Flame temperature

July 29, 2006 2:14 PM

What is the lowest possible flame temperature? No, not your cold-hearted ex, but actual fire. Google-search provides a host of information suitable for engineers and mathmeticians, but not for me. Help?
posted by jsteward to science & nature (20 comments total)

Your question is ambiguous.

Do you mean the lowest temperature which is possible in a steady-state flame, or the lowest temperature at which spontaneous ignition can take place?

The latter temperature is much lower, because there are chemical compounds which spontaneously burn when brought into contact pretty much irrespective of their temperatures at the time. A simplistic case is sodium metal and water; the sodium will burn even in water which is just above the freezing point.

When two such compounds spontaneously ignite when brought into contact, they're referred to as being "hypergolic". The "reaction control system" (RCS) steering rockets on the shuttle use nitrogen tetroxide and monomethyl hydrazine, which are kept liquified in the fuel tanks, so presumably they are at very low temperatures when mixed before they combust.

Of course, once they light off, the temperature rises quite rapidly. But even if it didn't, they'd combust anyway. Combustion is not maintained by temperature in such reactions.
posted by Steven C. Den Beste at 2:29 PM on July 29, 2006

See? I don't even know how to ask the question. The lowest temperature of a steady-state flame (i.e. look at that flame, what is it's temperature?).
posted by jsteward at 2:34 PM on July 29, 2006

A small correction: I assumed those two hypergolic fuel components were liquified gasses. Actually, nitrogen tetroxide's boiling point is 21 degrees C and various hydrazines have boiling points greater than water.
posted by Steven C. Den Beste at 2:37 PM on July 29, 2006

I would guess that the lowest temperature of a flaming compound would be its Autoignition temperature. If you could pump heat away from a flame, then the lowest temperature at which the reaction could continue would be at this temperature. This of course applys only to mixtures that need energy added to combust.
posted by MonkeySaltedNuts at 2:54 PM on July 29, 2006

I am reading the question differently than some of you. I think jsteward is asking "what is the lowest temperature at which a flame can be visible?" I am not a physicist and cannot give the best answer to that question, but I do know that a theoretical limit is given by the properties of black body radiation. This is the term for radiation, including light, given off by a theoretical body that absorbs all light falling on it (doesn't reflect any light so all you see is what it given off because of the temperature of the object. According to the linked article:
Black bodies below around 700 K (426.85 ºC) produce very little radiation at visible wavelengths and appear black (hence the name). Black bodies above this temperature, however, begin to produce radiation at visible wavelengths starting at red, going through orange, yellow, and white before ending up at blue as the temperature increases.

With a flame you are seeing radiation from gasses produced by combustion and smoke particles rather than a theoretical black body, but the temperature of about 425 C should be the theoretical minimum for a visible flame.
posted by TedW at 3:06 PM on July 29, 2006

This question is like asking what the minimum number of people in a singles bar is necessary for sexual coupling to exist. It's a very artifically-restricted notion of the phenomena it's trying to understand. Is sexual coupling only what happens as a result of a sufficient number people in a singles bar?
posted by Ethereal Bligh at 4:07 PM on July 29, 2006

TedW's point is well taken. It's possible for combustion to take place without there being a visible flame.

High temperature in the area of combustion doesn't guarantee that there will be a visible flame. Burning hydrogen can't be seen (by us); all the light that's emitted is infrared.

TedW has it wrong, I think, in that the light emitted by a flame isn't necessarily black body radiation. Some or all of it can be emission lines from the elements or compounds involved in the combustion.
posted by Steven C. Den Beste at 5:07 PM on July 29, 2006

Actually, I'm an idiot. "Black body radiation" is spectral lines.
posted by Steven C. Den Beste at 5:29 PM on July 29, 2006 [1 favorite has favorites]

SCDB is, I think, referring to the common middle school experiment in which a metal is dipped into a metal-containing solution and then put into a flame, where it burns a characteristic color.

This color is based on transitions within the metal (or emission lines). What happens is electrons are excited from a lower energy level to a higher level. Photons are emitted when the electrons relax to a lower energy level.

In order for the electrons to be excited, they need sufficient thermal energy to excite them into the higher energy level. (This thermal energy can take the form of photons emitted through blackbody radiation.) Therefore, it is necessary to have a flame hotter than the color of the transition in order to observe that tradition.

I think TedW's comment is a very worthwhile one; the minumum temperature for a flame depends on the wavelength of the flame. You can have an infrared flame at very low temperatures, but for visible light, I would agree with TedW's number.
posted by JMOZ at 6:18 PM on July 29, 2006

Ethereal Bligh...it's not meant to be an understanding of the physics of fire. It's a question that arose while laughing hysterically at a friend burning his finger with a candle. OK, that's not what made us laugh, it was the "damn, that's hot" comment he made. "Yeah, to bad it wasn't one of those cold flames..." It kind of went downhill from there.

In other words: There are no dumb questions, only dumb people.
posted by jsteward at 7:16 PM on July 29, 2006

I find it hard to believe that even the smallest candle flame has a temperature of 425 C (800 °F). Perhaps a discussion of the structure of a flame (in air, in gravity) would be in order. There's the small central blueish part where the combustion is happening (in this case, just above the wick), and then there's the taller yellow part, where it's radiating light but I bet it's cooled down considerably. So I propose that the yellow part is much cooler than 425 C. I have no science to back that up, I just put it out there.

Oh man I have the munchies.
posted by intermod at 8:05 PM on July 29, 2006

I believe that I've read that there is a special mixture someone came up with that burns with a visible flame without actually producing huge amounts of heat. My memory is rather vague about this, but I believe it was developed for use on the stage, or in movies. Does that strike a bell with anyone?
posted by Steven C. Den Beste at 8:21 PM on July 29, 2006

Not to throw more science-nerd-talk on this discussion, but lets not forget the difference between temperature and heat. Water at 100 degrees Celsius has considerably less heat (internal energy) than steam, even though both are at the same temperature the steam is going to hurt a lot more.
posted by onalark at 8:55 PM on July 29, 2006

I didn't mean to imply it was a dumb question. Sorry! Just that the notion of "flame" seemed very parochial and including a misunderstanding of how temperature is involved.

"Water at 100 degrees Celsius has considerably less heat (internal energy) than steam, even though both are at the same temperature the steam is going to hurt a lot more."

I don't think this is true. Liquid water has in general about twice the heat capacity that steam does. Furthermore, in your "hurt a lot more" example, your comparison is difficult to make, but if we assume you mean what you write and no more and no less, then of course steam being considerably less dense than liquid water has much less energy to give up to the skin of someone's arm per square centimeter, assuming both water and steam are very close to the same temperature.

Anyway, I do appreciate your point to differentiate heat and temperature to the layperson because it seems to me that the modern near-universal familiarity with "temperature" disguises the fact that it's a synthetic concept, something people don't perceive directly, and few are taught explicitly. Heat is the more fundamental quality, but we don't experience that directly, either. Our sensory experience is closer to rate of heat transfer.
posted by Ethereal Bligh at 12:08 AM on July 30, 2006

Water at 100 degrees Celsius has considerably less heat (internal energy) than steam, even though both are at the same temperature the steam is going to hurt a lot more.

Since we are all fully channeling our P-Chem geeks, here, I have to add that the above is true. Steam at 100 degrees has the same heat content as water at 100 degrees, plus the heat of vaporization. This is why pressure cookers and autoclaves work-they use steam rather than water to cook food/bacteria.

An example I like (although is also relies on differences in heat conductivity, not just content) is a 400 degree oven. You can reach in the 400 degree air without danger, but the metal rack at the same temperature will burn you, in part because of the higher heat content of the rack.
posted by TedW at 6:06 AM on July 30, 2006

Ethereal Bligh - don't worry, you didn't. I'm the one admitting it's late-night-we-had-the-munchies-too kind of question.
posted by jsteward at 8:22 AM on July 30, 2006

"I have to add that the above is true."

Yeah, I was wrong and I realize why. There's something about the example I think is misleading, though. (Above and beyond my point about "hurts more" neglecting the per surface-area variable.)

The air and metal in an oven example is good for pointing out the difference between heat and temperature, but I think you're underplaying the importance of conductivity, though. It's particularly easy to burn yourself on metal because of how heat conductive it is while, in contrast, a lot of things we use as insulators are not that extremely low in heat capacity as much as they are low in conductivity.
posted by Ethereal Bligh at 11:56 AM on July 30, 2006

Conductivity is certainly important.
posted by TedW at 12:23 PM on July 30, 2006

I believe that I've read that there is a special mixture someone came up with that burns with a visible flame without actually producing huge amounts of heat. My memory is rather vague about this, but I believe it was developed for use on the stage, or in movies. Does that strike a bell with anyone?

I can vouch for the existence of such a chemical.

I was the dupe audience member called up on stage by a flaming torch juggler in Montreal. To make sure I didn't freak out when he did what appeared to be some dangerous stuff to me, he took me behind the curtains and showed me that the flames were not at all hot to the touch.

I touched it myself and indeed there was no heat.
posted by event at 2:50 PM on July 30, 2006

Well, vaporization can look an awful lot like a flame but not produce heat.

For example, bromine has a rather high vapor pressure at room temperature, so when you pour some in a beaker (not recommended without fume hood/safety precautions/a damned good reason to be messing with the stuff since it's kinda nasty), there's a brown gas which evolves and appears somewhat flame-like.

I'm not sure what high-vapor-pressure chemical would look like conventional flame and be relatively harmless, but I would guess that's what event saw.
posted by JMOZ at 6:13 PM on July 30, 2006