Its cold enough to freeze fire!

[Traab]

Its a term ive come across reading various fantasy stories. Obviously an exaggeration, but it made me wonder. Is it possible to lower the temperature so much in an area it extinguishes an open flame? The oxygen remains, the material burning isnt covered with ice, etc. Just the ambient temperature gets low enough that it cant burn anymore.

[Strigon]

This, obviously, would depend on a lot of things. Air pressure, wind speed, what you're burning, what's in the air, and so on.
But, if the air were so cold that heat were being lost faster than the fire was producing it, theoretically it should eventually go out.

[Radar]

Its a term ive come across reading various fantasy stories. Obviously an exaggeration, but it made me wonder. Is it possible to lower the temperature so much in an area it extinguishes an open flame? The oxygen remains, the material burning isnt covered with ice, etc. Just the ambient temperature gets low enough that it cant burn anymore.

It will verily depend on burning material and the size of the fire. I would say that blowing out a candle mimics a very clod surrounding.

The question can be quantified as such: can the environment take away more heat then the fire produces? Any sustainable fire will be from a burning solid or liquid. If we assume that the shape of the burning object (let's say a ball of some flamable substance/oxidizer mix in zero gravity) is always proportional as we make the fire bigger or smaller, then the heat produced scales with square of the radius r. Heat taken by the environment can be calculated on any arbitrary suface encapsulating the fire as a flux of energy through that surface. This flux depends on the temperature gradient dT/dr, which in our simple case will be proportional to the difference between temperature at the surface of our ball T_S and the temperature of the environment T_E, and inversly proportional to the square of the distance from the ball r. There is also obviously a thermal conduction coefficient D and I am ignoring convection thanks to zero gravity clause.

Thanks to the symmetry of our system and the fact that the area of our surface at which we calculate the flux is proportional to r², the total heat given away in a unit of time will be

D(T_S-T_E)

In an equilibrium, this will be equal to the heat production rate of the ongoing oxidation

C r²

Where C is containing all the relevant constants and in general is temperature-dependent.

The fire stops, when the temperature of the ball is not high enough to sustain the reaction. From the relation between heat produced and given away we have

T_S = T_E+C r²/D

So in principle yes, you can do that (since T_E can obviously be below the combustion threshold), but it highly depends on the size of the buring object, since we cannot drop the temperature of the environment below absolute zero.

I also just learned, you can make freezing cold "fire".

[Brother Oni]

The fire stops, when the temperature of the ball is not high enough to sustain the reaction. From the relation between heat produced and given away we have

T_S = T_E+C r²/D

So in principle yes, you can do that (since T_E can obviously be below the combustion threshold), but it highly depends on the size of the buring object, since we cannot drop the temperature of the environment below absolute zero.

In my opinion, you're overthinking this. Fire requires three things to burn*: a fuel source, heat and oxygen.

You've tackled the heat portion and come away with a theoretical answer. The other obvious one is removing the oxygen - lowering the ambient temperature enough that oxygen simply condenses (-183C) or freezes (-218.8C) out of the atmosphere so that the fire is starved that way.

That said, at that temperature, not being able to use an open flame is the least of a person's worries (breathing is topping the list, shortly followed by hypothermia).

*in the classical sense at least.

[Traab]

In my opinion, you're overthinking this. Fire requires three things to burn*: a fuel source, heat and oxygen.

You've tackled the heat portion and come away with a theoretical answer. The other obvious one is removing the oxygen - lowering the ambient temperature enough that oxygen simply condenses (-183C) or freezes (-218.8C) out of the atmosphere so that the fire is starved that way.

That said, at that temperature, not being able to use an open flame is the least of a person's worries (breathing is topping the list, shortly followed by hypothermia).

*in the classical sense at least.

Holy crow I hadnt even thought about how at a certain temp, oxygen stops being a gas. Now my mind is blown.

[NichG]

Though liquid oxygen is going to be a much more potent oxidizer than atmospheric levels of oxygen would be, so if you extinguish the flame by having the oxygen rain out its more likely because you sped it up to the point of explosion rather than because liquid oxygen can't enable fuel to burn...

[veti]

There's a temperature threshold below which the fire will go out.

Hypothetically, we can imagine a fire that's only just hot enough to keep itself burning. If that's the case, then even a small drop in the ambient temperature would be enough to stop it.

And if that's right, then a hotter fire should be put out by a correspondingly greater chill. Until we find some material that releases so much energy on combustion that it can keep burning even at - 273°C.

[Radar]

In my opinion, you're overthinking this. Fire requires three things to burn*: a fuel source, heat and oxygen.

You've tackled the heat portion and come away with a theoretical answer. The other obvious one is removing the oxygen - lowering the ambient temperature enough that oxygen simply condenses (-183C) or freezes (-218.8C) out of the atmosphere so that the fire is starved that way.

That said, at that temperature, not being able to use an open flame is the least of a person's worries (breathing is topping the list, shortly followed by hypothermia).

*in the classical sense at least.

As was pointed out, liquid oxygen can still burn things. Aside from that there are other options as well, since it is not the only oxidizer available. Besides, the precise question was about cooling the fire out with a restriction that oxygen remains.

It is also worth mentioning that more often then not you do not need to go as low with temperature as -183C to extinguish the fire.

[Anymage]

If you think about absolute zero for a moment, it's impressive how it isn't that far from us. Temperatures can go up to four digit degrees without being too exceptional - lava can do it - but the difference between us and absolute zero is only about twice the difference between us and a usual baking temperature.

I'm sure that in a fantasy world where negative kelvin temperatures exist you could get some very interesting physical properties. "Cold enough to freeze fire" tends to be supernatural, after all. I wouldn't know what that would entail, however.

[factotum]

I may be wrong, but I'm pretty sure the reason water puts fires out is precisely because having to boil it away robs the fire of heat and thus makes it go out? As Brother Oni said, you need heat, fuel and oxygen for a regular fire, and taking any one of them away will make the fire go away as well.

[Brother Oni]

Though liquid oxygen is going to be a much more potent oxidizer than atmospheric levels of oxygen would be, so if you extinguish the flame by having the oxygen rain out its more likely because you sped it up to the point of explosion rather than because liquid oxygen can't enable fuel to burn...

As was pointed out, liquid oxygen can still burn things. Aside from that there are other options as well, since it is not the only oxidizer available. Besides, the precise question was about cooling the fire out with a restriction that oxygen remains.

I've never actually seen LOX put near a flame where it's cold enough for oxygen to stay in a liquid state. If you guys say that LOX will still enable a flame to burn at that temp (bearing in mind that there is no atmospheric oxygen at that temperature), then I'll take your word for it.

[Radar]

I've never actually seen LOX put near a flame where it's cold enough for oxygen to stay in a liquid state. If you guys say that LOX will still enable a flame to burn at that temp (bearing in mind that there is no atmospheric oxygen at that temperature), then I'll take your word for it.

You do not have to take our word for it. Here are some examples of how burning things react with liquid oxygen. It is not potent enough to ignite things (you would need fluorine or its compounds for that), but it makes any ongoing fire far worse. And since oxygen starts condensing at a higher temperature then nitrogen, cooling down the air results in an almost pure oxygen rain before the neutral nitrogen joins in.

[Tvtyrant]

If you think about absolute zero for a moment, it's impressive how it isn't that far from us. Temperatures can go up to four digit degrees without being too exceptional - lava can do it - but the difference between us and absolute zero is only about twice the difference between us and a usual baking temperature.

I'm sure that in a fantasy world where negative kelvin temperatures exist you could get some very interesting physical properties. "Cold enough to freeze fire" tends to be supernatural, after all. I wouldn't know what that would entail, however.

Paraphrasing XKCDs toaster vs. freezer: Hot things are hotter then cold things are cold.

Another way to put it would be that life survives in borderline freezing conditions.

[Telok]

Fire is an exothermic chemical reaction. The reason paper does not spontaneously combust at room temp is because triggering that reaction requires more heat than is available at room temp. Fire is self sustaining when the heat put out by a burning mass is enough to heat adjacent non-burning mass above it's ignition point.

To get a non-sustaining reaction the adjacent mass must be at such a low temp that the heat from the burning mass is not enough to bring the adjacent mass up to the ignition point. So there should be a point at which a piece of paper cannot sustain combustion because however much gets burnt cannot raise the temp of the rest of it to the ignition point.

Your critical temp for that is going to vary wildly depending on the combustable materials and oxidizers that are used. I would be hesitant to bet on atomic flourine being safe at any temp.

[Strigon]

If you think about absolute zero for a moment, it's impressive how it isn't that far from us. Temperatures can go up to four digit degrees without being too exceptional - lava can do it - but the difference between us and absolute zero is only about twice the difference between us and a usual baking temperature.

I'm sure that in a fantasy world where negative kelvin temperatures exist you could get some very interesting physical properties. "Cold enough to freeze fire" tends to be supernatural, after all. I wouldn't know what that would entail, however.

Paraphrasing XKCDs toaster vs. freezer: Hot things are hotter then cold things are cold.

Another way to put it would be that life survives in borderline freezing conditions.

There's a very good reason for this.
Actually, there are several; the most obvious being that liquid water - a prerequisite for all life that we know of - only occurs at relatively cool temperatures. However, it would be short-sighted to consider life without water as being impossible.
Another reason is that life requires strictly controlled chemical reactions, and the hotter the ambient temperature, the faster and less controlled chemical reactions become. At the relatively narrow band of temperatures nearly all life needs to survive, most chemicals are stable enough that they won't spontaneously react with whatever they touch without enzymes to help the process.

[Yora]

When a fire is blown out, it doesn't loose either fuel or oxygen. It goes out because of a loss of heat. You remove the hot air around the burning material and replace it with ambient temperature air. Since the fire reaction only takes place at the interface between the fuel and the air, it probably takes a long time for a sufficiently large fuel source to get an internal temperature high enough to sustain the reaction.

Anyone got liquid nitrogen? Can you lit a frozen candle that is standing in a beaker of liquid nitrogen?

[factotum]

Anyone got liquid nitrogen? Can you lit a frozen candle that is standing in a beaker of liquid nitrogen?

Problem with a test like that is that you couldn't be sure the cold was preventing the candle lighting, or the nitrogen boiling off the beaker was displacing the oxygen and stopping it lighting that way.

[Brother Oni]

You do not have to take our word for it. Here are some examples of how burning things react with liquid oxygen.

I can't see the video at work, but I should clarify - I've seen how LOX can help things burn, but only at ambient temperatures, i.e. the LOX is already boiling/evaporating off anyway. I've not seen thermally stable LOX (ie where the ambient temperature is between -218C to -183C ) introduced to a flame source.

[NichG]

I can't see the video at work, but I should clarify - I've seen how LOX can help things burn, but only at ambient temperatures, i.e. the LOX is already boiling/evaporating off anyway. I've not seen thermally stable LOX (ie where the ambient temperature is between -218C to -183C ) introduced to a flame source.

Well, think about it this way. If you already have a working flame source at that temperature, adding LOX can only accelerate things compared to the ambient oxidizer.

The molarity of atmospheric oxygen is about 0.01, while the molarity of LOX is ~35. So LOX+fuel has a reaction rate 3500 times higher than atmospheric oxygen+fuel at any particular temperature.

[Radar]

I can't see the video at work, but I should clarify - I've seen how LOX can help things burn, but only at ambient temperatures, i.e. the LOX is already boiling/evaporating off anyway. I've not seen thermally stable LOX (ie where the ambient temperature is between -218C to -183C ) introduced to a flame source.

This unfortunately is difficult to set up properly, since any installation capable of providing proper ambient temperature will not handle well an open flame inside. The closest you could make on the cheap is to put a tall, open container with liquid oxygen inside a larger container filled with liquid nitrogen. Given proper height of the thermal shielding you should be able to prevent oxygen from boiling. I am still not sure how convection would affect the temperature, but this is why you would want a tall inner container. There is another point though that, when we introduce a burning object to liquid oxygen, the latter will not be at this ambient temperature, since at least some of it will heat up immediately and start making things worse.

Well, think about it this way. If you already have a working flame source at that temperature, adding LOX can only accelerate things compared to the ambient oxidizer.

The molarity of atmospheric oxygen is about 0.01, while the molarity of LOX is ~35. So LOX+fuel has a reaction rate 3500 times higher than atmospheric oxygen+fuel at any particular temperature.

To add to that: which means that the fire produces heat 3500 as fast as in air. In order to compensate, one would need to extract it just as fast. Since thermal conductivity of liquid oxygen in the stable temperature range is roughly only 6 to 8 times higher then that of air in room temperature and the temperature difference between fire and surrounding will not increase significantly enough either (any visible light requires at the very least around 500C), one can conclude that liquid oxygen will most likely only make the fire more violent.

[Lvl 2 Expert]

I may be wrong, but I'm pretty sure the reason water puts fires out is precisely because having to boil it away robs the fire of heat and thus makes it go out? As Brother Oni said, you need heat, fuel and oxygen for a regular fire, and taking any one of them away will make the fire go away as well.

This is actually a very important point. Fire is not very hard to put out by cooling (although literally freezing fire is a little more fantastical), it's hard to put out by cooling it with air. Air is a heat insulator, which means it's slow to take up heat, and it has a very low heat capacity per liter (or any other measurement of volume), which means it takes the fire little energy to heat the air (when it finally does take up heat). Water is a heat conductor with a high heat capacity. You can douse a fire even with hot water, and the water doesn't need to cover the fire and cut it off from fresh oxygen for that to happen. Water of near boiling temperatures is already cold enough to put out a fire. Even a cloud of boiling water vapor could still work.

In a fantasy setting maybe gasses get a lot denser as they cool down, increasing their heat capacity (and making walking through the cold soak up even more energy through several different effects)? Or maybe air is more conductive to heat because that comes with it being conductive to magic? It's not that far fetched fo have a setting in which this is a thing.

[Radar]

In a fantasy setting maybe gasses get a lot denser as they cool down, increasing their heat capacity (and making walking through the cold soak up even more energy through several different effects)? Or maybe air is more conductive to heat because that comes with it being conductive to magic? It's not that far fetched fo have a setting in which this is a thing.

The first option might also help to explain all the ice magic, where ice crystals just appear out of thin air. Essentially we might assume that fantasy air can easily contain much more water then ours. Or some other substance with similar thermal capacity and conductivity.

[wumpus]

I can't see the video at work, but I should clarify - I've seen how LOX can help things burn, but only at ambient temperatures, i.e. the LOX is already boiling/evaporating off anyway. I've not seen thermally stable LOX (ie where the ambient temperature is between -218C to -183C ) introduced to a flame source.

This sounds a lot like the issue of starting a rocket engine in space. In the early days of space travel, lighting all stages on the ground (and ditching some of the rocket engines halfway up) was the preferred method (and I think Soyuz still does this. Four boosters surrounding a central sustainer rocket). In more recent (and primitive) testing, theregister.co.uk's LOHAN project had issues with low-temperature ignition (although I believe they simulated the temperature with liquid nitrogen and were using solid rockets and not liquid oxygen). While this doesn't seem to be an issue anymore, that might just be that they are using sufficiently reactive chemicals to make *sure* that the fire gets started: I'm not saying it is easy, just routine now.

Note that true vacuum doesn't have a temperature, so once things in space get hot they'll stay hot until they emit enough blackbody radiation to cool off (and cooling is a real problem, even on the ISS thanks to zero gravity == zero convective cooling). But that still means that if the stuff is already cold you have to get that fire started somehow.

[Brother Oni]

In a fantasy setting maybe gasses get a lot denser as they cool down, increasing their heat capacity (and making walking through the cold soak up even more energy through several different effects)? Or maybe air is more conductive to heat because that comes with it being conductive to magic? It's not that far fetched fo have a setting in which this is a thing.

I remember doing some number crunching on how hot dragonfire would be to roast someone in a second and based on some assumptions using the thermal transfer rate of natural gas, it was something ridiculously hot, lending credence to some of mythology and fantasy tropes that dragonfire was hotter than anything that man could make.

[factotum]

it was something ridiculously hot, lending credence to some of mythology and fantasy tropes that dragonfire was hotter than anything that man could make.

"It is said that the flame of dragons could melt the Rings of Power, but there is no dragon left in whom the old fire is still hot enough; nor was there ever any dragon, not even Ancalagon the Black, who could harm the Ruling Ring, because it was made by Sauron himself". I wonder if Tolkien started that particular trope or if he borrowed it from older myths?

[Knaight]

I remember doing some number crunching on how hot dragonfire would be to roast someone in a second and based on some assumptions using the thermal transfer rate of natural gas, it was something ridiculously hot, lending credence to some of mythology and fantasy tropes that dragonfire was hotter than anything that man could make.

What did you assume for density and airflow velocity when determining the convection coefficient, and for density when determining the cooling rate of the gas? There's a lot of play in those variables that lets you use lower temperatures than you otherwise would.

[Brother Oni]

What did you assume for density and airflow velocity when determining the convection coefficient, and for density when determining the cooling rate of the gas? There's a lot of play in those variables that lets you use lower temperatures than you otherwise would.

I can't remember - I'd have to look it up. Given I'm biochemist by training, I probably didn't take those into account and just used the propane heat transfer values from heating/HVAC systems.

I vaguely remember calculating the amount of energy it would require to inflict fatal burns via calculating the volume of human flesh (third degree burns to 50% of the human body surface area, so ~ 1 cm deep) then multiplying that up by the specific heat capacity of human flesh, but I can't remember what else I did.

[Vinyadan]

Its a term ive come across reading various fantasy stories. Obviously an exaggeration, but it made me wonder. Is it possible to lower the temperature so much in an area it extinguishes an open flame? The oxygen remains, the material burning isnt covered with ice, etc. Just the ambient temperature gets low enough that it cant burn anymore.

Yes

Yes

Yes

Spoiler

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[Knaight]

I can't remember - I'd have to look it up. Given I'm biochemist by training, I probably didn't take those into account and just used the propane heat transfer values from heating/HVAC systems.

I vaguely remember calculating the amount of energy it would require to inflict fatal burns via calculating the volume of human flesh (third degree burns to 50% of the human body surface area, so ~ 1 cm deep) then multiplying that up by the specific heat capacity of human flesh, but I can't remember what else I did.

Yeah, I wouldn't trust those values at all - my training involved a fair amount of mass/energy flow work, including heat transfer specifically, and while there's a lot where specific geometries come into play there are some general trends in what those heat transfer values would look like with speed and density.

[Brother Oni]

Yeah, I wouldn't trust those values at all - my training involved a fair amount of mass/energy flow work, including heat transfer specifically, and while there's a lot where specific geometries come into play there are some general trends in what those heat transfer values would look like with speed and density.

Interesting.

I'm wondering if it's possible to calculate the potential heat of dragonfire using some D&D assumptions - for example a large dragon has a cone breath range of 50ft, so you can figure out the initial velocity required to expel out a burning gas that far out.

Given a standard 6 second round, you have the heat transfer time. I can dig up the energy required to roast someone - would you think it's possible to ballpark some of your other required values like density and back calculate to a temperature of the dragon breath?