How would fire appear in different oxygen content or pressure,for example, in an atmosphere with 6, 13 , 30 , 50 , 70 or 100 percents of oxygen instead of 21 percents?

Even less, less, more, even more, very, and explosively bright.

Did you skip science class the day the teacher burned steel wool in lesser and higher oxygen concentrations?

YouTube: Oxygen Safety Demonstration (and check related videos)

PS: Oh, yeah, how could I forget: George Goble LOX BBQ video

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Ok, but I know how LOX or pure gaseous O2 can accelerate fire.

Then I don't understand your question. What are you after? What do you mean by how "fire would appear"?

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For example, if there was a puncture on aircraft in flight 10 km above ground so air will be the same inside and outside after a few hours and the fuel would catch fire, will it burn?

I mean that I am interested in fire in various concentrations of O2, not jus pure oxygen, for example, fire in the high altitide or in a mixture of 50:50 N2 and O2

Mixtures and concentrations aren't really as important as the amount of O2 available to the fuel. The concentration, mixture, pressure, etc, all help determine the amount of O2 available, and the speed at which it can get to the fuel.

But; more fuel will burn faster as the O2 amounts rise. This continues until the amount of O2 available to react with the fuel reaches the amount of O2 that can be consumed by the fuel.

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That's part of what I don't get. Are you interested in normal air mixtures at reduced pressure, like in an airplane at 10000 feet, or are you interested in modified mixtures of normal air, with less and more oxygen. Are you interested in different mixtures at different pressures?

And then, presuming it's some sort of table by 1) various mixtures, 2) various pressures, or 3) a matrix of mixtures and pressures, what entries do you expect? Whether a given standard object will burn at all? How fast a given object will burn? How bright a given object will burn?

Go beyond what you're interested in. Please describe what you seek.

Is it like the diagrams in NASA: Pressure Effects on Oxygen Concentration Flammability Thresholds of Materials for Aerospace Applications (PDF)?

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The concentration of oxygen is the SAME at different altitudes, from sea level to the stratosphere.
John

? I think you mean the percentage of oxygen in relation to the total gases in the atmosphere remains the same. The absolute concentration of oxygen decreases with altitude.

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Keeper of the Jabberwock

This is an interesting question, and although I don't know the answers, I think I can elaborate on the question.
If the oxygen content of an atmosphere remains the same but the pressure varies, does flammability increase with total pressure? That is, does the increase in partial pressure of oxygen increase flammability? It looks like that is the case, if I am interpreting the graphs from 01101001's link correctly. That means that flammability on the top of Everest is reduced, but is slightly greater on the shores of the Dead Sea when compared to the situation at sea level.

Alternately, the total pressure of an atmosphere might remain the same, but the oxygen level might be greater; this is believed to have been the case in the Carboniferous era on Earth, when the oxygen level might have been as much as 35%.
http://en.wikipedia.org/wiki/Carboni...Rocks_and_coal
The forests at that time must have been particularly fireproof; since they were probably wetland forests in many cases, this seems to be possible. But the flammability of the biosphere probably sets an upper bound on the oxygen content of an Earth-like planet.

I do not know if the question of the total pressure of Earth's atmosphere at that time has ever been addressed; if the total pressure of the atmosphere was greater, that might have allowed a greater partial pressure of oxygen without changing the proportion of oxygen in the total mix.

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Well AFAIK that was also reason why giant insects were living back then, they
have worse respiratory system than animals with lungs and so they cannot be very big in today's atmosphere.

http://en.wikipedia.org/wiki/Aurelia...Moon#Blue_Moon

Well, you can have even 35 percents and about 3x as much pressure, and still have stable biosphere.

I don't think you have yet. Are you satisified with the information you received?

Speaking of demos missed in school (Lurker made me do it), did you ever see the one where a candle is burned to extinguishment, say inside a sealed bottle? As the oxygen that the flame requires is consumed more and more, over time the flame dims until at some point -- well above 6% oxygen -- the fire is unsupportable. Is that not the sort of thing you'd expect for: how fire would appear in various oxygen contents?

Here's a table of 3 flaming things in constant pressure but some lower-than-normal percentages of oxygen from Carbon Dioxide, Caves and You:

Match: 21% - 18% easily burns all of match ; 17.5% Burns head and flame transfers down paraffin to wooden splint on most occasions ; 17% - 16.5% ignited head and on nearly every occasion, burns down onto paraffin coating then extinguishes ; 16% - 15.5% ignited head just ignites paraffin coating on splint (some matches only) ; 15% - head burns briefly with whispery flame & goes out ; 14% match head burns very briefly & goes out (burns due to the O2 in potassium chlorate contained in the head ; <13% head flares & extinguishes immediately (less than 0.5 seconds)

Candle: >19% normal flame ; 17% - 16.5% burns with elongated flame ; 16.5% - 16% flame begins to shrink, but candle remains alight ; 16% burns slowly with small flame ; < 15.0%, A burning paraffin candle is extinguished

Butane Cigarette lighter > 15% O2, A Butane Cigarette Lighter can easily be lit and will stay alight ; 14.5% - weak blue flame with orange top, just stays alight ; <14.25% - Flame will extinguish ; 14% - 13% Large flashes of flame but will not stay alight ; 12.5% sparks with partial ignition, small fireballs ; <10% - no ignition, only hot sparks from flint

Is that the sort of information you desire?

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That youtube video is classic... typical british understatement!

I think a useful phrase that people may be looking for here is oxygen partial pressure. Take the absolute pressure of the atmosphere, and then multiply that by the percentage of the gas in it - that's the partial pressure of the gas. If you have any scuba diver friends, ask them about partial pressures, they should know all about it.

So in our own atmosphere, we have 21% oxygen at 1 atm pressure. So the oxygen partial pressure (also known as "O2 ATA") is 0.21. Go up to a height of 5000 metres, where the absolute pressure is about 0.84 atmospheres, and the O2 ATA is 0.21*0.84 = 0.1764. That's the equivalent then of 17.64% oxygen.

What I find really interesting is that between about 0.10-0.15 O2 ATA, fires have trouble staying alight (even flammable gases can't stay alight below about 14%). Why would that be? I'd have thought that the flame would consume the available oxygen but then new air would circulate in and it could use that to sustain itself.

All this is very useful for considering the environment on hypothetical alien worlds that have less O2 than earth, or lower O2 ATA. Another factor though is the absolute pressure. For example - how would flame behave in an atmosphere that had the same O2 ATA as Earth (0.21) but was physically denser? In other words, an absolute atmospheric pressure of 2 atms, but an O2 concentration of 10.5% (to make an O2 ATA of 2 * 0.105 = 0.21). Is it just the O2 percentage that's important (in which case, the flame would die pretty much straight away) or is it the O2 ATA (in which case it would remain alight like it was in a normal earth atmosphere). Would the thicker air cause the flame to change its characteristics? If something was on fire, would it burn out quicker in such an environment?

Take a look at this paper, it's about how fires varied in the past on Earth as the O2 concentration changed - 35% seems to be the practical maximum, as then pretty much anything can catch fire (even wet wood!) and it can't be extinguished. Conversely, 13% seems to be the lower limit, where even the driest of material can't ignite. (at 17%, dry paper won't even catch fire).

http://www.pnas.org/cgi/content/full/103/29/10861
(or http://www.pnas.org/cgi/reprint/0604090103v1.pdf for a PDF version).

That seems about right for 5000 feet. At 5000 metres, the absolute pressure is about 0.53 atm, according to the formulas at http://en.wikipedia.org/wiki/Atmospheric_pressure

Yes, that is a nice link.

But if the total pressure of Earth's atmosphere has also been changing, those figures would need to be converted into oxygen partial pressure instead. In the Palaeozoic, Earth might have had an atmosphere twice as thick as today's; how could we tell?

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Ooops. Stupid non-metric american websites! Yeah, I did quote it in feet, I didn't notice that.

I think the earth's primordial (well, secondary) atmosphere was a lot thicker, but that all got locked up in the oceans and carbonates etc (see http://en.wikipedia.org/wiki/Earth's_atmosphere ). The current N2/O2 atmosphere is what's left after all that, and it's likely been about 1 atm thick since the Cambrian at least. Though I can't seem to find anything to definitively back that up... maybe we don't know for sure?

There's an extremely intuitive way of knowing how fire will behave at different altitudes - and you don't even need a fire to test it.

Just breathe.

Combust slowly.

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Oxygen levels and combustion are a concern for those interested in terraforming mars. As these things go it seems fairly easy to release carbon dioxide from the crust and then use plants or machines to extract the carbon and leave oxygen, but there isn't much in the way of nitrogen or other gases to act as a buffer and prevent explosive fires caused by high oxygen levels.

The thing I'm not sure about is whether it's the O2 partial pressure that's important, or the actual percentage of oxygen. I suspect it's the former... but then what happened if you had an atmosphere that was 100% O2 but with a absolute pressure of 0.1 atmospheres (so the O2 partial pressure is 0.1 bars)? Would fires not ignite there, even though it was pure oxygen??

There was a paper that eburacum45 linked to sometime back on fire in a spacecraft oxygen environment. It seems that fire can travel somewhat faster without a buffer gas, but things don't catch on fire radically easier. So if there was (say) 3 psi oxygen atmosphere on Mars with a little CO2 and nitrogen, things aren't going to burn that easily.

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It is about the same through the eons because N₂ is too heavy to escape the atmtmosphere of Earth and it is an endothermic reaction to fix nitrogen. Therefore, the total amount of N₂ is reasonably constant and the primary constituent of our atmosphere.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

Another point to consider for life and fire. . .

Our fourth grade teacher did the classic jar over the candle bit and put the candle out. We had a mouse in a cage. I think he may have been prompted by the teacher (though I didn't see it) but one student asked whether the mouse would die when the candle went out.

So the teacher put the mouse next to the lit candle and put the jar over both. The candle went out and everyone held their breath as they watched the mouse scurry around without seeming ill effect. The teacher then told us that mice and men require far lower oxygen content than open flame.

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Keeper of the Jabberwock

From what I've gathered, humans can survive (with acclimatisation) down to about 0.10 O2 ATA, though they are probably going to suffer from some long-term physiological effects as a result.

That said, I'm guessing the mouse wasn't under the jar for long - if it'd been left in there for longer than an hour or so I think it probably wouldn't have been too active!

What I heard is, humans fall into unconsiousness after an hour, or so at 16% O2 = 2.4 psi O2 partial pressure. Most flamable materials stop burning at about 16% = 2.4 psi O2 partial pressure. That may be incorrect as the several percent carbon dioxide may be more important than the low O2. With Increasing altitude, most people can adapt to less than 2.4 psi partial pressure. Perhaps as low as 7 psi air presure = 1.5 psi O2 partial pressure which a mountain climber experiences at 20,000 feet. I believe climbers typically grab a breath of pure oxygen (about once per minute) above 20,000 feet, even if they are well aclimated.
While some things will burn at 20,000 feet, most burn about as well as beer will burn at sea level = not at all. Beer, however, burns quite well at 80% or more O2 or at 13 psi oxygen partial pressure. Common flamable materials burn feriously at 13 psi partial pressure, or so II have been told. Neil

There is partial pressure corresponding to 17 percent O2 on sea level in the La Paz and people happily live there.