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H2O For A Lighter Than [Dry] Air Balloon

I think I know what you're trying to say. When you said, "You let the excess air out when the water evaporated", it didn't make any sense. There's no air getting in or out. But perhaps you intend for the bag to expand during evaporation.

A black bag in the sun is different. You are literally heating the air inside. This phenomenon will work whether there is moisture present or not. The atmosphere exerts a pressure over the surface area of the bag. The total force is this pressure multiplied by the surface area. To expand the bag will do work against the ambient pressure, which requires energy--in this case from the sun. The interior air is now at the same pressure, but is less dense, causing buoyancy.

In the case of an evaporating drop of water, the bag will also expand, requiring energy to do the work. At first I thought this was impossible...where does the energy come from? Then I realized that the drop will draw energy through the bag membrane from the ambient air that it needs for evaporation. The air just outside the bag near the water droplet will become a little cooler as a result--like the evaporator in a refrigerator. Unfortunately, as has already been mentioned, the minuscule expansion and difference in density would not be enough to provide much lift.

Don Kansas City

Reply to
Don A. Gilmore
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The general statement above on economies of scale is only true when the material is limited, when the only material you have is too heavy for a small balloon.

As with the vacuum structure lighter than air idea, scaling doesn't change the feasibility if you assume an unlimited range of material thicknesses.

Bret Cahill

Reply to
Bret Cahill

The balloon would fly either way, purely by thermal differences or purely by water vapor density differences, IF you had a large enough bag or thin enough material.

Assuming thin film plastic like in cheap garbage bags, it would need to be several hundred cubic feet -- the size of a room -- using only one tactic or the other by itself.

If you could find a silvery material that would reflect all heat like a thin Mylar and place it in a mideast desert where the temperature gets to 130 F, it would rise due to water vapor alone. The temperature inside could be the same as outside.

If you could get a clear bag with a black panel then the abs. humidity inside could be the same inside as outside yet still lift the balloon due to a temperature difference -- a hot air balloon.

If you combine both effects -- both happen to be about equal at 40 C outside and 65 C inside -- then the size of the balloon drops by an order of magnitude and a one cubic yard bag will fly.

You put 4 oz of water into a 1 yd^3 bag @ 150 F and as it evaporates it will displace 4 (29/18) or 6.5 oz of dry air.

That's a net 2.5 oz /cu yd less than dry ambient air @ 150 F.

If ambient is 100 F and the inside is 150 F then the temperature ratio is 1.09 or 0.09 X 1.9 lbs air/cu yd. = 2.6 oz.

That's over 5 oz total available for bouyancy on a cu yd basis -- enough to lift six panels of 1 square yard dry cleaner bag material + a black panel.

If you double the diameter the volume increases by 8, the bouyancy increases to 40 oz but the material wt. only increases by a factor of 4 or 20 oz.

Something just over a cubic meter should fly. When the wind stops I'll stuff a thermometer in some clear and black bags to get a clue of the heat transfer situation. Sometimes it is easier to just do it, especially when it doesn't cost more than a few cents and 9 Tbsp water.

Bret Cahill

Reply to
Bret Cahill

This is true, that it is often easier to just do it than calculate. The important/interesting thing though, is to understand exactly what is going on and why is this balloon flying or not. Which, I think, has been pretty much achieved in here!

Take care and let us know the results of your experiments!

Stefanos

Reply to
Stefanos D.

pretty much

I stuffed a black bag into a clear bag and only got a 8 C increase in temperature. Everything was flattened out so things might warm up more in a inflated bag.

On the other hand I just realized the vapor would condense on the plastic if the temperature difference was too great and/or the humidity too high so I may need to go back in the original direction of most bouyancy coming from the vapor. A 1 yd^3 bag would be too small.

The difficult part will be sealing all leaks without adding much weight and waiting for a completely windless day.

Bret Cahill

Reply to
Bret Cahill

Dear Bret Cahill:

If the surface is heated, it won't condense on the surface. You might get some "roll" as it condenses on the unlit side.

You might try the same with a "space blanket". Highly reflective surfaces are lousy at radiating their own heat. Also the mylar will retain water better than a garbage bag. Might be easier to seal, too.

David A. Smith

Reply to
N:dlzc D:aol T:com (dlzc)

That's correct... don't forget oxygen is O2 ( 16 + 16 = 32 )and nitrogen is N2 ( 14 + 14 = 28 ) and water is H20 ( 16+1+1=18).

Reply to
Jonathan Barnes

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