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chamber shape

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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chamber shape

Unread postAuthor: <WOG> » Thu Aug 30, 2007 5:49 pm

just a quick question

does theshape of the chamber effect how the gun shoots or its recoil or anything?

thanks
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Unread postAuthor: frankrede » Thu Aug 30, 2007 5:52 pm

Performance wise, maybe.
But recoil depends on the projectile.
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Unread postAuthor: SpudBlaster15 » Thu Aug 30, 2007 6:06 pm

Quite simply, the closer your chamber is to spherical, the faster the fuel will burn, and the better performance will be.
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Unread postAuthor: BC Pneumatics » Thu Aug 30, 2007 6:40 pm

SpudBlaster15 is correct. A spherical chamber should be ideal, but the cost is generally too high to justify it's use.
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Unread postAuthor: Fnord » Thu Aug 30, 2007 7:02 pm

Quite simply, the closer your chamber is to spherical, the faster the fuel will burn, and the better performance will be.


Salad bowls, anyone?
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Unread postAuthor: trae08 » Thu Aug 30, 2007 7:13 pm

found this a few months back. i was gonna make a sweet pneumatic but never got around to it. and these are a lil pricey.

http://www.aeroconsystems.com/plumbing/ ... pheres.htm


kinda small and rather expensive.

but they would be great for a small hybrid..


"""The sphere on the left is 3.5" OD with 1/8 NPT fitting.
Pressure rating from manufacturer is 700 PSI with wall thickness is 20 gauge.

The sphere on the right is 5.5" OD with 1/4 NPT fitting.
Pressure rating from manufacturer is 400 PSI with wall thickness of 20 gauge.

Both vessels have been proof tested over 1,500 PSI. """
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Unread postAuthor: iknowmy3tables » Thu Aug 30, 2007 7:30 pm

the opening is really small other than that it really does looks like a nice hybrid
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Unread postAuthor: frankrede » Thu Aug 30, 2007 10:06 pm

trae08 wrote:found this a few months back. i was gonna make a sweet pneumatic but never got around to it. and these are a lil pricey.

http://www.aeroconsystems.com/plumbing/ ... pheres.htm


kinda small and rather expensive.

but they would be great for a small hybrid..


"""The sphere on the left is 3.5" OD with 1/8 NPT fitting.
Pressure rating from manufacturer is 700 PSI with wall thickness is 20 gauge.

The sphere on the right is 5.5" OD with 1/4 NPT fitting.
Pressure rating from manufacturer is 400 PSI with wall thickness of 20 gauge.

Both vessels have been proof tested over 1,500 PSI. """


"These spheres are just terrific for your mono or bi-propellant project."
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Unread postAuthor: jimmy101 » Fri Aug 31, 2007 12:27 pm

Not so sure about the spherical chamber being optimal.

A closed spherical chamber burns faster than other closed chamber shapes.

But a spud gun is not a closed chamber.

Here is a thought I've been toying with...

A laminar flame front moves very slowly (~15"/second). A turbulent flame front moves much faster, perhaps 10X faster or more. In a spud gun the gases start to move when the spud starts to move. When the gases get going fast enough the gases' movement transitions from smooth (laminar) to turbulent. Not exactly sure what the speed needed is for this transition but I believe the Reynolds number of a typical spud gun combustion is reached at pretty low velocities, e.g., tens of fps.

When the flow of the gases in the chamber exceeds the Reynolds number and switches to turbulent flow, the flame front should also switch to turbulent and accelerate substantially.

The simple modeling I've done suggests that a short fat chamber is better than a long skinny one of the same total volume. But my modeling assumed laminar flow and flame fronts and doesn't take into account laminar to turbulent transitions.

So, who knows? Anybody have any data?

Lots of possible affects, hard to tell which is/are most important;
1. Spherical closed chamber burns faster than other shapes.
2. The most practical shape for spuders would be a cylinder with length=diameter.
3. A long skinny chamber has a much higher surface to volume ratio and will loose heat faster than a shorter fatter chamber of the same volume.
4. A long skinny chamber will have higher gas velocity and will transition to turbulent flow sooner than will a short fat chamber.

It is possible that a chamber fan will negate the "skinny chamber, early turbulent flow transition" affect since the fan is already making the gases (and hence the flame fronts) turbulent. Even in a closed spherical chamber a fan will substantially increase the burn rate.
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