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Humid-air balloons; a demonstration of buoyancy AND convection?


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So I was recently thinking; hot air balloons, as a travel mechanism and leisure activity, use hot air for buoyancy on a large scale, and party balloons use helium on a small scale.


Is there a way to form a middle ground between these traits? Not between helium and a large scale, but between hot air and a small scale.


Is there any substance out of which a small-scale balloon can be made that can withstand steam (or at the very least warm, humid air) with which it is inflated and/or be well-insulated to retain its higher temperature for a non-trivial amount of its ascent through cold winter air?


I ask this partly out of curiosity; partly because I'm considering trying it myself if I can carry out with proper safety precautions; and partly because I'm thinking, if this works, this will serve not only as a good demonstration of buoyancy (warmer than surroundings = ascent) but convection as well. (Once it cools, the liquid water weighs it down and it falls back to the ground.) This will also serve as a form of balloon that, even if let go into the air, could theoretically fall back down on its own before reaching an altitude at which it breaks, as opposed to a helium balloon which would just keep rising and rising. A re-usable balloon, in other words.

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The issue will be how much buoyancy you can achieve, and whether this will exceed the weight of the balloon, so you get lift. The balloon mass depends on surface area, while buoyancy will depend on volume. So the ratio gets bigger with larger balloons. Helium allows you to lift ~1g per liter, IIRC. A hot air balloon’s lift derives from (approximately) PV = nRT, and depends on the temperature difference you can achieve. 

Aluminized mylar balloons might be able to withstand some heating. Inflate it partway (or put a few drops of water inside), seal it, and then warm it up. 

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Hot air balloons can be quite small. So presumably since they can be large, you can design something in between with appropriate heat source and materials.


You just need to heat the air enough that the air inside is lighter than the air it displaces by as much or more than the mass of the rest of your balloon and anything it might carry.

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