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What are your thoughts on this ratio?

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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What are your thoughts on this ratio?

Unread postAuthor: VRietscha » Mon Apr 02, 2007 9:33 am

I'm thinking about upping the ratio on my propane combustion gun. I currently just run a 1x ratio. I was wondering if you thought it would be safe to use a 1.5x or 2x ratio but, here's the kicker... I was going to use O2 for the air part. Just a little spritz of pure O2 in the ratio so that I can bump up my propane ratio. I am using SCH 40 4" fittings w/ SCH 40 fittings sleeved inside to make this one bulky double walled gun so I'm assuming that the gun will be strong enough. I just want to check with you guys before I blow the gun up. I will remote detonate it anyways but I just about have it done and don't want to see my work go up in schrapnel.
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Unread postAuthor: cerberus » Mon Apr 02, 2007 9:40 am

Things get interesting when you start playing with oxygen. Not only will the explosion be much stronger but it will also be a lot hotter (almost goes without saying). The chamber volume ratio doesn't matter so much in this case as the diameter ratio. Make sure that you keep the diameter ratio at 2:1 at the absolute most and I'd use an externel bevel on the cutter so the potato isn't too tightly fitted within the barrel (if it can escape so can the exhaust gasses and you're less likely to end up with a pipe bomb). Be careful and stick with the remote detonation.
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Unread postAuthor: boilingleadbath » Mon Apr 02, 2007 6:36 pm

1) Gaseq shows the following results for oxygen enriched air...
http://img.photobucket.com/albums/v611/ ... dTvsOx.png

The axis with large numbers (pink line) is temperature in kelvin, the blue line is pressure in bars.
The X axis is the percent oxygen in the mixture... 20% is normal.
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Unread postAuthor: paaiyan » Mon Apr 02, 2007 6:50 pm

Does that 100% on the graph mean that you're combusting pure oxygen? If so then I think the graph is pretty much useless for the purposes of combusting propane and oxygen together. Other than showing that it will make a bigger boom of course. I think what we really need is a graph that shows % oxygen and propane present.

EDIT: C3H8 + 5 O2 → 3 CO2 + 4 H2O + heat represents the molecular equation for the combustion of propane and oxygen. Assuming you completely fill the chamber with pure samples of both gasses, the correct ratio would be 16.6% propane and 83.3% oxygen.

Do however keep in mind that propane produces co2 when burned clean. When not enough oxygen is present it produces carbon monoxide.

Carbon monoxide is a significantly toxic gas with poisoning being the most common type of fatal poisoning in many countries.[13] Symptoms of mild poisoning include headaches and flu-like effects; larger exposures can lead to significant toxicity of the central nervous system and heart. Following poisoning, long-term sequelae often occur. Carbon monoxide can also have severe effects on the fetus of a pregnant woman. (per wikipedia)

Just be safe
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Unread postAuthor: boilingleadbath » Tue Apr 03, 2007 4:44 pm

Eh... I'm not stupid, and gaseq probably would give me an error message if I tried to burn O2 all by it's self.

...I meant that the percent was <b>the percent O2 in the</b> air <b>mixture</b>; 100% represents pure O2 and propane, and 0% represents pure N2... and propane.

Gaseq, by the way, wouldn't give me an answear if I used "air" with only 1.5% (or so) oxygen.

And it's NOT useless; it gives you an idea <i>how much</i> extra performance you should expect.
Not an exact number, mind you, as the increased burn rate complicates things and combustion guns are not a simple system to start with, but it's better than your previous guess might have been. (I was expecting a curve looking something like "y = sqrt(x)", for the pressures - not the roughly linear thing that we got.)
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