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Combustion 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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Unread postAuthor: Insomniac » Mon Oct 10, 2011 7:15 pm

I think he means something along the lines of 'guide the combustion products towards the barrel'.

In that case, it's not really necessary... The gasses have no-where else to go so they have no option but to force the projectile from the barrel. Though I suppose removing any sharp transitions in diameter for smoother, gradual ones could reduce drag on the combustion gasses, and perhaps give a fraction of a percent increase in velocity.

Case in point: Use whatever's pressure rated and don't worry too much about 'guiding' the combustion gasses to the barrel. Far more important is the pressure rating of the pipe, using a metered fuel system, and a chamber fan.
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Unread postAuthor: jimmy101 » Mon Oct 10, 2011 7:29 pm

saefroch wrote:Spherical chamber with ignition in the center is the ideal. shoot for that. Try to avoid DWV for extra safety should the projectile fail to exit the barrel. You're looking for something that says "NSF-PW" or has a pressure rating stamped on it. If it says "DWV" or "not safe for pressure," I'd personally advise against it, though you're probably okay with just a combustion.

Saefroch should know better. The presence or absence of "DWV" means nothing. The ONLY thing that matters is that it has either "NSF-PW" or an actual pressure rating (like 220 PSIG) printed on it. Virtually all pressure rated PVC will also be labeled "DWV".

All pressure rated pipe can be used as DWV. It is the reverse that isn't true.

Having said all that, DWV is fine for any propane (or butane) + air fueled combustion gun (even an "advanced" one) as long as it is 1X fuel load.
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Unread postAuthor: sacklunch » Mon Oct 10, 2011 8:02 pm

Directing the charge is what I was going for in theory. In combustion engines the transition of combustion is a factor in making power ie how fast the exhaust gasses exit directly affects the speed and force of the piston in motion as well as the force of the explosion within the chamber......so in theory a more efficient design has to be a possible. Not trying to make this a big debate or anything, just figured this would be an interesting idea to try.
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Unread postAuthor: sacklunch » Mon Oct 10, 2011 8:15 pm

Ok, scratch the 83" barrel. Going with 62" x 2" on a 24" x 4" chamber. still end up with a 1.55 ratio, roughly.
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Unread postAuthor: saefroch » Mon Oct 10, 2011 8:40 pm

What Technician is talking about is pretty much a DeLaval nozzle, which only works to produce more thrust from superheated gases (they convert thermal energy to kinetic energy, from heat to faster flow). They are only applicable for rocket engines, for a potato cannon they're just a flow restriction (we need maximum flow, not just flow velocity, from a relatively cool and low-pressure environment). We've had this debate many times.

There isn't a magic number. Larger chamber is always better, but with diminishing returns.

"Explosion" is a poor way to talk about the combustion in an air-propane mix. What you're thinking of is applicable to solid explosives, don't think it applies here at all.
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Unread postAuthor: sacklunch » Mon Oct 10, 2011 9:00 pm

So does the fan inside the chamber create an obstruction of flow? I understand the purpose of having one, just curious if it causes a disturbance or if it's not considered a factor. What about having a bowled endcap?
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Unread postAuthor: saefroch » Mon Oct 10, 2011 9:50 pm

It creates turbulence, which stirs the mix and also drastically decreases the burn time. If it's at the back it's a negligible flow restriction. They're also very useful for venting; exchanging the combustion products for air, which importantly contains oxygen.

The shape of the endcap will not have any observable effect. I'm sure D_Hall can explain that it technically does.

My apologies about the DWV, it's been about two years since I even considered buying plastic parts.
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Unread postAuthor: sacklunch » Mon Oct 10, 2011 10:05 pm

No harm or foul just trying to get some new ideas for my first build and figured this would be an interesting topic to discuss considering we are striving for flow and power. My have to try a few things I have on my idea list.
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Unread postAuthor: Technician1002 » Tue Oct 11, 2011 12:52 am

saefroch wrote:What Technician is talking about is pretty much a DeLaval nozzle, which only works to produce more thrust from superheated gases (they convert thermal energy to kinetic energy, from heat to faster flow).


Actually I was talking about 1/2 of one. Just the chamber and the chamber shape to couple into the barrel. Chamber transition to the barrel might be able to use the same theory to cram more high pressure gas into the barrel behind the projectile instead of dealing with the 90 degree surfaces between chamber and barrel. Less insertion loss in the connection.

How this would compare to a sphere or a cylinder with flat ends is the question.
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Unread postAuthor: sacklunch » Tue Oct 11, 2011 8:01 am

All I'm saying is that there has to be a measurable/ noticeable difference in the performance of the cannon due to shape. Just like technician said smooth vs 90 degree angles. I think its at least worth exploring. I was thinking of the shape of a co2 cartridge, round on one end and gently tapered on the other leading into the barrel.
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Unread postAuthor: D_Hall » Tue Oct 11, 2011 8:05 pm

sacklunch wrote:All I'm saying is that there has to be a measurable/ noticeable difference in the performance of the cannon due to shape. Just like technician said smooth vs 90 degree angles. I think its at least worth exploring. I was thinking of the shape of a co2 cartridge, round on one end and gently tapered on the other leading into the barrel.


There ARE effects. However, the definition of "measurable/noticeable difference" is up for debate here. True, you could intentionally make a chamber design that was deliberately draggy/lossy and it would be easy to see such differences. However, in anything that resembles a traditional design you're going to be talking about a percent or three difference at best.

So one day you measure 100 fps. The next you measure 103 fps. Is that measurable? Yes. But is it noticeable? Not really.


As for the "half DeLaval nozzle" discussion. Technician is right. It *is* a more efficient design. For choked flow, a smooth transition to the barrel will yield almost twice the effective flow as a blunt transition. But the vast majority of designs never see anything that resembles choked flow at the breech. So once again, you're dealing with a design feature that may yield "measurably" faster results, but it's very unlikely to be noticeable for the vast majority of the designs out there.
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Unread postAuthor: cfb_rolley » Wed Oct 12, 2011 7:08 am

sorry to jump in to this, but i was about to post a thread with almost the exact same question. I'm going for a chamber that forms the stock (pressure rated pipe, don't worry...) and I'm guessing it's not going to have great flow, but it's the most chamber volume I can fit in a small area. but more importantly, where do I put the spark gap?

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Unread postAuthor: sacklunch » Thu Oct 13, 2011 9:10 am

Interesting design, I like it. Lends to some good ideas. Let us know how it turns out.
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Unread postAuthor: saefroch » Thu Oct 13, 2011 3:37 pm

A centrally located spark gap is best. Place it such that it's the same distance in both directions around the stock to the barrel. Ideally though, add multiple spark gaps and space them out evenly around the length of the chamber.
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Unread postAuthor: cfb_rolley » Fri Oct 14, 2011 6:49 am

that woud make sense, that would mean (idealy) the flame front would go both directions and meet back at the T. I don't know why i hadn't thought like that earlier...
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