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Rich Oxygen/Fuel Mixes

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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Rich Oxygen/Fuel Mixes

Unread postAuthor: DYI » Mon Oct 17, 2011 5:06 pm

Looking at GasEq adiabatic combustion calculations, a mix of 2.2 C<sub>3</sub>H<sub>6</sub> + 5O<sub>2</sub> will yield a deflagration pressure of ~23*p<sub>pre-ignition</sub>. The optimal ratio varies somewhat with pressure, but the point is as follows:

0.91 : 5 ("stoichiometric")
Pressure multiplication factor: ~17
Sound speed: ~1200m/s

2.2 : 5
Pressure multiplication factor: ~23
Sound speed: ~1450m/s

(For comparison, the sound speed in an air/propane mix immediately after deflagration is approximately 950m/s).

The rich combustion produces large quantities of CO and H<sub>2</sub>, whereas the "stoichiometric" mix produces chiefly water and carbon dioxide. This explains the higher sound speeds, and is certainly a better use of the elements available. Another useful characteristic of this mix is that oxygen/alkene mixes are, in general, more difficult to detonate as they get richer. Detonation being the major concern that it is for us, more hobbyists might be willing to work with oxygen/fuel mixes such as this one, where the ignition energy is higher and there is less concern over chamber geometry. This mix, in theory at least, performs so well that it approaches air/propylene in psi/X performance, despite the fact that an air/propylene mix has a starting pressure over three times as high.

There is one disadvantage which is immediately apparent to me, and a few which could make the implementation problematic or even impractical;

- The maximum mix achievable is only a little over 100X, for roughly 11kpsi. Considering that the first and last 100X+ hybrid was built over three years ago, this doesn't appear to be an immediately pressing problem.

- Soot buildup may be an issue if it was too excessive. This would have to be determined through testing.

- One member suggested in the thread where this idea came up that the flame propagation rate may be too slow to be useful. I doubt this very much, but testing would clarify that point as well.

- DDT of this mix would, of course, have to be characterized. As an addendum to my comment in one of the above paragraphs, it's not likely that this mix would be as nearly totally immune to DDT as air/propane, but it'd certainly be better than typical oxygen/fuel mixes which have seen use around here for the past few years.

- Finally, the mix may turn out to perform poorly for one reason or other in an actual launcher situation, which is considerably different from what GasEq does calculations for.


If performance is as high as these preliminary calculations indicate, it could be very useful to all hybrid builders - lower starting pressures mean cheaper fueling systems and fewer issues with leaks, and both gases required are quite readily available. A fueling pressure of only ~450psi could provide HyGaC20-level performance and even higher maximum projectile speeds, with much less expense on gauges and fuel lines. There are certainly mixes with much higher sound speeds or better pressure multiplication ratios, or perhaps even both. This one, however, struck me as being highly applicable by just about anyone, with very little extra cost. If it works out, it could provide quite a boost in typical hybrid performance.


If anyone is interested in its potential, I may run some preliminary tests with it on the Christmas break to at least partially resolve some of the issues noted above (I won't be able to measure combustion pressure waveforms directlyyet, but everything else is just a matter of measuring speeds, running DDT runup distance tests, and inspecting the chamber and barrel).
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Last edited by DYI on Tue Oct 18, 2011 7:31 am, edited 1 time in total.
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Unread postAuthor: saefroch » Mon Oct 17, 2011 6:30 pm

I am VERY interested in this topic. I read an article a while ago that mentioned a 45% fuel efficiency increase by using laser spark plugs to ignite heavy methaine:air mixes. You could also probably get away from greenhouse gases and leave the carbon as a solid... But that's a different topic.
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Unread postAuthor: Fnord » Mon Oct 17, 2011 6:34 pm

Have you spoken to SB15 about this? It seems like something that could be tested quite easily with his existing hybrid platform.
Although I don't have the combustion/math skill to back it up, I don't see why this wouldn't work so long as the mix was reasonably homogenous.

Soot, I wouldn't worry about too much. Given the expense of oxygen vs. air I doubt you'd be doing more than a few dozen shots within the lifespan of a typical launcher.

You'd think flame front propagation wouldn't have much of an effect, assuming the valve remained closed until near-maximum pressure was reached... Playing with HGDT seems to indicate heat losses are less of a concern with mid-to-high mixes.

As to DDT, I'd probably leave one atmosphere of buffer gas in to be safe. E.g. the burnt gasses from the previous shot.
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Unread postAuthor: DYI » Mon Oct 17, 2011 7:01 pm

I was planning to leave one atmosphere of buffer in there anyway during my testing - it's difficult to fully purge it without a chamber design different from mine, and it only makes a big dent in post-combustion sound speed in mixes too low to approach that speed.

SB15's hybrid platform would be well suited to testing the actual performance improvements over a standard oxygen/fuel mix. As to the "high speed" end of things, I was thinking of testing at about 55X using my miniature hybrid chamber (I don't have an oxygen tank at the moment, so I'm limited by the starting pressure of the Bernzomatic disposable tanks).

Saefroch: I'd take that paper with several very large grains of salt - this mix actually decreases fuel efficiency, it's just optimized for spudgun use where fuel is practically free and the pressure increase and sound speed are the main elements of concern. Please expand on what you were saying about "leaving the carbon as a solid", I feel like I'm missing something.
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Unread postAuthor: saefroch » Mon Oct 17, 2011 7:14 pm

Soot can exists as a suspension in a gas, with little bits of solid carbon. These can be filtered out, and removed. Therefore, one can in theory burn hydrocarbons in very heavy mixes and leave <b>most</b> of the carbon still sequestered as solid, not as greenhouse gases or pollutants.
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Unread postAuthor: MrCrowley » Mon Oct 17, 2011 9:25 pm

I'm interested. Can't really add much else to the topic so I'll just ramble on until The Forum lets me submit this reply.
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Re: Rich Oxygen/Fuel Mixes

Unread postAuthor: SpudBlaster15 » Thu Oct 20, 2011 7:23 pm

DYI wrote:DDT of this mix would, of course, have to be characterized. As an addendum to my comment in one of the above paragraphs, it's not likely that this mix would be as nearly totally immune to DDT as air/propane, but it'd certainly be better than typical oxygen/fuel mixes which have seen use around here for the past few years.


You may be interested in this paper, which gives experimental results for the DDT run up distance of propane/oxygen mixtures at rich, lean and stoichiometric ratios in tubes of differing diameters.

If I'm reading the first graph correctly, it appears that the DDT run up distance is minimized at slightly rich ratios, especially in small diameter tubes.

That said, the mixture you described in the OP is so oxidizer deficient that it may not even deflagrate efficiently, never mind reach DDT in anything resembling a standard pressure vessel, so detonation is not likely to be an issue.

As far as performance is concerned, I'm not entirely sure what to expect here. Under adiabatic conditions, the increased molar quantity and decreased heat capacity of the products certainly looks favorable, but in practice the slower reaction rate may cause excessive heat to be lost to the chamber walls, thus lowering internal pressures. My testing with 40% oxygen hybrid mixes seemed to indicate that any significant excess of fuel resulted in a failure of the valve to open, however this may not be an accurate reflection of the behaviour of higher mixes with no buffer gas present.

Looking forward to the testing; I may also throw together a similar experimental rig later in the year if time permits.
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