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Sphereical combustion chamber

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Sphereical combustion chamber

Unread postAuthor: metalmeltr » Sun Sep 05, 2010 7:04 am

It is gennerally said that a shorter fat combustion chamber is more efficient than a long skinny one. Would a sphere provide an even more efficient chamber? A hemispherical combustion chamber is more efficient than a normal one in an engine, so would a whole sphere provide a noticeable diference in a cannon?
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 05, 2010 7:16 am

Technically a spherical chamber with central ignition would be the most efficient, however not as easy to obtain as tubing...
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Unread postAuthor: metalmeltr » Sun Sep 05, 2010 7:24 am

I know that is would be dificult and likely impracitical to make a sherical chamber

i had thought about ignition in the center as well for the most efficient use of the chamber
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Unread postAuthor: ilovefire » Sun Sep 05, 2010 7:58 am

wouldnt it be just as efficient to use same volume chamber with multiple sparks?
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Unread postAuthor: metalmeltr » Sun Sep 05, 2010 8:10 am

My thought is that the combustion will expand evenly fron the starting point, if the spark is in the middle of a spherical combustion chamber the fire will reach the walls of the chamber at the same time and theorectially more efficiently increase the chamber pressure.
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Unread postAuthor: Technician1002 » Sun Sep 05, 2010 8:28 am

A sphere contains the largest possible volume with the least surface area. This contains the maximum fuel air volume with the least surface for cooling.

A sphere also provides the shortest distance for a flame front initiated in the center to reach the farthest distance.

As for a chamber, many short cylinders are close to a sphere and are a good compromise between a cylinder and a sphere. I use a short fat cylinder shape for my air cannons due to the high flow I can get with them. There is little acceleration from the outside toward the valve so the valve is fed with high pressure air. Expansion takes place in the barrel so the projectile sees higher pressure during launch.
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Unread postAuthor: jimmy101 » Sun Sep 05, 2010 1:43 pm

A cylindrical chamber with multiple spark gaps can burn faster than a spherical one. And it is a lot easier to build the cylindrical chamber and to install the multiple sparks.

A sphere is good because it is strong and minimize the surface area for a given volume (like Tech said).

A cylinder has the advantage of being able to use multiple sparks. A cylindrical chamber with a spark gap every chamber diameter will burn about as fast as a spherical chamber with a diameter equal to the cylindrical one's diameter. Since you can have a long cylindrical chamber, with multiple spark gaps, much more easily than a larger spherical chamber with multiple gaps, you can probably get a cylindrical chamber to outperform the very difficult to build spherical one.

Also, in combustion spud gunning (or in guns in general) "efficiency" is rarely a significant design criteria. Performance is almost always more important. That being the case, you can increase performance by just using a larger chamber. Really isn't any advantage to going to the very difficult to build spherical chamber. Just increase the length of the cylindrical chamber by say 50% and you'll get a bigger boost in performance for a tiny fraction of the cost. The cost to lengthen the cylindrical chamber would be essentially zero.
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