Keeping helium in an unsealed balloon?

  • #1

Main Question or Discussion Point

Along the same lines as an earlier post about helium and materials that wouldn't allow it to permeate it, I wondered if a helium balloon with a fill tube would actually need to be sealed.

What I mean is, imagine a balloon that has been filled with a 20'-30' very thin tube that is still attached to the balloon, trailing below it. There are at least the forces of pressure on the balloon itself, and the helium density against the surrounding air pressure to create lift, but I wonder whether such a balloon & fill tube assembly would need to be sealed, or even sealed well, at the base of the tube (lowermost portion).

In the larger scope, I'll be filling a helium balloon of relatively small dimensions (less than 6' dia.), tethered to as much as 30' of very thin tubing (think fish-tank size or thinner). I'll be filling it through said tube at the time of deployment, and I've been wondering how to design the components that will keep the helium gas in the balloon & fill tube.

So will the gas escape out the bottom of the hanging fill tube?
Will there be too little pressure on the helium throughout to push the lighter than air gas out the bottom?

If there is a condition that the gas would escape, and then with different dimensions it would not, then I could construct experiments accordingly. Ultimately, I'd like to cut down on weight, so even down-sizing components to avoid over-engineering would be significant.
 
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Answers and Replies

  • #2
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It might be difficult to predict how much helium might leak from the open end. Transient conditions like wind would have an effect.

But all you need to close it off is a piece of string and a knot. Why not close it?
 
  • #3
@anorlunda, thanks for writing!

There is no problem sealing the hose. I tried to say that knowing, or even calculating, how much pressure there would be would allow me to cut weight down.

As for string or manually securing, this balloon will be deployed from a filling tank and then automatically disconnected from the tank. You can think of its operation as similar to a signal-except balloon & fill tank disengage when filling stops.

The fill tube serves as tether as well as filler, permanently attached to balloon, but portable once filled. I found no containers of helium light enough to allow permanent attachment, and I've been using a 7:1 volume ratio for containment at 2000psi.

If there is virtually no He at pressure at base (of fill tube), perhaps a simple check valve will do, which needs no manual operation or electric actuator, and would be vastly lighter.

Thanks!
 
  • #4
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Why would you have pressure differences anywhere? Does the balloon have a pressure that differs notably from the atmospheric pressure? That would be odd.
With a long thin tube you don't get much diffusion, if the balloon is stable I don't see a reason for directed flow either (maybe wind temporarily changing its volume?).
 
  • #5
Baluncore
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Why would you have pressure differences anywhere?
A parcel of low density lifting gas is wrapped in a fabric bag, the balloon envelope.
The envelope slides down the contained bubble of lifting gas, until there is tension in the upper envelope surface, sufficient tension to carry the mass of the balloon envelope and any payload hanging below.
That surface tension implies a small pressure difference across the fabric wall on the balloon upper surface. That pressure difference is the lifting force on the balloon.
 
  • #6
Baluncore
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With an open filler tube, the He will remain in the balloon, if the balloon is sufficiently stable to maintain attitude.
The balloon will also maintain it's inflated shape with the open hole, just like a hot air balloon does.

I will consider here only small vertical height differences, so that density changes due to pressure differences can be safely ignored. I will also neglect internal stratification of the lifting gas. My analysis goes something like this.

As you rise from the open base of the balloon, the hydrostatic pressure due to the columns of gas above falls. The pressure difference is zero at the open base. The difference in gas density results in different hydrostatic pressure gradients inside and outside the envelope. The absolute pressure falls faster in the air outside the balloon, than in the less dense lifting gas within. The pressure inside the envelope will therefore always be slightly greater than outside the envelope. So, while the absolute hydrostatic pressures are falling with height, the differential envelope pressure increases with height. That causes a differential pressure to appear across the envelope wall, proportional to height above the opening at the base.

At sea level the difference in density of air and He is about 1.2754 - 0.1786 = 1.1 kg/m3.
For each metre rise above the open base, the differential envelope pressure will increase by 1.1 * 9.8 N/m2 = 10.78 Pa.
At the top of a 93 metre high balloon, that gives about 1 kPa, only 1% of the atmospheric pressure at sea level.

The shape of the envelope material and the distribution of the differential envelope pressure will decide the tension in the envelope fabric. Considering a balloon shaped like an ice cream cone, the greater differential pressure will be in the upper hemispherical envelope, while the cone with an open base will hang from the sphere with sufficient internal pressure to maintain the conical shape. It will also be stable with the open vent remaining low. The conical section is necessary to maintain stability while avoiding pulling in the upper hemispherical lifting surface.

Apart from the longer gas diffusion time, I see no advantage in a longer filler tube. The square root of molecular weight is important. Air will take longer to diffuse into the balloon than the He will take to diffuse out.

Integrating the pressure vector over the entire envelope will give the lifting force. Positive lift must come from the inside of the upper envelope surfaces. The lower surfaces subtract from the lift, but less so, because the differential pressure there is less. It is still simpler to find the total lift available by multiplying the volume of the balloon by the differential gas density. As the balloon is allowed to rise, the potential energy of the system will be reduced as expected. Lifting gas will be lost as the balloon rises and the external absolute pressure falls.
 
  • #7
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Most balloons have significant payload weights below. That acts as ballast keeping up-up and down-down. The OP did not mention that. Without ballast, the balloon can roll in the wind.
 
  • #8
DaveC426913
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Most balloons have significant payload weights below. That acts as ballast keeping up-up and down-down. The OP did not mention that. Without ballast, the balloon can roll in the wind.
The OP mentions 30 feet of fill tubing attached to the balloon. That'll act as ballast up to a certain amount of air turbulence.
 
  • #9
Baluncore
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@Pat Trainor
There is a very good reference book on this subject called; Scientific Ballooning; Technology and Applications of Exploration Balloons Floating in the Stratosphere and the Atmospheres of Other Planets. By Nobuyuki Yajima & Naoki Izutsu & Takeshi Imamura & Toyoo Abe. The Japanese 2000 edition was translated to English in 2009. Springer Science+Business Media, LLC 2009. ISBN: 978-0-387-09725-1

The 2'nd chapter on; Engineering Fundamentals of Balloons, is available for download as a .pdf
https://www.springer.com/cda/content/document/cda_downloaddocument/9780387097251-c1.pdf?SGWID=0-0-45-740923-p173843909
You may notice that; load tapes are used between gores to remove tension from the envelope. See part; 2.2.3.1 Load Tape. Also, venting of zero pressure balloons, is not always from the base.
 

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