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?
Then, typically, fires exist in a state of increasing starvation? If the oxygen around a fire were to be deliberately added at a steady rate, the fire would continue to flare?
I can be like that for three days out of every month.
And hair. It's remarkable, yes.
Pressure also. On a cake of ice hang weights on both ends of wire and suspend the wire on the cake of ice. The scientific explanation is that ice expands upon freezing and the weighted wire applies enough pressure to reverse the freezing.
ML/NJ
Basically, though, I'm concluding that even though fire may spread, it is really dying -- fortunately. Nascentes morimur, so to speak: "from the moment we're born we begin to die."
It’s called a match head.
Pay your taxes and do not mock the rightful recipients of your tribute!
Depends upon the ‘fire’ you are referring to, but let’s say you are referring to a common match.
What you see initially is an accelerant being ignited and burned quickly. When the fuel is gone, the cardboard of the match continues to burn, but at an even rate of combustion, since there is no accelerant.
To watch how most fire burns, minus the accelerant, take a common cotton rag, and then use a magnifying glass to get it to burn. Make sure it’s clean first. You’ll find it smolders, and then you have to blow on it in order for it to turn from an ember to a flame.
Most times if it hasn’t burned, you will light it so that a big gas bubble doesn’t form that could be a real problem.
Take a small smoldering fire at a campsight, and then take the carton that your six-pack (or 12-pack) came in and open both ends and square it up, then place it like a chimney in the center of the smoldering coals and watch what happens.
You will recreate the pillar of fire scene from The Ten Commandments!
-PJ
Great question...never had thought about it. Some good answers on this post, too. I’m leaning towards oxygen availability.
That makes even better sense — the flare is when the accelerant gets eaten up, which happens rapidly? and then the subsequent flames are the fire eating up whatever else is there?
Again, basically, fire looks lively but is actually moribund? Or non-sustainable as people are lamentably inclined to say these days.
*sniff* Reminds me of my husband, an avid camper and volunteer fireman. RIP.
The rate at which a combustion reaction proceeds may be limited by the concentration of oxygen, the concentration of fuel, or the amount of available heat. When the speed of a reaction is limited by the concentration of oxygen or fuel, it can be controlled by regulating the rate at which oxygen or fuel are made available, since the faster the reaction occurs, the less oxygen or fuel will be available to it. When the speed of the reaction is limited by the amount of available heat, however, it is very unstable. The faster the reaction occurs, the more heat will be available to it; this process will often accelerate until the rate of combustion is limited either by the availability of oxygen or fuel.
Many materials are not directly flammable, but when heated will decompose into other materials that are. Often, the decomposition reactions may occur at temperatures below the ignition point of the substances produced, and may absorb a considerable amount of heat. As a consequence of this, it is entirely possible for a fire to give off substances which decompose into flammable gasses or particulates, but which lose so much heat doing so that they can accumulate without igniting. This accumulation may continue until some of the materials are hot enough that--even after decomposition--they are hot enough to auto-ignite. This will quickly heat up nearby materials which were almost--but not quite--hot enough to auto-ignite. This process will proceed very quickly until it consumes nearly all the immediately-available oxygen or fuel, whichever runs out first. Once that occurs, the rate of the reaction will be limited to the rate at which oxygen or fuel is made available to it.
That would be my guess, also all the fuel vapor will be consumed in a fraction of a second, leaving only the liquid fuel left to slowly vaporize and feed the flames.
It is a good question, isn’t it. A young relative asked me, so I cannot take the credit.
Fire is like a flower. It blooms, then dies. As freedumb said, it is magic. :)
It doesn't ALWAYS happen, it depends on the type of combustible material that is put on top of the initial flame.........A lot of times there is an abundance of flammable material put on top of the initial flames. Such material will not burn until it has reached its temperature of flammability.
If all the material reaches that temperature at relatively the same time then it will all flame up at once........
Excluding electrical fires, why does water not always extinguish fires? The answer is, water only decreases the temperature of the burning object below the temperature required to stay inflamed. Thus, burning buildings that have built up so much heat will require a tremendous amount of water to lower the temperature below the point of flammability......It's not an issue of water blocking any oxygen.
Thank you, supercat. I’m going to have to read that several times more just to grasp it, but it sounds brilliant!
Thanks for reminding me, oxygen and fuel are two different things! I have been mixing them up here. :(
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