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The mighty dragon is a giant hexapod animal with hollow bones and bat-like wings, bearing a superficial resemblance to a lizard. Being an exothermic animal, it produces an obnoxious amount of heat. Therefore, to get rid of all this heat, the dragon conducts heat from all parts of its body to heat up the air in its lungs before exhaling it, resulting in breath hot enough to light objects on fire. How does the dragon conduct heat from all parts of the body to be exhaled using this method?

It doesn't necessarily have to heat its exhalations, but the basic idea of getting rid of heat through expelling some kind of heated medium must remain. Also, I understand from the replies that you can't get a breath that can light things on fire without the dragon's body temperature being the same as the ignition temperature. If that's the case, then it just has to be hot enough to cause skin burns, which should be much more manageable. This doesn't invalidate any existing answers as far as I can tell.

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    $\begingroup$ The same way that every one else, from mammals to the space rockets, heat their exhaust gas: by an oxidization reaction, which is strongly exothermic. We basically burn sugar into CO2 and water; you may want to consider bleaching some specific secretion with peroxide instead. Bombardier beetles bleach hydroquinone. en.wikipedia.org/wiki/Bombardier_beetle#Mechanism_of_defense $\endgroup$
    – François Jurain
    Commented 22 hours ago
  • $\begingroup$ you can't without violating thermodynamics, if you can concentrate heat like that you have a perpetual motion machine. $\endgroup$
    – John
    Commented 16 hours ago
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    $\begingroup$ @John you can concentrate heat; that's what a heat pump does. But only if you have a source of power. Entropy of the universe as a whole always increases but it is fine for localised areas to have it decrease $\endgroup$
    – Richard Tingle
    Commented 5 hours ago
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    $\begingroup$ @John mammals and birds do concentrate heat. To a (very) slight extent, they even cool down their environment in the process. What the 2nd Law says is, they must use up chemical potential energy and/or mechanical work, i. e. convert it to heat, to achieve this. $\endgroup$
    – François Jurain
    Commented 2 hours ago

7 Answers 7

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  1. Most animals, most likely including dragons, have a built-in heat management system using a pervasive network of heat pipes circulating a red thermal agent called blood. Collecting the heat is not a problem.

    For example, humans use these heat transfer pipes to transfer the heat of the internal organs to their wonderful cooling system called the skin.

  2. That dismal second law of thermodynamics says that heat won't flow from a colder body to a warmer body without some external help. This means that without a heat pump, the air in the lungs cannot be hotter than the blood. With a heat pump the dragon can push heat from the blood into the air; but heat pumps which can push heat against a temperature differential of more than about 100 K (180 °F) are special designs, very expensive and out of reach for the average dragon.

    And of course in order for the exhaled air to be hot enough to set stuff on fire it must go above the self-ignition temperature of the stuff. Which, for most usual stuff, is much more than 100 K (180 °F) above any resonable blood temperature.

  3. But thermodynamics only applies to the muggle world. Dragons are magical, so that there is nothing stopping well-off dragons from installing the HTD® Heat Pumping Lung Liner™ system, which, using esoteric arcane wizardly means, patent pending, will pump heat from the blood into the air in the lungs, bringing it to a roasting 1,500 °C (2,700 °F). Temperatures up to 2,000 °C (3,600 °F) are available with the Magignitor™ upgrade, sold separately. Some restrictions apply. Not for the use of underage dragons. The HTD Heat Pumping Lung Lining system is not a toy; dragons are to exercise due care.

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  • $\begingroup$ Observation: this would mean the blood of the dragon is cooled significantly if I am not mistaken. $\endgroup$
    – Parrotmaster
    Commented 2 mins ago
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Introducing Nature's Tiniest Dragon: The Bombardier Beetle!

Your dragon doesn't need to expel something hot, it needs something that becomes hot

A violently exothermic reaction could do it. The bombardier beetle already does this, in nature. Unfortunately, from the wrong end, but that's just plumbing. When threatened, they release a scalding, noxious spray.

The beetle has two separate reservoirs, one containing hydroquinones and one hydrogen peroxide. In the beetle's case, small quantities of this mean it only nearly reaches the boiling point of water, but given some heavy evolution towards more deadly breath, and your dragon should be producing large amounts of rocket fuel grade hydrogen peroxide in no time.

It almost doesn't matter if it ignites - violently exothermic is enough to boil the dragon's prey alive.

High test hydrogen peroxide is rocket fuel, so you should be able to achieve a full fantasy dragon effect. Blowback will be fatal without proper shielding, and your dragons, a la Pratchett, may explode from time to time.

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  • $\begingroup$ Dragons belch super-heated steam under stress, when bombardier beetles fart it. Explosive belching evokes more the Monty Python than Terry Pratchett: as illustrated in this documentary, After-Eights catalyze fermentation in the stomach, to the point that gas effusion becomes explosive. youtu.be/GxRnenQYG7I?t=330 Accordingly, dragon explosion should only occur when they mistake mint chocolate thins for anti-stress medication. $\endgroup$
    – François Jurain
    Commented 1 hour ago
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Lets convert some mechanical energy to heat

Your dragon has a complex lung system and diaphragms of steel. It inhales a volume of air that is transported to smaller alveoli which compress it. Then it keeps shoving air in by trapping it in compressible chambers in its lungs.

By compressing the air, you are heating it up and changing mechanical energy from the dragon's muscles to heat in the compressed air. The dragon can't hold its breath for too long, otherwise the compressed air will cool and your exhale will have the opposite effect. (It would be an ice dragon)

When the dragon exhales, all alveoli/lung chambers are opened at once. The air will expand and cool, but at the nozzle/throat and for some distance around the mouth, you will have a superheated and pressurized air stream.

Or lets convert some heat into chemical energy

Biology is typically stingy when it comes to energy. It doesn't want to waste it. As such, mechanisms to store chemical energy are very common. Humans store ATP for bursts of energy, glycogen in our muscles for short term exercise, and fats around our bellies for long term needs.

If your dragon has a mechanism of creating glycogen from the stuff he eats, then all he has to do is poop it out and there goes an enormous amount of energy.

But dragons are cool and our answer has to be cool too

Lets put them both together then and our dragon's lungs turn into a combustion chamber. Compress in some air, create a mechanism to add in some carbohydrate (or hydrocarbon if your physiology allows) with the correct mixture. Then create a spark and the whole thing will burn out of the dragon's mouth like a chemical rocket while getting rid of excess energy.

(If you want to avoid the spark, there are fuel mixtures that will spontaneously combust when compressed, like diesel)

Fun edit: If your dragon had one or more blowholes, like a dolphin, around the rib cage and, umm, the backside, that energy could be directed during flight for thrust vectoring or speed bursts. It could also assist in initial takeoff like a JATO.

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  • $\begingroup$ you missed the part about this being a cooling mechanism. $\endgroup$
    – John
    Commented 16 hours ago
  • $\begingroup$ @John Cooling = net energy loss. This is the case for all options above. If all they wanted was a homeostasis mechanism, the dragon would use its scales as radiators during flight or some equally mundane sweating scheme. $\endgroup$
    – JustAnotherUser
    Commented 16 hours ago
  • $\begingroup$ you can't compress air without generating heat, you can't compress air to cool the thing doing the compressing. If oyu could we would not have to build heat pumps with radiators. You can't convert heat into chemical energy for cooling, heat can only generate work when coupled with a gradient, you need seperate cooling. if you could you would again have a perpetual motion machine. $\endgroup$
    – John
    Commented 15 hours ago
  • $\begingroup$ @John Look, I get what you are describing, but this is not that complicated. Breathe in, breathe out. Cold air goes in, hot air comes out. The bigger the difference in temperature between our very hot dragon and the air temp, the better this works. $\endgroup$
    – JustAnotherUser
    Commented 15 hours ago
  • $\begingroup$ which cannot cool you any more than doing it normaly, the problem is the "heat concentration" step thats the thing that is impossible. the only way it works is if the dragons body temprature is even hotter than the ignirion temprature. $\endgroup$
    – John
    Commented 14 hours ago
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So, we have "working" examples in biology. But its not fire. https://en.wikipedia.org/wiki/Bombardier_beetle

And its survivable. But its abblative, as in diggests and denatures parts of the containing living entity (read up on the valve necessary to protect internal organs).

So your dragon can generate chemicals that react- but.. it needs a chamber that has a cell lining that can regenerate after controlled damage.

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    $\begingroup$ you beat me to it, by 5 minutes - I was writing my answer as you posted :P $\endgroup$
    – lupe
    Commented 3 hours ago
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    $\begingroup$ I was mixing answer enzymes while you where still digesting! $\endgroup$
    – Pica
    Commented 2 hours ago
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Dragon blood is a highly heat-conductive liquid metal like gallium.

In addition to getting oxygen from its lungs to every part of its body, a dragon's blood collects heat from every part of its body and returns it to its lungs. Every fresh breath brings in cool air, so the temperature differential between its lung tissue and the air in its lungs is very high—facilitating rapid heat transfer. All of that lung surface area that exists to aid oxygen transfer also helps heat transfer. A dragon's blood and breath is sort of like a computer's heatsink and fan: efficient at dispelling heat.

Liquid gallium is about 80 times more thermally conductive than blood. It's about 6 times heavier though, so the dragon will need some powerful wings. The wings could be full of capillaries for heating the air beneath its wings, providing some compensatory lift.

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    $\begingroup$ this can't heat the air warmer than the dragons body temprature. $\endgroup$
    – John
    Commented 12 hours ago
  • $\begingroup$ @John That's not a requirement. It doesn't need to. The dragon's body produces an "obnoxious amount of heat" to start with. The breath could be cooler than the dragon's body but still hot enough to ignite things. $\endgroup$
    – Sean
    Commented 16 mins ago
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You can't without magic.

You can't without generating more heat than you move, not without violating thermodynamics.

If you can concentrate heat like that you have a perpetual motion machine.

generating heat through reaction is fine, moving around like can't be done for a net heat loss without magic. you only make the dragon hotter this cannot work as a cooling mechanism.

lets be clear you Can net cool the dragon and you CAN heat the air hotter than the dragons body temprature. You cannot do both without violating the laws of thermodynamics. You are asking how to make a biological perpetual motion machine. If you don't care about physics than just use magic.

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I agree to other answers that it is nearly impossible to collect the heat from the body to cool down the dragon while heating up the breath above body temperature.
So it may not work to combine a the cooling with a extremely hot breath into one feature. But You may be able to achieve both effects by two separate features.

As for how to produce an extremely hot breath, I simply refer to the other answers.

As for how to cool the dragon down:
Panting, like dogs do could be one way to cool down your dragon.
This seems to be a pretty common way to cool down for mammals which cannot sweat enough.
But I am not sure if a panting dragon meets your desired dragon behavior or if that reminds too much of a dog. (Who is a good boy? Yes you are.)

So basically panting uses the evaporation of water (spit) to cool the body.
The air won't be hot enough to ignite something, but when we assume that the inner body temperature of a dragon is way higher than other living animals, then it is easy to imagine that the dragon will exhale a lot of hot and humid air.
With a bit of imagination this could be hot enough to scald the skin of a human.

Though the range of the hot breath will be very limited (it will be easier to bite someone than to scald) and your dragons will be pretty thirsty.

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