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hydrogen power

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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hydrogen power

Unread postAuthor: rp181 » Fri Dec 21, 2007 3:41 pm

is there any formula/way to find out the psi burning h2 and o2, ignitions point in the middle of the chamber?

i made my first spud gun, works but i wanna make sure the PVC wont fail after repeated use. The fastes ile be shooting is a shot every ~3min.
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Unread postAuthor: LikimysCrotchus5 » Fri Dec 21, 2007 3:45 pm

Umm o dont reccommend using a h2 and o2 mix in pvc, unless you enjoi shrapnel in you face whizzing by you. You can do it, but just with metal.

P.S. idk what you mean with "ignitions point in the middle of the chamber.

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Unread postAuthor: paaiyan » Fri Dec 21, 2007 4:05 pm

It's been a well-known fact on this site that PVC and H2/O2 don't mix. Maybe he wants to know the max PSI of the combustion if the ignitor is in the middle of the chamber? I really don't have a clue.

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Unread postAuthor: daberno123 » Fri Dec 21, 2007 4:08 pm

For some reason i think he thinks that the placement of the ignitor in the middle would give a different psi than say in the front or back. i don't think it makes a difference.

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Unread postAuthor: paaiyan » Fri Dec 21, 2007 4:09 pm

Well, if the placemetn of the ignitor really does make a difference in cannon efficiency, it might well make a difference in the peak pressure. Suffice to say that either way, the peak pressure is too much for PVC.
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Unread postAuthor: LikimysCrotchus5 » Fri Dec 21, 2007 4:10 pm

hey you give me some time and ill be there :D
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Unread postAuthor: paaiyan » Fri Dec 21, 2007 4:12 pm

Yea I can't really figure out the peak pressure of the H2O2 mix because I don't know what the temperature of the products would be. If anyone knows that I can figure it out. Well, I guess if you can figure that out you can probably find the peak pressure too.

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Unread postAuthor: rp181 » Fri Dec 21, 2007 4:16 pm

first of, its a very small gun, can hardly shoot a battery :)

on spud wiki, it said placeing the ignitor in the middle would have the potential to have twice the flame front speed, as it burns forward and backward. would that not effect the psi?

ile just try several shots away from the gun
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Unread postAuthor: paaiyan » Fri Dec 21, 2007 4:26 pm

The placement of the ignitor is the subject of many debates. The fact is, nobody really knows one way or the other, and most of us don't really care.
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Unread postAuthor: fred » Sat Dec 22, 2007 3:45 am

I built 2 hybrids both the exact same dimensions and fittings. One with the spark in the back of the chamber and one with the spark in the middle. With the back placed spark I got a number of misfires. The one in the middled fired every time. This is only what I have found. I also use a spark plug and bbq ign. Never had a problem with that. I did open the spark gap to about 30 thou. though.
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Unread postAuthor: jimmy101 » Sat Dec 22, 2007 2:30 pm

fred wrote:I built 2 hybrids both the exact same dimensions and fittings. One with the spark in the back of the chamber and one with the spark in the middle. With the back placed spark I got a number of misfires.

That suggests that you don't have a chamber fan.

Back to the OP, H<sub>2</sub> in air, according to GasEq , P<sub>max</sub> of 8.0 ATM at 2767K. That is a bit less than propane in air. Pure H<sub>2</sub> plus pure O<sub>2</sub> gives 9.6 ATM at 3600K.

Both of those peak pressures are within the limits of pressure rated PVC.

But, Hydrogen burns considerably faster than propane and hydrogen is much more likely to reach DDT than propane.

Hydrogen only gives a little more total energy in the chamber so it is probably not worth the effort.
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Unread postAuthor: rp181 » Sat Dec 22, 2007 8:17 pm

before i started making the gun, i tried hydrogen vs propane, and hydrogen seemed much more power full, though that could be due to insufficient ration of propane to air.

im mainly using h2,o2 because its already at the perfect ratio, easy to ignite, clean burning, easy to get, and nothing else is needed, like a propane meter.

thanks for you help.
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Unread postAuthor: BigGrib » Sat Dec 22, 2007 9:02 pm

we are veering away from the point that this big dummy is using hydrogen to power his spud gun made out of pvc, you're gonna blow your ass up guy
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Unread postAuthor: Novacastrian » Sat Dec 22, 2007 9:30 pm

BigGrib wrote:we are veering away from the point that this big dummy is using hydrogen to power his spud gun made out of pvc, you're gonna blow your ass up guy


Holy Crap, what mixes do you plan on taking this to, hopefully not over 1.5x.
Even then :bom: Ask guys that use pvc Hybrids and they will tell you that they don't feel safe around one (most of them anyway)

I agree with BigGrib.
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Unread postAuthor: Stormy_2021 » Sun Dec 23, 2007 1:27 am

I don't know a whole lot, but I would assume if you know the heat of formation for H2, O2, and H20, you could figure out the temp. Then I suppose if you used the ideal gas law or something like that you could figure it out.
I'm on break and I really don't feel like doing chemistry homework, but heres the formula for combustion of H and O. Ideal gas law is this.
PV=nRT, n is the number of mols of product, R is .08206, T would be temperature, P would be pressure in atmospheres, and V would be volume. I'm not sure abou this, but it might work.

2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ/mol

This is in no way a recommendation for combusting hydrogen. That crap is one of the most reactive elements out there.
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