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Oxy/fuel Mixture DDT Runup Distances

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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Oxy/fuel Mixture DDT Runup Distances

Unread postAuthor: DYI » Tue Apr 22, 2008 8:43 pm

I've got something of a handheld in the works, and I don't want what my username abbreviates to happen yet, so here's the question:

How does oxygen enrichment affect the DDT runup distances of gaseous hydrocarbon mixtures?

We all know that it will be decreased, but to what extent? As far as most of us know, DDT runup distance, assuming perfect mixing, no turbulence, and no restrictions, is about 35 feet in air/propane mixes. It's also been tentatively determined that increasing the mix does have an effect on the runup distance, but not a linear one, due to the decreased burn rates caused by higher preignition pressures.

I suspect that oxygen enrichment will have a much more pronounced effect on the runup distance of the mix, for the reasons of increased burn temperature (and therefore increased burn speed) and the fact that more fuel can be burned with a much lower preignition pressure, decreasing the dampening effect that protects traditional hybrids from detonation. Also, the nitrogen in air acts as a kind of buffer, as it undergoes very little change during the burn, and cannot actually support combustion. While the initially present nitrogen will remain in the chamber, it's percentage will steadily decrease as the mix increases, unlike in traditional air/propane mixes where it remains constant as the mix increases.

I believe that, for these reasons, DDT runup distance will be affected by the amount of fuel being burned much more strongly than a traditional hybrid is, because of the decreasing percentage of buffer gas, as well as the reduction of the preignition pressure dampening effect by a factor of roughly 5. I also believe that the runup distance in atmospheric pressure oxy/propane should be less than 8 feet in a 4" pipe. And if the roughly linear and direct relationship between combustion vessel diameter and DDT runup distance remains the same for oxygen enriched mixtures, this would suggest that the runup distance of oxy/propane in a 2" diameter cylinder is less than 4 feet. The vessel in my design is 27" long, and I'm beginning to suspect that even 2x oxy/propane mixes in it would be unneccesarily dangerous.

The only problem is, sealing becomes rather more difficult with the higher pressures required when using air as an oxidiser, and metering becomes less accurate, since the 0-15psig gauge cannot be used for oxidiser metering. Obviously safety trumps ease of use, but I'm wondering whether the two are indeed mutually exclusive in this situation.

Any thoughts?
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Unread postAuthor: D_Hall » Tue Apr 22, 2008 8:45 pm

Only comment: Runup distance is also affected by pipe diameter and ignition method so your 35' statement requires some sort of qualification.
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Unread postAuthor: DYI » Tue Apr 22, 2008 8:51 pm

I think I stated this somewhere, but the 35 feet is for 4" diameter pipe. I know I didn't state that the ignition method was a low energy high voltage electric spark, but now I did. This is also the ignition method I intend on using.
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Unread postAuthor: MrCrowley » Tue Apr 22, 2008 11:11 pm

I don't know a helluva lot 'bout this subject, but you should read <A HREF="http://209.85.173.104/search?q=cache:ZlgVykp4Q_4J:www.galcit.caltech.edu/EDL/publications/reprints/jannaf99_paper.pdf+deflagration+detonation+transition&hl=en&ct=clnk&cd=1&gl=nz">this.</a>

I linked it to D_Hall awhile back, I do remember it talking about oxygen mixtures and run-up distances, but I can't remember the scenarios.
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Unread postAuthor: jimmy101 » Wed Apr 23, 2008 2:04 pm

Wouldn't multiple sparks effectively reduce the chances of DDT? For each spark, and assuming the ammo isn't moving, the effective length of the chamber is reduced. A 40' long chamber with a spark every foot is not going to reach DDT since the chamber burns as if it is only 1' long?
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Unread postAuthor: DYI » Wed Apr 23, 2008 6:31 pm

Wouldn't multiple sparks effectively reduce the chances of DDT? For each spark, and assuming the ammo isn't moving, the effective length of the chamber is reduced. A 40' long chamber with a spark every foot is not going to reach DDT since the chamber burns as if it is only 1' long?


That's what I would assume. However, the chamber in question only has one gap. When I said the runup distance was 35', I meant that the flame front had to proceed through 35' of the un-ignited gas mixture before it reached detonation.
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Unread postAuthor: boilingleadbath » Sat Apr 26, 2008 3:48 pm

How much does the lack of nitrogen (or other buffer gas) affect the run-up distance?

A lot. The propane-O2/run-up-distance/spark-ignition/pipe research I remember seeing was indicating one or two pipe diameters for a C3H8-O2 mixture. (centered ignition)

I think. I'd have to check to be sure.
I think I linked to it before. Feel free to search my post history for "O2 run" or something like that...
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Unread postAuthor: DYI » Sat Apr 26, 2008 4:58 pm

A lot. The propane-O2/run-up-distance/spark-ignition/pipe research I remember seeing was indicating one or two pipe diameters for a C3H8-O2 mixture. (centered ignition)


:shock: Uh oh... I've been running my 4" x 12" steel hybrid chamber at 3x with oxy/propane, and there's only one ignition point at the back of the chamber. I guess the nitrogen that was already in the chamber saved it from detonating.

I'll be trying the same thing at 4x tomorrow, and if it explodes, we'll have gained some valuable insight. This makes me wonder though, SB15's oxy/MAPP hybrid was a lot longer and skinier than the chamber on my hybrid, and he actually flushed the chamber out with oxygen before shooting. Seems like it should have detonated.
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Unread postAuthor: Fnord » Sat Apr 26, 2008 5:43 pm

You'll probably see stress cracks and deformities before it actually "explodes". I doubt there's enough energy density in your chamber to make it go off like a pipe bomb.

Still, I wouldn't want to be standing by it.
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Unread postAuthor: SpudFarm » Sat Apr 26, 2008 6:14 pm

_Fnord wrote:You'll probably see stress cracks and deformities before it actually "explodes". I doubt there's enough energy density in your chamber to make it go off like a pipe bomb.

Still, I wouldn't want to be standing by it.


would those cracks be showed if you put 150psi in it and dump the chamber in water?
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