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Combustion containment

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Combustion containment

Unread postAuthor: Moonbogg » Sun May 24, 2009 11:33 am

Quick question here...I was thinking about getting a camlock plug and doing some combustion containment with a pressure gage connected to the chamber for testing purposes. I was going to test 1x and 2x contained combustions, possibly 3x. I was wondering if the heat inside the chamber would become insane enough to melt the chamber fan, spark strip etc. Does the heat dissipate like it does when not contained? Or will the pressure remain for a few moments along with the intense heat?
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Unread postAuthor: Carlman » Sun May 24, 2009 11:50 am

There has been some testing like this performed and posted maybe on burntlatke. (need confirmation from someone)
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Unread postAuthor: jackssmirkingrevenge » Sun May 24, 2009 12:01 pm

Carlman wrote:(need confirmation from someone)


Almost ;)
thank jimmy101
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Unread postAuthor: Carlman » Sun May 24, 2009 12:24 pm

jackssmirkingrevenge wrote:
Carlman wrote:(need confirmation from someone)


Almost ;)
thank jimmy101


Trust you to stick ya head in the door :D Thanks mate
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Unread postAuthor: jackssmirkingrevenge » Sun May 24, 2009 12:28 pm

I have a reputation as an annoying pedant who spend time at his PC dreaming up unlikely designs when he should be building them to maintain ;)
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Unread postAuthor: Brian the brain » Sun May 24, 2009 3:21 pm

Is any of the above not true?

Well, Moonbogg, seems you don't have to do it all yourself...
The benefits of a forum that has been running for years...
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: jimmy101 » Sun May 24, 2009 6:16 pm

Moondog:

Go for it. More data is always good. Heck, in the world of spudguns, data is a rarity.

As to your questions:

The pressure won't last long at all. Not long enough for you to see anything on the gauge by eye, other than that the needle moved. I used a generic digital camera in it's crappy video mode. Works OK and you can measure how quickly the chamber pressure drops. It drops pretty fast, within a second or so you're back to about ambient conditions. Getting the peak pressure is tricky since the temperature drops so fast.

A peak reading pressure gauge (either a cheapo "pen" type tire gauge or an engine cylinder compression testing gauge) has been discussed and tried a few times. The problem is usually that the gauges just can't respond fast enough. If they do respond fast enough then they tend to pretty grossly overshoot. Or, they leak like a sieve.

The temperature is pretty high, for 1X it is over 2200C (~4000F). But the mass of the gasses is so small compared to the mass of the chamber (fan, spark strip...) that the actual temperature rise of the chamber is pretty minimal. The chamber won't melt, nor will the fan, spark strip etc. Indeed their temperature will ony rise by several degrees C for 1X. Figure twice as much for 2X and so on.
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Unread postAuthor: Technician1002 » Sun May 24, 2009 8:22 pm

jimmy101 wrote:The pressure won't last long at all. Not long enough for you to see
The temperature is pretty high, for 1X it is over 2200C (~4000F). But the mass of the gasses is so small compared to the mass of the chamber (fan, spark strip...) that the actual temperature rise of the chamber is pretty minimal. The chamber won't melt, nor will the fan, spark strip etc. Indeed their temperature will ony rise by several degrees C for 1X. Figure twice as much for 2X and so on.


Thin small mass items may see higher temperatures. The curled edge of the label peeling off the fan may become scorched for example.
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Last edited by Technician1002 on Mon May 25, 2009 1:57 am, edited 1 time in total.
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Unread postAuthor: Moonbogg » Sun May 24, 2009 8:35 pm

Good feedback guys, thanks. I'll do some burst disc testing and get some fully contained shots in there...how I will get an accurate pressure reading is beyond me. Sounds like it requires expensive equipment to get a good reading.
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Unread postAuthor: Technician1002 » Sun May 24, 2009 9:29 pm

Moonbogg wrote:Good feedback guys, thanks. I'll do some burst disc testing and get some fully contained shots in there...how I will get an accurate pressure reading is beyond me. Sounds like it requires expensive equipment to get a good reading.


Many times accurate pressure testing is done with burst disks. Use a series of small ones that you know the calibrated failure of and with many tests find the pressure that last one fails.

Another variant is disk deformation. A test disk is deformed (domed) by the pressure and the pressure to reproduce the dent is the pressure that caused it. It is similar to the bite sticks used to check shark bite strength.
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Unread postAuthor: starman » Mon May 25, 2009 12:20 am

If you can get some good data for 1x you should be able to extrapolate that out for 2x and 3x...not sure it's really necessary to test those unless you just want to. Double the burst disk layer for 2x, triple them for 3x.

I did my burst disk testing pneumatically, pumping air in until it bursts and videoing a pressure gauge. Jimmy got 90-100 psi-ish peak (possibly 15 psi higher) performing combustion tests.
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Unread postAuthor: Moonbogg » Mon May 25, 2009 1:51 am

starman wrote:If you can get some good data for 1x you should be able to extrapolate that out for 2x and 3x...not sure it's really necessary to test those unless you just want to. Double the burst disk layer for 2x, triple them for 3x.

I did my burst disk testing pneumatically, pumping air in until it bursts and videoing a pressure gauge. Jimmy got 90-100 psi-ish peak (possibly 15 psi higher) performing combustion tests.


I just did that tonight with a 125psi air compressor. I got right to around 120psi and BOOM! It let loose. I think this cannon would make a pretty powerful burst disc pneumatic as it has a lot of volume and no dead space. I bet I could even install a really small and simple hole poking thingy inside to actually be able to fire the cannon pneumatically without having to fill it until it blows.
I will do more testing later for fun and use your burst disc advice Starman. Oh, at mc master I found some aluminum foil discs that come in various diameters. They are adhesive backed and would probably be excellent burst discs. Just peel and stick. It would also be easy to layer them up as well. Oh, did I mention they are cheap? Like $20.00 for 100 or something like that.
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