Avoiding Detonation

Postby SPG » Tue Oct 10, 2006 9:40 am

I guess it was D Hall's posts on improving hybrids that set me thinking last night. We've often heard that conditions can move propane from the deflagration stage to a detonation stage, and talk of ever more advanced and powerful hybrids always makes me worry that one will go boom.

So what should we do to avoid detonation, what exactly brings it about? And how can we eek as much power out of combusting propane without it going off in our faces?
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Postby D_Hall » Tue Oct 10, 2006 10:56 am

I'd have to double check my readings but...

IIRC, in order to sustain a detonation a vessel must be of a certain size or larger. The size varies for each fuel/oxidizer combination and is dependent upon the shockwave structure characteristic of that combination. That said, at 1 atm the critical diameter for propane/air was on the order of 12 inches. HOWEVER, I suspect that diameter varies with pressure as well. Double the pressure, half the diameter? I don't know.....
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Postby joannaardway » Tue Oct 10, 2006 1:47 pm

Perhaps double the pressure, half the cross sectional area.....

If people had to know the likely limits, then surely someone could delibrately attempt to achieve detonatation in a controlled situation, with a helluva tough launcher.

Would I be correct in stating that no-one has yet achieved detonation, accidentaly or otherwise?
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Postby boilingleadbath » Tue Oct 10, 2006 2:34 pm

The gexcon <a href="http://www.gexcon.com/index.php?src=handbook/GEXHBchap6.htm">"gas explosion handbook"</a> says that the cell size of propane is ~55 mm at stoicometric, and that a detonation can proceed in a pipe with a diameter > cell size/3.

...they also say that the diameter of the pipe has to be at least 13x that of the cell for the pipe to transmit the shockwave into a medium, though (or at least as a sphericaly) - which, combined with their statment that measured cell sizes sometimes vary by a factor of two, could explain your statment.

I also found <a href="http://www.galcit.caltech.edu/detn_db/html/HC-Ox2.html">this</a>, which (although using O2-C3H8) might be useful.
(that website seems to have alot of data on detonations, acctualy...)

However, the run-up distance to get a detonation (asuming no chamber obstructions - which would quicken it) is crazily long, on the order of 12 meters for a 4" pipe at 1 atm. (gexcon, chapter 9)
And a closed end (or semi-closed, in our case) would probably make that take even longer.
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Postby spuzi14 » Tue Oct 10, 2006 7:49 pm

This has always been a fascinating concept to me. To my understanding, detonation occurs when the flame front causes fuel in front of it to compress and then explode. If that's wrong then what I'm about to ask probably won't count.

It was stated above that the lenght of a chamber for detonation would be very long. Wouldn't that length be shortened once the compressed air was added?
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Postby boilingleadbath » Tue Oct 10, 2006 9:42 pm

Yeah, the gexcon handbook mentioned that effect... I don't think they quantified it, though.

Put a moment of searching into it:
In H2/O2: run-up = P^-1.17
In hydrocarbon/[assumed to be O2]: run-up = p^-x (where x is .4 to .8)
(found <a href="">here</a>)

I could put more effort into it, but it's late... so I'll just provide you with my search terms: "run up distance" [and] "initial pressure"
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Postby tim jones » Wed Oct 11, 2006 2:41 pm

Check Figures 9.7 and 9.8 in the Gexcon Gas Explosion Handbook:

Figure 9.7 gives us a nice correlation between pipe diameter and run-up distance for stoichiometric mixtures of propane in air and ethylene in air. Reading from the propane curve, run-up distance for a 100mm (4 inch) pipe is about 12m (over 39 feet). That is unrealisticly long for a 4" dia. spudgun combustion chamber. The ratio between pipe diameter and run-up distance appears to be constant for each fuel.

Figure 9.8 gives us correlation between %fuel in air and run-up distance in a 50mm pipe for mixtures of propane in air and ethylene in air. Again reading from the propane curve, run-up distance for a stoichiometric mix is about 4.5m (about 15 feet). For 3% and 6% mixes, the run-up distances are 7m (23 ft) and 8.4m (27 ft), respectively.

Even though detonations can occur in propane-air mixtures in pipes smaller than 2" dia., these charts indicate that our small propane-fired combustion launchers lack sufficient run-up distance for flame propagation to transition to detonation. D_Hall's project is another story.
tim jones
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Postby clide » Wed Oct 11, 2006 5:30 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by joannaardway
Would I be correct in stating that no-one has yet achieved detonation, accidentaly or otherwise?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Fuzzyscorpian had his SCH80 hybrid explode, can't say for sure if it was a detonation, but it seems very possible that it was. IIRC the projectile went through more boards than normal despite the chamber blowing.

http://forums.spudtech.com/topic.asp?AR ... IC_ID=6095
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