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Cold air balloon

Chilly!
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Everyone knows that a hot air balloon floats because hot air is less dense than cold air. Key to the function of a hot air balloon is the fact that it is open on the bottom - this heated air can expand and become less dense.

The cold air balloon is closed at the bottom. The air within stays the same volume. The cold air in the name is generated above the balloon. Atop the cold air balloon is a reservoir filled with dry ice or liquid nitrogen. Extending out from the reservoir are aluminum wires, forming a halo over the top of the balloon. These wires conduct the coldth up and out of the supercooled gas, cooling the air above the balloon. The consequent increase in density of air above the balloon causes the balloon to become relatively more buoyant, and ascend.

bungston, Aug 09 2017

Here's [bungston] composing this idea https://www.youtube...watch?v=1USRDkF6NKs
[normzone, Aug 09 2017]

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       As you cool the atmospheric air, it will immediately flow down around the balloon and under any warmer air in the vicinity. In the absence of wind, it will stratify close to the ground.   

       Warmer air at ambient temperature will then flow in to correct the pressure differential thus created, and the air above the balloon will still be at ambient temperature. Thus there will be no buoyancy.   

       This can only work if the movement of air outside the balloon is constrained.   

       It will be necessary to construct a cylinder of a diameter larger than the balloon and as tall as the highest desired altitude. Cold, dense gas (we suggest SF6*) can then be injected into the base of the cylinder, causing the balloon to rise.   

       Lateral movement is achieved by moving the cylinder as required, keeping it in a vertical orientation at all times.   

         

       *Note that since SF6 is asphyxiant, without supplementary oxygen the occupants of the gondola will rapidly lose consciousness and die shortly afterwards.
8th of 7, Aug 09 2017
  

       /it will immediately flow/ It will not instantaneously flow. But the movement of floating thing upward and displacement of medium downwards is key to buoyancy. If only the cold air below is chilled then the balloon will sit atop that, like a rubber duck in your bath. Which makes me think...   

       In any case - the column _above_ must be chilled to produce upwards movement.
bungston, Aug 09 2017
  

       No.   

       Consider a lightweight rigid sphere filled with air, floating on the surface of a body of water.   

       It will rest in equilibrium between its weight and the displaced volume of water supporting it.   

       If the sphere is pushed down into the water, the buoyancy force increases in proportion to the difference between the weight of the sphere and the weight of water it displaces.   

       When the sphere is fully immersed, as long as the crush depth is not reached, then the force is only related to the difference in weight of the sphere and the water it displaces at that level - the height of the water column above is irrelevant.
8th of 7, Aug 09 2017
  

       [+] there's something there...
FlyingToaster, Aug 09 2017
  

       Hmmm, with a sufficent power supply and water, it should be possible to make a balloon that flies by water pressure.   

       At rest the balloon is a smallish closed capsule above the gondola. To make the damn thing actually work the water pump in the centre of the capsule is started and a gazillion jets of water forces the capsule's skin to stretch it out, lowering the mean internal pressure, thus raising the balloon from the ground.
not_morrison_rm, Aug 10 2017
  

       Dry ice on top of a pinwheel would make a good youtube video
beanangel, Aug 10 2017
  

       hmm... how'bout an array of Peltiers on top, cold side up : heat the air inside the balloon while producing a Coanda effect on the outside.
FlyingToaster, Aug 11 2017
  

       Wouldn't the density (cold) be at the bottom, filling upwards, lifting the balloon? Else it's just a vessel with a weight on the top which falls off.   

       Now if you freeze the molecules and they gain dead weight and drop out of the way creating a less dense volume... Nope, the air above and to the sides will just replace it.   

       Sorry, I would have liked it to work.
wjt, Aug 11 2017
  

       Maxwell's demons with very small pingpong bats could create a vacuum above the balloon, into which it would be continously sucked up.
pertinax, Aug 11 2017
  

       Just have dry ice on top of a pinwheel, the cool air dropping turns the pinwheel then the thread attached to the pinwheel spools up and pulls a balloon upwards.
beanangel, Aug 11 2017
  

       This would be improved by cooling the air *beside* the balloon, instead of above. So the whole column of air above and below the balloon rises, while the surrounding air sinks to displace it. That way you don't have heavy air sitting on the balloon weighing it down. Diagonally above-and-beside might be even better.   

       So if you cool the air at the same height as the balloon, how far does that have to extend? Cooling a spherical shell around the whole planet would obviously be difficult, but should be sufficient if you could do it. Cooling a small area by the balloon may just cause a down draft that the balloon also gets caught up in.   

       If you could somehow separate the balloon into one part that causes the down draft in one place and another part that takes advantage of the rising displaced air elsewhere, that could help.   

       Another issue, the liquid nitrogen you're carrying might have as much excess weight/density as the air it cools, so you only have normal buoyancy after it's used up.
caspian, Aug 11 2017
  

       [pertinax] Or one big ping pong bat called a rotor.
wjt, Aug 12 2017
  

       // the liquid nitrogen you're carrying might have as much excess weight/density as the air it cools, so you only have normal buoyancy after it's used up. //   

       ... and energetically, it's a joke. The energy density of liquified gas is tiny, compared to combustible fuels.
8th of 7, Aug 12 2017
  

       With an aircraft wing, the heat would be applied to top and cooling to the bottom, right? More air speed, less density, above the wing and slower. more density, below.
wjt, Aug 12 2017
  
      
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