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curious question

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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curious question

Unread postAuthor: jon_89 » Tue Sep 18, 2007 10:31 pm

does the diameter of the combustion chamber matter? does a one inch chamber have a different effect than a two inch chamber.
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Unread postAuthor: frankrede » Tue Sep 18, 2007 10:40 pm

obviously.
ha.
Yes it makes a big difference.
Essentially, bugger is better.
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Unread postAuthor: jon_89 » Tue Sep 18, 2007 10:42 pm

i now bigger is always better but i mean if they are the same volume.
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Unread postAuthor: Lentamentalisk » Tue Sep 18, 2007 10:50 pm

what matters more, is how instantaneously the gas is lit, though that is not very much.

i do however suspect that having an incredibly skinny chamber wouldn't be too good.
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Unread postAuthor: spudgunnerwryyyyy » Tue Sep 18, 2007 10:58 pm

It has to do with the speed of the flame front. Combustion occurs more rapidly in a short fat chamber than in a long skinny chamber. The ideal shape is a sphere but that would be way too expensive.
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Unread postAuthor: CannonCreator » Wed Sep 19, 2007 12:16 am

you need to have a good Cahmber to barrel ratio. Haveing a huge chamber and small barrel is wasting power. And if you have a extremely long barrel and small chamber, thats just bad. Get a C:B calcualtor off of http://www.BurntLatke.com
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Unread postAuthor: MrCrowley » Wed Sep 19, 2007 12:45 am

jon_89 wrote:i now bigger is always better but i mean if they are the same volume.


I do remember I think it was BoilingLeadBath saying that the diameter does matter even if it is the same volume, I can't remember if he said bigger diameter is better or smaller, but I'm pretty sure the bigger diameter is better so the combustion is as close to spherical as possible.

Anyone feel free to correct me, this is just my memory saying this :P
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Unread postAuthor: mark.f » Wed Sep 19, 2007 5:36 am

spudgunnerwry.... and MrCrowley brought up the key point.

A flame front travels faster in spherical mode. If the ignition is in the center of the chamber, and the chamber is as close to the same diameter as the length, while keeping the right volume, then the flame front travels more in spherical mode, and progresses faster before transitioning to laminar mode.

Jimmy101 made a great series of articles on things like this that can be found on the Spudtech archives. I have to admit I c/p'ed each to word and printed them out, (Don't worry Jimmy, it's because it's good information, not any *other* reasons), after he made them each.
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Unread postAuthor: Redcoat » Wed Sep 19, 2007 5:42 am

frankrede wrote:bugger is better.


One could take that in a different context, frank. :wink: .

Anyway, yes bigger is better but to a degree you don't see many people with Chambers the size of a Truck. :D
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 19, 2007 5:55 am

One could take that in a different context, frank.


It depends on which side of the Atlantic you're on, "buggery" doesn't mean much in America... on the other hand, sucking on a fag isn't a big deal in the UK, but doing it in the US is something entirely different :D

Anyway, yes bigger is better but to a degree you don't see many people with Chambers the size of a Truck.


The topic isn't bigger or smaller chamber volumes here, but how that volume is distributed. A 1/2 inch diameter chamber 50 inches long has the same volume as a 5 inch diameter chamber 2 inches long, but the latter will give you more power.
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Unread postAuthor: jon_89 » Thu Sep 20, 2007 12:17 am

thanks for the answers. any ideas for a sphere chamber?
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Unread postAuthor: jackssmirkingrevenge » Thu Sep 20, 2007 2:03 am

jon_89 wrote:any ideas for a sphere chamber?


have a look here.
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Unread postAuthor: jimmy101 » Thu Sep 20, 2007 10:58 am

You really don't need a spherical chamber. Just use a chamber that has length approximately equal to the diameter. Much easier than a spherical chamber.

With a cylinder of length=diameter the percent of fuel burned in the spherical mode is;
(4/3)PiR<sup>3</sup>
----------- = (4/3)/2 = 67%
2PiR<sup>3</sup>

For a more typical chamber shape, say 3"ID by 12"L, about 34% of the fuel is burned in the spherical mode.

A chamber fan running during combustion probably does more to speed things up than does a spherical chamber.
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