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?
Water gives off a considerable quantity of vapor when it boils. This can be useful when extinguishing a fire, though the fact that the vapor is lighter than air may at times make it less effective than CO2. Soda water seems to work pretty well, though. 8-)
When initially lighting something on fire and it flashes (”FWOOOOSH!”), in my experience it is due to an inadvertant accumulation of somthing “very-combustible” that you are trying to light.
Take my mom at the beach, trying to light the hot-water heater on the camper. Opened the gas valve and tried striking a match. The on-shore breeze kept blowing them out. So, she got the great idea to drape a big beach towel over her head to shield from the wind. Well, by the time she got the towel situated just like she wanted it, groped around to find the box of kitchen matches and tried to strike one, “FWOOOOSHH”!! she got knocked back, sprawling wide-eyed & flailing into the campsite due to the flash of the inadvertant accumulation of propane while she was getting herself all situated with that towel.
Luckily she didn’t get burned, but the front of her hair caught on fire for a moment and it stunk for the rest of the camping trip. We all learned a lesson that day (after we could stop belly-laughing long enough to think about it).
BINGO.
With wood kindling or paper (or liquid accelerants), smaller cross section fuel sources are easier to bring up to gas phase and ignition temperature. Second is the quantity of O2 saturation in the immediate vacinity prior to initial ignition, which once the smaller cross section fuels are consumed are only replaced as consumed.
Try to light a regular sized log, it will not flare unless you are using a torch to ignite.
Take paper for an instance. Paper will not burn until it's temperature reaches 451 degrees farenheight. At 450 degrees it gives off smoke which contains flamable gasses but are not yet hot enough to burn. Raise the temperature one degree and it flashes into flame.
The flash you see is the gasses reaching the ignition point.
Indeed...I emailed an old high school friend who is a Harvard physicist and he said what some posters have said: there is a build-up of gasses just before ignition which causes the big flamer; it then settles down to a stable amount of combustibles.
More oxygen accelerates the rate of combustion, so yes, I think you’re correct.
Remove the accelarant - and you will have a dramatically slow combustion. If you were a boy scout, and had to spend time rubbing sticks, or using a flint with kindling - you would see that under 'normal' conditions, fire starts quite slowly, and builds. What you are seeing is the application of accelerants - which "accelerate" the combustion. Again, light a candle, light some leaves, light paper, cardboard or kindling. Without an accelerant, it's a slow, blistering and painful process.
“Again, basically, fire looks lively but is actually moribund? Or non-sustainable as people are lamentably inclined to say these days.”
Sort of.
Fire is something that results from a chemical process. When Oxygen gets together with something that will combine readily with it, a couple of things happen:
1. You get heat
2. You get some light
This happens in chemical processes that result in a lower thermal energy state.
Right now, your body burns food. Oxygen is combined with the corn flakes (which is turned in to blood sugar) in your cells which result in waste heat and fuel your muscles use to move, etc.
That’s all ‘burning’ is - oxidation.
You can tell what’s burning by the color in a lot of cases. Cobalt burns blue. Copper burns green. Iron burns red. Guess what makes those fireworks on the 4th of July colorful?
So, instead of thinking of fire as burning, think of it as oxygen mixing with something ready to mix with it.
If you add fire (heat) to a piece of binder paper, it raises the temperature of the paper beyond 451 degrees, at which point the paper will sustain the chemical reaction of oxidation occurring in the paper.
Some fire burns without a flame (Indy car fuel, for example, burns so you can’t see it - believe me it burns just the same as any other fire). Some fire burns ‘cool’ (You can dip your hand in pure alcohol and light your hand on fire and it won’t burn. You can try this with someone elses $20 bill and it will burn the alcohol off without burning the bill.)
Some fire burns so slowly you can only see it over time. Visit any rusty swingset or gate - the iron in the gate is turning into Iron Oxide, or Oxidized Iron, or burned Iron.
As such, don’t be fooled by ‘fire’. Fire’s just something that results from certain kinds of oxidation processes.
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