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Hydrogen Hybrid?

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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Hydrogen Hybrid?

Unread postAuthor: Alster370 » Thu Jun 02, 2011 4:15 am

Im sure I read somewhere that given enough pressure, a Hydrogen/Oxygen mix can ignite itself. Would this pose a significant danger if one wanted to build a hydrogen hybrid with mixes of 5x+? Ive been looking but there seems to be no accurate pressures at which it could happen. (Im assuming there are many factors that have to be taken into account).

Update:

Just found out a 4:1 - 5:1 mix will prevent an oxidizing flame whilst still leaving the mix combustible by manual ignition.

http://en.wikipedia.org/wiki/Oxyhydrogen
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Last edited by Alster370 on Thu Jun 02, 2011 8:26 am, edited 1 time in total.
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Unread postAuthor: DYI » Thu Jun 02, 2011 8:22 am

Threads from forums where "HHO" is an accepted term are not "relevant" to anything, ever. Thank you for not using it here.

Utron uses oxygen/hydrogen/buffer (where the buffer is often simply excess hydrogen) mixes at up to ~10000psi pre-ignition. While these are not pure, stoichiometric oxygen/hydrogen fuel mixes, they are at extremely high pressure compared to what you're likely to be working with. Further, the Space Shuttle's liquid fueled rocket engines also run oxygen/hydrogen, mixing high pressure gaseous hydrogen with liquid oxygen. They still need an ignition system.

Hydrogen is an excellent fuel choice, and also an excellent buffer gas for hybrid mixes (surpassed by helium in that regard for most applications). The use of rich mixes not only decreases chances of auto-ignition, it reduces likelihood of detonations and significantly improves sound speed in the propellant gases.
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Unread postAuthor: Heimo » Thu Jun 02, 2011 1:17 pm

DYI wrote:Threads from forums where "HHO" is an accepted term are not "relevant" to anything, ever. Thank you for not using it here.



1+ on that...

but here is what I am worried about, won't the combustion chamber be susceptible to hydrogen embrittlement?? would this be cause for concern in this applecation, or am I being paranoid...
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Unread postAuthor: Labtecpower » Thu Jun 02, 2011 2:14 pm

Low tensile strength steel isn't susceptible to hydrogen embrittlement.

I was also thinking about a hydrogen/oxygen hybrid. I will first have to build myself a better electrolysis cell.

What power gain would it have over a conventional air/propane hybrid? the gas itself has a far higher burning speed and SOS, but a stochiometric mixture of oxygen and hydrogen will only produce water (probably as steam) as a reaction product. Will it give enough pressure to push a projectile down a barrel of about 3 m?
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Unread postAuthor: Alster370 » Thu Jun 02, 2011 3:24 pm

I may be in a little over my head here anyway, Im only just covered fuel metering and such. I might just try a combustion for now, to keep things simple as I doubt im going to accomplish anything other than injuring myself if I attempt a Piston hybrid. Now I imagine copper is not suitable for a hydrogen combustion due to hydrogen embrittlement? Also would i keep the Hyd/Oxg ratio 2:1 for the most pressure?

thanks
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Unread postAuthor: DYI » Thu Jun 02, 2011 5:04 pm

Won't the combustion chamber be susceptible to hydrogen embrittlement? Would this be cause for concern in this application, or am I being paranoid...


In some cases, it could be. Stainless (not usually austentitic stainless though) and high strength, low alloy steels are quite susceptible, as are many nickel or titanium based alloys. As Labtec mentioned, lower strength, softer steels are not so badly affected.



What power gain would it have over a conventional air/propane hybrid?


For a given quantity of oxygen (which is what we typically measure the "mix number" by), propane mixes will produce higher pressures. However, the low propellant gas SOS in propane/air hybrids will start to cut back efficiency as you exceed 800m/s or so, and much higher than that they become less useful. Compare to 1550 m/s for 2:1 hydrogen/oxygen and 1870m/s for 4:1, which produces similar pressure per X and is less prone to detonation (and accidental ignition, of course).

If you're trying to compare 2:1 hydrogen/oxygen to stoichiometric air/propane at the same starting pressure, the hydrogen/oxygen mix will produce very slightly higher pressure than air/propane, and for 4:1 the pressure will be almost exactly the same.

Will it give enough pressure to push a projectile down a barrel of about 3 m?


How long is a piece of string? :P



Now I imagine copper is not suitable for a hydrogen combustion due to hydrogen embrittlement?


Copper doesn't suffer from conventional embrittlement, but it can be attacked by hot hydrogen if there is sufficient oxygen content in the copper. I don't think this would be of particular concern in your application, especially when using a 2:1 ratio.

Also would I keep the Hyd/Oxg ratio 2:1 for the most pressure?


I must admit to experiencing a moment of confusion upon reading that, what with the amount of time I seem to spend trying to achieve the opposite situation... The answer is that you'll actually want about 2.3:1 for the highest pressure. Also, I highly recommend downloading GasEq. It's free software, and would allow you to investigate many of these queries for yourself.
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Unread postAuthor: ramses » Thu Jun 02, 2011 9:24 pm

Did we ever reach consensus on if it is safe to compress an stoichiometric mix of oxygen and hydrogen with a shock pump? I think it's safe to assume that a fridge compressor or other mechanical pump would be a bad idea.
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Unread postAuthor: Alster370 » Fri Jun 03, 2011 4:23 am

I might go with 4:1 or 5:1 then, Id rather have more safety than pressure this time round.

Also, I highly recommend downloading GasEq


I downloaded it recently, Although I will say it does confuse me a little :?
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Unread postAuthor: DYI » Fri Jun 03, 2011 8:01 am

...or 5:1...


Considering the UEL of hydrogen in air is 75% (5% oxygen in the mix), you shouldn't have any trouble igniting an 83% hydrogen mix with 17% oxygen. The pre-ignition pressure will be getting pretty high here though - at 18.5 psi/X (including the oxygen), you'd better have a high pressure hydrogen source handy.
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Unread postAuthor: Alster370 » Fri Jun 03, 2011 10:26 am

A well built Electrolysis cell could produce 18.5 psi of hydrogen right? :?
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Unread postAuthor: ramses » Fri Jun 03, 2011 3:34 pm

well, you'd probably have to compress it. Thus my question.
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Unread postAuthor: Alster370 » Fri Jun 03, 2011 3:39 pm

I assumed that the pressure would build inside the chamber, as the unit would continue to produce hydrogen. It wouldn't stop producing hydrogen because there was pressure in the air above the water? would it?
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Unread postAuthor: ramses » Fri Jun 03, 2011 4:02 pm

pressure would build, and some hydrogen and oxygen would dissolve in the water. It is worth noting that you would get "wet" hydrogen and oxygen in your chamber, as opposed to "dry" gasses. This makes detonation less likely, but would reduce the temperature and pressure of the gas in your chamber. You'd have to collect the hydrogen and oxygen separately, and somehow dispose of the pressurized oxygen if you wanted anything but a 2:1 ratio.
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Unread postAuthor: Fnord » Fri Jun 03, 2011 4:21 pm

I'd think any water vapor present would act as a buffer gas, turning to steam during ignition. Of course, high concentrations may be counterproductive (I know from experience they are with propane and acetylene), but some experimentation would be necessary to see what levels you actually end up with in practice.

Also, Without doing any research at all, I remember seeing a few electrolysis rigs blowing up on youtube without an apparent source of ignition energy. Any thoughts on this?
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Unread postAuthor: ramses » Fri Jun 03, 2011 5:34 pm

Fnord wrote:I'd think any water vapor present would act as a buffer gas, turning to steam during ignition. Of course, high concentrations may be counterproductive (I know from experience they are with propane and acetylene), but some experimentation would be necessary to see what levels you actually end up with in practice.

Also, Without doing any research at all, I remember seeing a few electrolysis rigs blowing up on youtube without an apparent source of ignition energy. Any thoughts on this?


according to gaseq, if about 1/7 of the moles of gas is water vapor in a stoich mix of h2 and 02, pressure will drop from 9.56 bar to 9 bar and SoS will drop from 1553 m/s to 1450 m/s. A 4:1 ratio has about the same pressure drop, and a SoS drop of about 200m/s.

I remember seeing videos (can't find them again) comparing igniting wet hydrogen from an electrolysis cell and the ignition of dry hydrogen from a bottle. The wet stuff had a dull thump at best, but the dry gas had a sharp crack.

As to the spontaneous ignition, there are a number of possible causes. Loose electrodes causing sparks, floating debris shorting electrodes, downstream ignition and flashback.
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