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Tricky 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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Tricky question.

Unread postAuthor: )DEMON( » Fri Jul 11, 2008 5:37 am

If, lets say, there was a pipe with an internal diameter of about 1 1/2'' filled with air and a continuous spark from a stungun was arcing across a gap in the centre.

If a large amount of propane in the form of gas started passing passed it, would the propane ignite?
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Re: Tricky question.

Unread postAuthor: Ragnarok » Fri Jul 11, 2008 5:44 am

)DEMON( wrote:If a large amount of propane in the form of gas started passing passed it, would the propane ignite?

It depends on if there were enough air for the propane to burn in.

A blowtorch works on a continuous stream of propane/butane, although it does need lots of air mixed in to get a really hot blue flame rather than a relatively cold yellow one.
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Unread postAuthor: Spitfire » Fri Jul 11, 2008 6:41 am

It should, if there was enough air for combustion to take place
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Unread postAuthor: SpudFarm » Fri Jul 11, 2008 8:29 am

the answer should be yes if you have some holes drilled in the section of the pipe before the gas ignite so it can suck air with it.
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Unread postAuthor: rednecktatertosser » Fri Jul 11, 2008 9:19 am

Demon you crazy sob, I think I see where this is going...

It should work if, likr others have said, there is the right ammount of Fuel and oxygen mixed as it flows over the spark.

now, are you looking for closed combustion, or more of a flow of ignited gas?
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Unread postAuthor: TurboSuper » Fri Jul 11, 2008 9:54 am

You really don't even need an arc, just a quick spark to get it going.
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Unread postAuthor: jimmy101 » Fri Jul 11, 2008 12:19 pm

Like others have said, as long as the mixture is correct it'll ignite. It may take a bit hotter spark though.

To keep it lit you have to get the mixture and the flow rate correct. For a stable flame the gas flow rate has to be the same as the flame propagation rate. At room temperature and pressure that's about 0.4m/s (16inch/sec). Through a 1.5" pipe that would be a heck of a lot of propane+air moving.
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Unread postAuthor: psycix » Fri Jul 11, 2008 1:32 pm

jimmy, about keeping the flame in the same place with the use of a gas flow just as fast as the flame propagation rate, do remember that turbulence might speed it up, and when DDT occurs there is no gas flow fast enough to hold it. :wink:
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Unread postAuthor: jimmy101 » Sat Jul 12, 2008 10:57 am

psycix wrote:jimmy, about keeping the flame in the same place with the use of a gas flow just as fast as the flame propagation rate, do remember that turbulence might speed it up, and when DDT occurs there is no gas flow fast enough to hold it. :wink:

Even with turbulance you can get a stable flame, it's done all the time to study flame propagation rates.

As for DDT, a stable flame pretty much implies constant pressure, as in a bunsen burner or a Bernzomatic torch. Constant pressure means DDT is impossible.

Since the OP didn't mention why they wanted flowing fuel its kind of hard to know if is is a constant pressure setup or not. Combustion spudguns will ignite even when a chmaber fan is moving the gases past the spark gap at a pretty good speed. Indeed, they work better with the fan running.
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Re: Tricky question.

Unread postAuthor: STHORNE » Sat Jul 12, 2008 11:05 am

Ragnarok wrote:although it does need lots of air mixed in to get a really hot blue flame rather than a relatively cold yellow one.


umm...last time I checked, a yellow flame usually ranges from 500-1,000*C using propane gas as your fuel source.

"Cold"? I think NOT! lol
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Re: Tricky question.

Unread postAuthor: Ragnarok » Sat Jul 12, 2008 2:11 pm

STHORNE wrote:"Cold"? I think NOT! lol

You're missing the point of the term "Relatively".

A blue propane flame is between 1500 to 2000 degrees C. Relative to that, a yellow flame is indeed cool.
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Unread postAuthor: starman » Sat Jul 12, 2008 2:38 pm

Fuel/air mixture is the key as has been stated. Jimmy's website has an excellent write-up on several aspects of fuel mixture and spud world tests in general. Here's one that you could start with, then another here.
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Unread postAuthor: D_Hall » Sat Jul 12, 2008 3:26 pm

Original poster should google for "SUE burner" (aka a Sudden Expansion Burner).
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