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Theoretical Combustion Cannon Power Improvement

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Theoretical Combustion Cannon Power Improvement

Unread postAuthor: Mr.Tallahassee » Wed Nov 02, 2011 9:10 pm

So I was watching Myth Busters tonight and learned something that pertains to combustion cannons. The myth was that the gas produced in sewers could send man hole covers sky high (and yes, it can!). The small scale was almost the equivalent to a super size combustion cannon. The first test had no debris and the deflagration was remarkably slow even with the fan mixing things up. They tried it again with a bunch of large springs and the combustion was easily ten times faster. The theory stated was that the coils increased the flame front area thereby increasing the speed of the burn. Maybe we should start creating these disturbances into every day cannons?
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Unread postAuthor: metalmeltr » Wed Nov 02, 2011 9:37 pm

I saw this as well. I intend to throw some random bs into my combustion and see what happens.
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Unread postAuthor: POLAND_SPUD » Wed Nov 02, 2011 9:39 pm

I think I've heard something about that here on SF.
Got a link to that episode ?
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Unread postAuthor: Mr.Tallahassee » Wed Nov 02, 2011 9:39 pm

I would say "crinkle" some wire and put it in there. That should easily do the trick.
By the way, I am a major Myth Busters fan.
PS, It's a new episode. It just aired today.
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Last edited by Mr.Tallahassee on Wed Nov 02, 2011 9:41 pm, edited 1 time in total.
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Unread postAuthor: mobile chernobyl » Wed Nov 02, 2011 9:39 pm

This has been a common finding in research related to deflagration to detonation transition in pipes, ducts, tube and other long items... Good to see the mythbusters at it proving it experimentally though! :D I'm sad I missed it, I'll have to see if it's on demand.

Edit - just so I don't say without showing... here's some previous DDT studies and a good "book" (more of a field book) on the concept.

Book
http://www.gexcon.com/handbook/GEXHBchap10.htm

Studies
http://www.lsc.phy.cam.ac.uk/research/ddt.shtml
(You'll have to google this url for it to work...) http://pbadupws.nrc.gov/docs/ML0717/ML071700076.pdf
http://www.hit.no/eng/HiT/Research/Rese ... cal-Safety

From this you can see why DDT is still something we don't 100% understand, It's something I've considered for a senior thesis since we have a combustion lab on campus! And professors willing to study it...
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Unread postAuthor: metalmeltr » Wed Nov 02, 2011 9:51 pm

This in a way goes back to earlier discussions of screen mesh in chamber similar to coal lanterns. Maybe it will help.
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Unread postAuthor: Mr.Tallahassee » Wed Nov 02, 2011 9:59 pm

This is the only link I could find. The "Drain Disaster Aftershow" actually has side by side footage of the small scale version.
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Unread postAuthor: mobile chernobyl » Wed Nov 02, 2011 10:11 pm

Mr.Tallahassee wrote:This is the only link I could find. The "Drain Disaster Aftershow" actually has side by side footage of the small scale version.


Cool... I love their high speed stuff, they did a great job on this one.
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Unread postAuthor: SpudBlaster15 » Wed Nov 02, 2011 10:19 pm

How large was the mixing fan in comparison to the test "chamber", and at what rate was it moving the gas? I'd assume that if the reaction proceeded slowly even with a fan present, the airflow was insufficient to generate the required turbulence to cause a transition to a faster combustion mode.

In a traditional atmospheric pressure air/fuel powered spudgun, a normal PC fan moves a ton of air relative to the volume of the chamber, which allows turbulent combustion to develop rather quickly. As a result, I don't expect that adding random objects to the inside of the chamber will do much other than provide additional surface area for heat loss to occur.
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Unread postAuthor: Moonbogg » Wed Nov 02, 2011 10:28 pm

It looks like we could totally use that to increase combustion speed. Although many combustions already have spark strips with multiple sparks and that would cause the entire flame to wrap around and speed up i would think.
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Unread postAuthor: Mr.Tallahassee » Wed Nov 02, 2011 10:28 pm

It was a relatively small fan but they let it run for two or so minutes straight so it may have done the same thing that a larger fan would do as to mixing the fuel and air. Either way, the debris boosted the burn rate quite drastically even with a well mixed chamber. This really requires a lot of testing that I am not capable of currently.
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Unread postAuthor: SpudBlaster15 » Wed Nov 02, 2011 10:41 pm

Mr.Tallahassee wrote:It was a relatively small fan but they let it run for two or so minutes straight so it may have done the same thing that a larger fan would do as to mixing the fuel and air.


In terms of mixing, yes, it may have worked as effectively as a larger fan running for a shorter duration, but the small fan would not move air at a rate sufficient to generate significant turbulence in the test chamber. Therefore, it's not a valid comparison with a fan-equipped combustion spudgun, which relies on a turbulent chamber flow to maximize the burn rate of the air/fuel mixture.
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Unread postAuthor: inonickname » Thu Nov 03, 2011 3:21 am

It works- the same thing is used in experimental PDE's (albeit with more reactive oxidants) to great effect.
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Unread postAuthor: MrCrowley » Thu Nov 03, 2011 3:33 am

I'm sure JSR would love to do some tests with the HS camera and some clear tube :wink:

If there's little merit for combustions equipped with fans, what about for hybrids where fans are unable to be used? Though, if it's significantly increasing the risk of DDT it might not be a good idea.

Edit: On reflection, I assume not. Multiple spark gaps didn't seem to make much difference when modelling my hybrid in HGDT (though this could have been effected by several variables). And of course, hybrids have valves.
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Last edited by MrCrowley on Fri Nov 04, 2011 8:34 pm, edited 2 times in total.
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Unread postAuthor: Goats spudz » Thu Nov 03, 2011 5:50 am

i watched that too the sewer explosion was a burn but they were aiming for a deflagration so uncoil a transformer or get lots of copper wire or wire and stuff up your combustion :D
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