity of combustible fuel that feeds the "excess" -- that is, the flare -- what was this quantity doing in the previous second? Why did it not ignite exactly when it was at the quantity, or the density, that feeds the later even flames? Why is it lying there waiting to feed a flare?
Would you say that the heat that is released when the fire ignites, reaches out to acquire more oxygen than subsequent flames have available? Or is something else going on?
It's curious that the change of state from non-fire to fire is explosive rather than gradual -- and yet it's gradual just before it actually combusts.
Initially you have to heat whatever you are burning to a gas that is at or above it’s flash point. This process generates a lot of combustible vapor that isn’t quite at the flash point yet. Once the fire starts and flares (burns up the excess vapor) it will die back to the point where it consumes the vapor as it is generated while not allowing an excess to build up like it did before the flare.
A fire doesn’t have to start with a flare. But two gases are in the equation and their abundance and ratios affect the intensity of the burning.
One is oxygen, or an oxidizing gas (chlorine is another oxidizer). The other gas is the fuel. With the exception of metal, just about any flaming fire is a gas that is burning. This is true of liquid fuel like oil or solid fuel like wood or paper. Prior to ignition, the fuel has to be heated to a point where a flammable gas is evolved. In liquid fuels, this means that the fuel must be at its flash point - the temperature at which flammable vapors are evolved. For gasoline, it is about minus 49 degrees Fahrenheit. For fuel oil (diesel fuel), it is over 100 degrees Fahrenheit. For solid fuels, the heat must be sufficient to pyrolytically decompose the material to a flammable gas. This is why when wood burns, you have a visible flame until it burns down to just the coals. At that point, the material that can be gasified and produce a visible flame is gone, but the remaining carbon is still oxidizing, burning, and giving off heat - just not a flame.
When something is first heated to the ignition temperature, the flammable gases and oxygen are at their maximum abundance. Whatever external heat source is applied to the fuel is causing it to give off gases that ignite when they hit the ignition temperature for that particular fuel. These gases ignite and burn rapidly. If this ignition stage fire produces enough heat to then cause the fuel to continue to give off flammable gas, the combustion is self-sustaining. If it is not, the fire will go out. Usually with solid fuel fires, combustion starts slowly and gradually increases as the the fuel temperature increases, reaching a vigorous free-burning stage. As the fuel is consumed, the flame dies down and eventually combustion stops as wither the fuel is all consumed, or the fire cannot produce sufficient heat to perpetuate the evolution of flammable gas.
Likewise at the moment of ignition, oxygen is at its maximum. It is consumed in the combustion. If the oxygen is not replaced - if the draft to the fire is restricted, the flame will diminish and maybe go out.
The secret is surface area.
If you will notice, kindling is small pieces of fuel with lots of surface area. Lots of flame, but it doesn't last long. The main fuel, (logs) has the burning distributed over a large surface area, by volume, and, as such, gives off more heat, (burns), over a longer period.
If you're a pyromaniac, it's simple.
"A brother or a sister, but then the passion flares again...
I wanna hold ya 'til I die..."
Gotta another for you...
Science question. What makes water ice melt?
Well. What would you pyromaniacs like to do first? ;)
Heat.
The remaining carbon after the gasses burn off or broken down and ‘went up in smoke’.
http://www.arboristsite.com/firewood-heating-wood-burning-equipment/160520.htm
Surface area. Hmm. That seems plausible....I think...but if so, then in the instant afterwards, the surface area burning is diminished, whereas you’d expect it to grow. I mean, if the flare diminishes, then the surface area burning also did.
Eh?
Many thanks to everyone, you are ALL Feynmans to me. :)
Well,. why does the shower curtain come into the tub/stall when you're showerin', smarty pants?
my guess as well, maybe because before the flare up the Oxygen available is normal, after the initial flare the air is heated and surrounding air would be less dense, so less Oxygen would be available. Not being a scientist, I really wouldn’t know, they probably don’t either, but I bet they could fill a book or two on the subject anyway.
Oh wait. I think I'm getting it now. Virtually immediate cooldown?
Sometimes you need more flare...............
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