What is the flame we see - whether from a candle, natural gas, or what have you? What is it made out of? Does it have mass/exert pressure?
What’s going on there - is it excited oxygen or nitrogen atoms/molecules, is it the actual material itself that is being combusted? What causes the glow? Are electrons releasing photons. Why does it have boundaries? Why the shape? Does the flame make the smoke?
I know all about the color equalling the heat. I’m trying to figure out what the flame/fire is.
I know this sounds really silly, but I’m really looking for a very scientific yet easy to grasp explanation of what we’re seeing when we see a flame, and what’s going on to get it that way. If you want to post your IMHO, fine, but I want the science! (Really, I’ll accept any answer that 's internally consistent!)
BTW, I am only a guest, so could a member please run a search for me to see if this question has already been addressed by the Master or one of his Minions? Thanks.
This is a very good question, but unfortunatelly I haven’t got the time (or knowledge) to answer in full.
Let’s look at a candle flame for the sake of simplicity. It is known that the temperature of the flame is about 1250K (sorry no cite), so the flame is not just a case of black-body radiation from the combustion gasses. I personally believe that the yellow flame associated with flames comes from characterisitc radiation of nitrogen in the air.
It definitely *feels * like it, although I’m not sure how to measure it without fancy equipment. Let me 'splain. I perform fire spinning (staffs sometimes, but mostly chains, also called poi.) When practicing, we don’t light things on fire - if you’re learning a new move, you don’t learn on fire - if you’re a brand newbie, you don’t do *anything * on fire. The strangest phenomenon happens to everyone when they “light up,” or try spinning with the wicks alight - everything gets heavier. There’s more drag, it’s harder to change the direction of the spin, your chains move slower and muscle fatigue sets in quicker. I can spin for 20 minutes or more with an unlit wick. I’m fatigued after 7 when lit.
Some of this may be actual weight of the burning gas (we use charcol lighter fluid - but it’s “heavier” lit then soaked and not lit), some of it may be convection currents fighting the direction of spin. Some of it, admitedly may be the extra heat produced by huge flameballs whizzing by inches from your head causing fatigue sooner. But every firespinner I know has noticed the extra “weight” of the fire.
Couldn’t it be that the extra “weight” you feel is just the extra the air resistance from the fire moving through the air? Now how that works is well beyond me; that’s why I asked the question.
A candle flame, for example, at its simplest, has an innermost region of freshly vaporized wax surrounded by a pale blue region of actual combustion, surmounted by a “cap” of brightly glowing yellow which is mostly the passive glow of fine carbon soot (from incomplete combustion of the hydrocarbon in the main combustion zone). Some of that soot burns up in the glow, but most of it cools too quickly.
Radiation (e.g. of heat from combustion to vaporize wax), conduction (e.g. cooling by contact or mixing) and convection (e.g. the rising tapered shape fo the flame) all play important roles. The important fact to keep in mind is that the hottest part (energy producing) of the flame is the pale blue conbustion zone. The brightest is just an incandescent glow. A miner’s or cavers acetylene lamp, an old style kreig lamp, and the “lime lights” of theaters in past centuries all produce more light by heating something to incandescence (glowing) in a flame than with the flame itself. That’s what happens with the yellow light of a candle flame as well
Michael Faraday gave a classic series of lectures and demonstrations ca 1860 called " ‘The Natural History of a Candle’ which was both simple enough for a modern child, but I think enlightening for an adult. I could have sworn that a copy was online, but I don’t see it anywhere. If you google it (or just >Faraday and candle<) you’ll find plenty of the demonstrations written up for modern classrooms. They’re worth actually doing, even if the conclusions are easily understood from the text.
I’m often amazed at how poorly people understand flame, both now and in Faraday’s time – and I suppose always. It bogggles my mind because, in the sheletered modern life, if you’re using a naked flame, you often have little to do but contemplate it (e.g. hurricane lamp and candles during a power outage, campfires, fireplaces) , and its hypnotic appeal is well noted. I guess most kids just think about other things than I did.
KP, it’s not that I haven’t sat and contemplated, it - I have! Especially around campfires, fireplaces, etc. That’s the point. I was just wondering about what the flame is that we see - is it energy? is it plasma?
Nope, it’s just plain old heated incandenscent gas, according to Cecil, and that’s good enough for me. None of our collective contemplation while gathered 'round the fire could ever answer this basic question for me.
Thanks for the Faraday info, KP. I have done the Google search and found it already. Thanks also to everyone for their answers and for pointing the way.
My apologies for horning in on a truly wonderful question but since the brain trust is already working on your behalf, honored guest, maybe they would spread a little “light” (snicker, snicker) my way.
This is less of a hijack than a sideways kinda…shuffle. But I’ve always wondered what happens to flame in zero gravity. I mean, the top of a flame always points up here on earth. What happens to flame on the space shuttle?
Chemist/Physicist/All-Around-Scientist Michael Faraday used to give a Christmas lecture on this over 150 years ago. It’s been republished as a book and it’sa classic. Faraday’s Chemical History of a Candle is a great read (the book gives all the experiments, with lots of illustrations). Unfortunately, it’s probably out of print, but you ought to be able to find a copy in a large library or through usedbook sites.
Sorry, I didn’t mean to imply you hadn’t. I was thinking of the many Great Unwashed I’ve met over the years who would never have asked the question, as you did–or worse, who had their own weird, easily disproved theories on how flames worked. We all laugh at the old saw about using a match for light to see if the gas tank is empty, but we all have our own Real Life Stupid Fire Stories, too.
Then again, if my childhood was any standard, understanding flames and combustion just opens you up to a higher level of Stupid Fire Stories.
One of my deep life lessons has been that we really haven’t come too far since the cave man; most humans will look back on our tiny slice of history and laugh.
A technique called atomic absorption spectroscopy typically uses a gas flame to analyze the concentration of various elements of a sample by shining a bright light through the flame and analyzing the change in color of the flame. Many elements give a characteristic color in the gas phase – copper atoms in a flame are green, and lithium is bright red. These colors are caused by the characteristic absorption spectra (which in turn are caused by the difference in energy between energy levels that are characteristic for each element) of the gases. In other words, the color of the flame is determined by the elements it contains.
Another phenomenon is blackbody radiation, which is not a sharply-defined emission of light of certain wavelengths, as an emission or absorption spectrum is. A blackbody spectrum is a broader curve, with the wavelength represented by the peak of the curve determined by the object’s temperature. Because of this, hot objects radiating as blackbodies appear first dark, then red, then yellow, then white. Some of the radiation from a flame is blackbody radiation.
Apparently the blue part of a (candle) flame is luminescent – caused by electron transitions, as in an emission spectrum. The yellow part is incandescent, caused by blackbody radiation of soot particles. (But I think it’s quite possible the hot nitrogen and other gases in the air may also be a factor.)
