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Theory on Fast Combustions

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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Theory on Fast Combustions

Unread postAuthor: noname » Tue Sep 04, 2007 7:44 pm

I just had this random idea, but since only H2O is left over from a hydrogen combustion, if you could meter in the stoichiometric volumes of air or oxygen and hydrogen, you could create a combustion gun that fires every second or so, or as long as it takes for you to fuel the gun. Using Gort's 1 touch meter device, or something similar, you could end up with a very high ROF. Of course, the chamber would need to be steel (maybe even Sch 80), and you'd need a feed system, but the idea is feasible. There's no way I'm going to make it happen, but I'm sure one of you combustion junkies could.
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Unread postAuthor: thespeedycicada » Tue Sep 04, 2007 8:18 pm

that should work but there mignt be to much H2O left over after a while so you would have to empty it but im not toatally sure.
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Unread postAuthor: Fnord » Tue Sep 04, 2007 8:22 pm

I'm not sure if this will be a problem, but air will be sucked back into the chamber after combustion takes place and mess with the mix, so you migth waste a bit of fuel before each shot.

Anyone got the explosive limits of hydrogen in pure oxygen?
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Unread postAuthor: iPaintball » Tue Sep 04, 2007 8:30 pm

Uhh, doesn't a stoichiometric mix of hydrogen and air detonate?
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Unread postAuthor: Velocity » Tue Sep 04, 2007 8:33 pm

Hydrogen = scary stuff
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Unread postAuthor: dongfang » Wed Sep 05, 2007 3:13 am

Hi,

My comments: Combustion products left in the chamber don't matter, as long as you get enough fuel and oxidizer into it when reloading. Whether the remaining gas in there is nitrogen, or it is CO_2 and H_2O has little significance..

Hydrogen isn't all that scary, really. In fact, it takes up a lot of space per joule of combustion energy. A good oxygen-hydrogen mixture will have less energy than the same volume of oxygen-propane mixture at the same pressure. Try it out in GasEq. Hydrogen could be interesting as a cold gas in a pneumatic because of its viscosity and sonic speed. As a fuel, it isn't of much value really.

For a repeating air gun, maybe somebody could try underdimensioning the chamber, and placing a large, fast check valve in it (flap valve). When the gun is fired, the inertia of the gas in the barrel will suck some fresh air into the chamber... ideally, this should start right after the projectile has left the barrel, but it might work better if it starts a little sooner??
The check valve, or the gunner, also controls a second valve that injects some fuel.
The thing becomes a simple pulse jet, except that other pulse jets don't launch potatoes.

This kind of simple check valve pulse jets were used in the German V1 cruise missiles in WW2. Worked fine, and were cheap.

Regards
Soren
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Unread postAuthor: jimmy101 » Wed Sep 05, 2007 9:10 am

I don't see how this differs substantially from the oft discussed metering of proapne (or butane or ...) and air (or oxygen).

It is true that hydrogen won't produce CO<sub>2</sub> but so what? The CO<sub>2</sub> doesn't do anything in subsequent shots, and you still need to get the oxygen needed for the next shot into the gun. So, there is no difference between the refueling of a chamber containing CO<sub>2</sub>+N<sub>2</sub>+H<sub>2</sub>O and one containing just N2+H2O.

Some stuff from the SpudWiki page on fuels.
1. Hydrogen burns much faster than do hydrocarbons like propane. The laminar flame speed of H<sub>2</sub> is 3.2m/s versus propane's 0.4m/s.
2. Theoretically, hydrogen produces about 15% more energy per volume of oxygen than does propane, but the peak pressure from Hydrogen is often quoted as being less than for propane by about 1 ATM.
3. Hydrogen has a very wide range of combustibility; 4% to 74% versus propane's 2.4% to 9.5%.

IIRC, because of hydrogen's very high burn rate there is an increased risk of DDT.

I doubt that the water formed by combustion would be much of a problem. A fair amount of the water (and the CO<sub>2</sub>) would be blown out of the gun each time it is fired, to be replaced by fresh air.

The trickiest part of a semi-auto that provides oxygen for each shot is taking into account the amount of oxygen that would be sucked back into the chamber after the preceeding shot. I wouldn't think the amount of oxygen that needs to be injected wouldn't be anywhere near the stoichiometric amount.
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Unread postAuthor: noname » Wed Sep 05, 2007 9:19 pm

The only real point I meant to bring up was that there would be no need for venting the chamber after firing, you could just keep metering in the hydrogen mixture and firing. Yeah, DDT, whatever. Sch 80 steel would be able to hold it for at least the amount of time it would take to see if the idea would work or not.
I already said, this idea is not perfect, nor am I ever going to make it.
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Unread postAuthor: jimmy101 » Thu Sep 06, 2007 10:14 am

If you are adding oxygen (or air) via an injection system than there is really no advantage (or disadvantage) to using hydrogen versus any other gaseous fuel. So, no reason to be using hydrogen since propane is cheaper and easier to get.
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